[Illustration: THE AUTHOR’S OBSERVATORY.]
THE
PRACTICAL ASTRONOMER,
COMPRISING
ILLUSTRATIONS OF LIGHT AND COLOURS--PRACTICAL
DESCRIPTIONS OF ALL KINDS OF TELESCOPES--THE
USE OF THE EQUATORIAL-TRANSIT--CIRCULAR,
AND OTHER ASTRONOMICAL
INSTRUMENTS,
A PARTICULAR ACCOUNT OF THE
EARL OF ROSSE’S LARGE TELESCOPES,
AND OTHER TOPICS CONNECTED WITH ASTRONOMY.
BY THOMAS DICK, LL.D.
AUTHOR OF THE “CHRISTIAN PHILOSOPHER,” “CELESTIAL SCENERY,”
“THE SIDEREAL HEAVENS,” &C. &C.
Illustrated with One hundred Engravings.
SEELEY, BURNSIDE, AND SEELEY,
FLEET-STREET, LONDON.
MDCCCXLV.
PRINTED BY L. SEELEY.
PREFACE.
The following work was announced several years ago in the preface
to the volume on “The Sidereal Heavens;” since which time numerous
enquiries have been made after it by correspondents in England, the
West Indies, and America. It was nearly ready for publication three
years ago; but circumstances over which the Author had no controul,
prevented its appearance at that period. This delay, however, has
enabled him to introduce descriptions of certain instruments and
inventions which were partly unknown at the time to which he refers.
The title “Practical Astronomer” has been fixed upon, as the shortest
that could be selected, although the volume does not comprise a variety
of topics and discussions generally comprehended in this department of
astronomy. The work is intended for the information of general readers,
especially for those who have acquired a relish for astronomical
pursuits, and who wish to become acquainted with the instruments by
which celestial observations are made, and to apply their mechanical
skill to the construction of some of those which they may wish to
possess. With this view the Author has entered into a variety of minute
details, in reference to the construction and practical application
of all kinds of telescopes, &c. which are not to be found in general
treatises on Optics and Astronomy.
As _Light_ is the foundation of astronomical science, and of all the
instruments used for celestial observation, a brief description is
given of the general properties of light--of the laws by which it is
refracted and reflected when passing through different mediums--and of
the effects it produces in the system of nature--in order to prepare
the way for a clear understanding of the principles on which optical
instruments are constructed, and the effects they produce.
As this, as well as every other physical subject, forms a part of the
arrangements of the Creator throughout the material system--the Author
has occasionally taken an opportunity of directing the attention of
the reader to the Wisdom and Beneficence of the Great First Cause, and
of introducing those moral reflections which naturally flow from the
subject.
The present is the ninth volume which the Author has presented to
the public, and he indulges the hope that it will meet with the same
favourable reception which his former publications have uniformly
experienced. It was originally intended to conclude the volume with a
few remarks on the _utility_ of astronomical studies, and their _moral_
and _religious_ tendency, but this has been prevented, for the present,
in consequence of the work having swelled to a greater size than was
anticipated. Should he again appear before the public as an author, the
subject of discussion and illustration will have a more direct bearing
than the present on the great objects of religion and a future world.
_Broughty Ferry, near Dundee, August, 1845._
CONTENTS.
PART I.
ON LIGHT.
INTRODUCTION.
Necessity of light to the knowledge and happiness of all sentient
beings--Its beautiful and enlivening effects--An emblem of the
Deity--Provision made for its universal diffusion
_page_ 1-7.
CHAPTER I.
GENERAL PROPERTIES OF LIGHT.
Interesting nature of this study--Different hypotheses which have
been formed respecting the nature of light--It radiates in straight
lines--Moves with amazing velocity--Flows in all directions from
luminous bodies--Duration of its impressions on the eye--Supposed
to have a certain degree of force or momentum--Experiments in
relation to this point--Its intensity diminished in proportion to
the square of the distance--Its reflection from opake bodies renders
objects visible--Intensity of reflected light--Subject to the law
of attraction--Forms a constituent part of certain bodies--_Solar
phosphori_, and the phenomena they exhibit--Produces certain effects on
planets and flowers, exemplified in a variety of instances--Supposed to
have an influence on the _propagation of sound_
_page_ 8-37
Reflections on the nature of light, and the multifarious effects
it produces throughout the universe--A representation of the
Divinity--Wisdom and Goodness of God displayed in its formation
_page_ 37-40.
CHAPTER II.
ON THE REFRACTION OF LIGHT.
Nature of refraction--Illustrated by experiments--Angle of
refraction--Familiar experiments illustrative of refraction--Refraction
explains the causes of many curious and interesting phenomena--Its
effect on the heavenly bodies--On the twilight--Illustrated by figures
_page_ 41-53.
EXTRAORDINARY CASES OF REFRACTION IN RELATION TO TERRESTRIAL OBJECTS.
Extraordinary appearance of the coast of France from
Hastings--Appearance of a ship seen by Captain Colby, beyond the
coast of Caithness--Scoresby’s view of his father’s ship when
beyond the horizon--Phenomenon near the Himalaya mountains--Bell
Rock light-house--Summary statement of the diversified effects of
refraction--Reflections on the beneficent and diversified effects
produced by the law of refraction--It increases the length of the day,
particularly in the polar regions--Is the cause of that splendour which
appears in the objects around us--Quantity of refraction in respect
to terrestrial objects, and its utility--Its effects may be more
diversified in other worlds
_page_ 53-63.
CHAPTER III.
ON THE REFRACTION OF LIGHT THROUGH SPHERICAL TRANSPARENT
SUBSTANCES, OR LENSES.
Refraction the foundation of optical instruments--Various forms of
lenses--_Parallel_, _converging_, and _diverging_ rays--Illustrated by
diagrams--_Concave_ lenses, their effects, and how to find their focal
distances--IMAGES formed by convex lenses--Illustrated by
experiments--Principles in relation to images formed by lenses--Their
magnifying powers, &c.
_page_ 63-75.
REFLECTIONS DEDUCED FROM THE PRECEDING SUBJECT.
Property of the rays of light in forming _images_ of objects--Wonderful
results and discoveries which have flowed from this property--in
relation to our knowledge of the scenery of the heavens and the minute
parts of nature--and of our views of the attributes of Deity
_page_ 75-80.
CHAPTER IV.
ON THE REFLECTION OF LIGHT.
Nature of reflection--Plane, convex, and concave speculums--Angle of
reflection--Reflection of objects from plane mirrors, illustrated
by figures--_Reflection by Convex and Concave mirrors_--Properties
of _convex_ mirrors, and the purposes to which they are applied.
Properties of concave speculums, and their utility--Of the _images_
formed by concave speculums--Illustrated by a variety of figures and
experiments--Their power of magnifying and burning--Amusing deceptions
produced by--Resemblance between the properties of convex lenses, and
concave mirrors--Quantity of light reflected by polished surfaces
_page_ 81-106.
UNCOMMON APPEARANCES OF NATURE PRODUCED BY THE COMBINED INFLUENCE OF
REFLECTION AND REFRACTION.
_Fata Morgana_--The Mirage--Inverted images of ships seen in the
horizon--Appearance of Dover castle at Ramsgate--Spectre of the
Brocken--Scenes in the Highlands of Scotland--Large cross seen at Migné
in France--Dr. Wollaston’s illustrations of such phenomena--Utility of
science in dissipating superstitious fears
_page_ 106-118.
REMARKS AND REFLECTIONS IN REFERENCE TO THE PHENOMENA DESCRIBED ABOVE.
Light, the beauty of the universe, and a symbol of the Divinity--In
other worlds it may produce an infinite variety of sublime scenery
_page_ 118-122.
CHAPTER V.
SECT. 1.--ON THE COLOURS OF LIGHT.
Colours, the beauty of nature--Opinions which were formerly entertained
respecting their cause--Sir I. Newton’s experiments with the
Prism--Colours and phenomena produced by the prism--Imperfection of
optic lenses--Various illustrations--Differently coloured rays have
not the same illuminating power--Heating and chemical properties of
some of the rays of the solar spectrum--property of communicating the
Magnetic power--Fraunhofer, and his discoveries in reference to the
spectrum--Experiments on white and coloured light
_page_ 123-137.
SECT. 2.--ON THE COLOURS OF NATURAL OBJECTS.
Colours not in the objects themselves, but in the light which falls
upon them--Illustrations of this position--Atmosphere the source of a
variety of colours--Various natural phenomena, in relation to colour,
explained
_page_ 137-143.
SECT. 3.--PHENOMENA OF THE RAINBOW.
Rainbow described--Experiments to illustrate its cause--Descriptions
of its various phenomena, and optical explanations of their
causes--Rainbows exhibiting _complete circles_--Their appearance in
different countries--Summary view of the principal facts respecting the
rainbow--_Lunar_ rainbows--Scriptural allusions to the rainbow--Whether
there was any rainbow before the deluge
_page_ 144-157.
SECT. 4.--REFLECTIONS ON THE BEAUTY AND UTILITY OF COLOURS.
Beauty and variety derived from colours in the scenery of
nature--Colours produced by the atmosphere in different countries--What
would be the aspect of nature, in heaven and on earth, were there only
one colour--How it would affect the common intercourse and employments
of society--Wisdom and Beneficence of the Creator displayed in the
diversity of colours--Throughout all the systems of the universe, a
diversity of colours prevails--This subject has a tendency to inspire
us with gratitude
_page_ 158-168.
PART II.
ON TELESCOPES.
CHAPTER I.
HISTORY OF THE INVENTION OF TELESCOPES.
The telescope a noble instrument--Effects it produces--Whether
known to the ancients--Friar Bacon’s ideas respecting
telescopes--First constructed in Holland--The invention claimed
by different persons--Galileo’s account of the construction of his
telescope--Discoveries which he made with this instrument--How his
discoveries were received by the learned--Specimens of learned nonsense
brought forward by pretended philosophers--Supposed length of Galileo’s
telescope--Various claimants to the invention of this instrument
_page_ 169-183.
CHAPTER II.
OF THE CAMERA OBSCURA.
Appearance of objects in a camera obscura--The dark chamber--This
instrument serves to explain the nature of a refracting
telescope--Particulars to be attended to, in exhibiting objects with
the Camera--It illustrates the nature of vision--Revolving camera
obscura--_Portable_ camera
_page_ 184-196.
THE DAGUERREOTYPE.
An important discovery for fixing the images produced by the
camera--Description of the Daguerreotype process--Preparation of
the plate, fixing the impression, &c.--Preparation of photogenic
paper--Beneficial effects which this art may produce--Representations
of objects in the heavens, &c.
_page_ 196-205.
CHAPTER III.
ON THE OPTICAL ANGLE, AND THE APPARENT MAGNITUDE OF OBJECTS.
Various illustrations of the apparent magnitude of objects--Fallacies
in relation to apparent magnitudes--Apparent magnitudes in the
heavens--Difference between _absolute_ and apparent magnitudes
_page_ 206-213.
CHAPTER IV.
ON THE DIFFERENT KINDS OF REFRACTING TELESCOPES.
SECT. 1.--THE GALILEAN TELESCOPE.
Construction and peculiar properties of this instrument
_page_ 214-217.
SECT. 2.--THE COMMON ASTRONOMICAL REFRACTING TELESCOPE.
Description of its nature and construction--How its magnifying power is
determined. Table of the linear aperture, magnifying powers, &c., of
astronomical telescopes from 1 to 120 feet in length--Summary view of
the properties of this telescope
_page_ 218-224.
SECT. 3.--THE AERIAL TELESCOPE.
This telescope is used without a tube--Description of the apparatus
connected with it, illustrated with figures--Huygens’ Hartsocker’s and
Cassini’s large telescopes
SECT. 4.--THE COMMON REFRACTING TELESCOPE FOR TERRESTRIAL
OBJECTS.
Arrangement of its lenses--Magnifying power--Manner in which the rays
of light are refracted through the telescopes now described
_page_ 228-231.
SECT. 5.--TELESCOPE FORMED BY A SINGLE LENS.
Various experiments in relation to this point--Experiments with a lens
26 focal distance, and 11-1/2 inches diameter
_page_ 232-235.
SECT. 6.--THE ACHROMATIC TELESCOPE.
Imperfections of common refracting telescopes--Dollond’s
discovery--Newton’s error--Explanation of the principle of achromatic
telescopes--Combination of lenses--Difficulties in the construction
of such instruments--Difficulty in procuring large disks of flint
glass--Guinaud’s experiments
_page_ 235-248.
NOTICES OF SOME LARGE ACHROMATIC TELESCOPES ON THE CONTINENT, AND IN
GREAT BRITAIN.
The Dorpat telescope--Sir J. South’s telescope--Captain Smyth’s--Rev.
Dr. Pearson’s--Mr. Lawson’s--Mr. Cooper’s--Mr. Bridges’,
&c.,--Achromatics in Cambridge and Paris observatories
_pages_ 248-254.
ACHROMATIC TELESCOPES OF A MODERATE SIZE, WITH THEIR PRICES, AS SOLD BY
LONDON OPTICIANS.
The 2-1/2 feet Achromatic--The 3-1/2 feet--The powers applied to
it--and the views it gives of the heavenly bodies--The 5 feet
achromatic--_Stands_ for telescopes, illustrated by engravings
_page_ 254-264.
PROPORTIONS OR CURVATURE OF THE LENSES WHICH FORM AN ACHROMATIC
OBJECT-GLASS.
Various tables and explanations
_page_ 265-269.
ACHROMATIC TELESCOPES COMPOSED OF FLUID LENSES.
Blair’s fluid telescope, with an account of its performance--Barlow’s
large refracting telescope with a fluid concave lens--Its construction,
and the effect it produces on double stars, &c.--Rogers’ achromatic
telescope on a new plan--Wilson’s telescope, &c.
_page_ 269-283.
CHAPTER V.
ON REFLECTING TELESCOPES.
SECT. 1.--HISTORY OF THE INVENTION, AND A GENERAL DESCRIPTION OF
THE CONSTRUCTION OF THESE INSTRUMENTS.
Gregory’s Reflector--Newtonian Reflector--Cassegrainian
Reflector--Magnifying powers of reflectors--Short’s Reflectors--Their
powers and prices--_General remarks on Gregorian reflectors_--Apertures
and magnifying powers of Newtonian telescopes--_Prices_ of Reflecting
telescopes
_page_ 284-301.
SECT. 2.--THE HERSCHELIAN TELESCOPE.
Description of Sir W. Herschel’s 40 feet telescope, with its machinery,
apparatus, and the discoveries made by it--Sir J. Herschel’s 20 feet
reflector _page_ 301-308.
SECT. 3.--RAMAGE’S LARGE REFLECTING TELESCOPE.
_page_ 308-311.
SECT. 4.--THE AERIAL REFLECTOR--CONSTRUCTED BY THE AUTHOR.
Construction of this telescope, and the manner of using
it--Illustrated by figures--Its properties and advantages--Tube not
necessary in reflecting telescopes--How a large reflector might be
constructed without a tube--How the form of a telescope may be used for
viewing perspectives
_page_ 311-325.
SECT. 4.--EARL OF ROSSE’S REFLECTING TELESCOPES.
His mode of forming a large speculum, &c., see also, Appendix
_page_ 325-328.
SECT. 5.--REFLECTING TELESCOPES WITH GLASS SPECULA.
Various experiments on this subject, with their results
_page_ 329-331.
SECT. 6.--A REFLECTING TELESCOPE WITH A SINGLE MIRROR AND NO
EYE-PIECE.
Experiments illustrative of this construction
_page_ 332-334.
ON THE EYE-PIECES OF TELESCOPES. ASTRONOMICAL EYE-PIECES.
Huygenian eye-piece--Ramsden’s eye-piece--Aberration of
lenses--Celestial eye-pieces with variable powers. _Diagonal_
eye-pieces--Various forms of them described--Various aspects in which
objects may be viewed by them
_page_ 335-347.
TERRESTRIAL EYE-PIECES.
Eye-pieces with four lenses--Proportions of the focal lengths of these
lenses--Dimensions and powers of several eye-pieces stated
_page_ 347-353.
DESCRIPTION OF AN EYE-PIECE, &C., OF AN OLD DUTCH ACHROMATIC TELESCOPE.
This telescope supposed to have been _invented_ in Holland before
Dollond’s discovery was known--Peculiarity of its eye-piece
_page_ 354-357.
DESCRIPTION OF THE PANCRATIC EYE-TUBE.
_page_ 357-360.
CHAPTER VI.
MISCELLANEOUS REMARKS IN RELATION TO TELESCOPES.
1. Adjustments requisite to be attended to in the use of
telescopes--2. State of the atmosphere most proper for observing
terrestrial and celestial objects--Average number of hours in the year
fit for celestial observations.--3. On the magnifying powers requisite
for observing the phenomena of the different planets--Comets--Double
stars, &c.--Illustrated at large from p. 369-380.--4. Mode of
exhibiting the solar spots--Eye-pieces best adapted for this
purpose--How they may be exhibited to a large company--Mode in which
their dimensions may be determined.--5. On the _space-penetrating_
power of telescopes--Herschel’s observations on space-penetrating
powers--Comparison of achromatic and Gregorian reflectors.--6. On
choosing telescopes, and ascertaining their properties--Various modes
of ascertaining the goodness of telescopes--General remarks and
cautions on this point--A circumstance which requires to be attended
to in using achromatics.--7. On the mode of determining the magnifying
power of telescopes--Various experiments in relation to this point.--8.
On cleaning the lenses of telescopes
_page_ 361-407.
ON MEGALASCOPES, OR TELESCOPES FOR VIEWING VERY NEAR OBJECTS.
Mode of adapting a telescope for this purpose--objects to which they
may be applied
_page_ 407-411.
REFLECTIONS ON LIGHT AND VISION, AND ON THE NATURE AND UTILITY OF
TELESCOPES.
Wonderful and mysterious nature of light--The organ of vision, and its
expansive range--Wonderful nature of the telescope, and the objects it
has disclosed to view--No boundaries should be set to the discoveries
of science and the improvement of art--The telescope is a machine which
virtually transports us to the distant regions of space--It enlarges
our views of the sublime scenes of creation--It has tended to amplify
our conceptions of the empire and the attributes of the Deity--Various
uses of this instrument in relation to science and common life
_page_ 411-431.
CHAPTER VII.
ON THE METHOD OF GRINDING AND POLISHING OPTICAL LENSES AND
SPECULA.
1. Directions for grinding lenses for eye-glasses, microscopes,
&c.--2. Method of casting and grinding the specula of reflecting
telescopes--Compositions for speculum metal--To try the figure of the
metal--To adjust the eye-hole of Gregorian reflectors--To center the
specula--To center lenses. _page_ 432-442.
PART III.
ON VARIOUS ASTRONOMICAL INSTRUMENTS.
CHAPTER I.
ON MICROMETERS.
Various descriptions of micrometers--_Cavallo’s_ micrometer
described--To ascertain the value of its divisions--Practical uses
of this micrometer--Problems which may be solved by it--Tables for
facilitating its use
_page_ 443-452.
CHAPTER II.
ON THE EQUATORIAL TELESCOPE, OR PORTABLE OBSERVATORY.
History of equatorials--Description of one of the simplest construction
of these instruments--To adjust the equatorial for observation--To
adjust the line of sight--Description of the _nonius_--To find the
meridian line by one observation--Manner of observing stars and planets
in the day-time
_page_ 453-464.
OBSERVATIONS, BY THE AUTHOR, ON THE FIXED STARS AND PLANETS, MADE IN
THE DAY-TIME, BY THE EQUATORIAL.
Object of these observations--stars of the first and second
magnitudes--General deductions from these observations
_page_ 464-469.
OBSERVATIONS ON THE PLANETS IN THE DAY-TIME.
Series of observations on _Venus_, when near the sun--Seen at the time
of her superior conjunction in 1843--Conclusions deduced from these
observations--phenomena observed during these observations--Remarkable
phenomenon during an eclipse of the sun
_page_ 469-480.
OBSERVATIONS ON JUPITER AND OTHER PLANETS.
General conclusions, &c.
_page_ 480-485.
UTILITY OF CELESTIAL DAY OBSERVATIONS.
_page_ 485-491.
ON THE ASTRONOMICAL QUADRANT.
_page_ 492-496.
THE ASTRONOMICAL CIRCLE.
_page_ 496-502.
THE TRANSIT INSTRUMENT.
_page_ 502-505.
CHAPTER III.
ON OBSERVATORIES.
Leading features of a spot adapted for celestial observations--Public
and private observatories--Greenwich observatory--Instruments with
which an observatory should be furnished--The Author’s private
observatory--Revolving domes for observatories--Cautions to be attended
to in celestial observations
_page_ 506-516.
CHAPTER IV.
ON ORRERIES OR PLANETARIUMS.
History of such machines--Sphere of Archimedes and Posidonius--Dr.
Long’s _Uranium_--Wheel-work of the common Planetarium--Figure
representing this machine--Problems which may be performed by it
_page_ 517-527.
DR. HENDERSON’S PLANETARIUM.
Section of its wheel-work--Number of teeth in the wheels and pinions
which move the different planets--Extreme accuracy of these movements.
_page_ 527-538.
ON THE VARIOUS OPINIONS WHICH WERE ORIGINALLY FORMED OF SATURN’S RING,
ILLUSTRATED WITH 13 VIEWS.
When and by whom its true figure was discovered.
_page_ 538-543.
ON THE SUPPOSED DIVISION OF THE EXTERIOR RING OF SATURN.
Kater’s, Short’s, Quetelet’s and Decuppis’s observations
_page_ 543-547.
APPENDIX.
1. DESCRIPTION OF THE EARL OF ROSSE’S LARGEST TELESCOPE.
Composition of the speculum, and the process of casting it--Mode of
grinding and polishing it--Manner in which it is filled up--Expenses
incurred in its construction--Results of observations which have been
made with it--Two views representing this instrument and the buildings
connected with it--Sir J. South’s remarks and anticipations
_page_ 548-562.
2. HINTS TO AMATEURS IN ASTRONOMY RESPECTING THE CONSTRUCTION OF
TELESCOPES.
_page_ 563.
LIST OF ENGRAVINGS.
_Figure_ _Page_
1. Representation of the diminution of the intensity of light. 22
2. Illustrative of the refraction of light. 43
3. Representing the angles of incidence and refraction. 44
4. The refraction of the atmosphere. 51
5. Various forms of lenses. 65
6, 7, 8. Parallel, converging, and diverging rays. 66
9, 10, 11. Passage of parallel, diverging, and
converging rays through convex lenses. 67
12. Passage of parallel rays through concave lenses. 69
13. Images formed by convex lenses. 71
14. Angle of incidence and reflection. 83
15. Images as reflected from a plane mirror. 84
16. Illustrative of reflections from a plane mirror. 85
17. Shewing how the image in a plane mirror is twice the
length of the object. 86
18. Reflection from _concave_ mirrors. 87
19. Reflection from _convex_ mirrors. 89
20. Parallel rays as reflected from concave mirrors. 91
21. Diverging rays as reflected from concave mirrors. 91
22. Images formed before concave mirrors. 93
23. Images formed behind concave mirrors. 96
24. Illustrating the magnifying power of concave mirrors. 97
25. Inverted images formed in the front of concave mirrors. 98
26. Illustrative of deceptions produced by concave mirrors. 100
27, 28. Experiment with a bottle half filled with water. 101
29. Effect of extraordinary refraction on ships at sea. 109
30. Experiment for illustrating the causes of uncommon refraction. 117
31. Prismatic spectrum. 127
32. Different foci of coloured rays in convex lenses. 129
33. Experiment to show the different foci of red and violet rays. 129
34. Illustrative of the prismatic colours. 136
35. Explanatory of refraction and reflection from drops of rain. 147
36. Explanatory of the rainbow. 149
37. Images of objects formed in a dark chamber. 187
38. The revolving Camera Obscura. 194
39, 40. The portable Camera Obscura. 195, 196
40, 41, 42. Illustrative of the angle of vision,
and the apparent magnitude of objects. 206, 207, 208
43. The Galilean telescope. 215
44. The astronomical telescope. 218
45, 46. The aerial refracting telescope. 226
47. The common refracting telescope. 228
48, 49, 50. Manner in which the rays of light are refracted in
telescopes. 231
51. Telescope with a single lens. 234
52. Illustrative of spherical aberration 236
53. Illustrative of the principle of achromatic telescopes. 241
54, 55. Double and treble achromatic object-glass. 242
57. Common stand for achromatic telescopes. 260
58. Equatorial stand for achromatic telescopes. 262
59. Dollond’s stand for achromatic telescopes. 264
60. Blair’s fluid achromatic object-glass. 271
61. Barlow’s fluid telescope. 274
62, 63, 64, 65, 66. Various forms of reflecting telescopes. 288
67. Gregorian reflecting telescope. 293
69. The aerial reflector. 313
70. Front view of the aerial reflector. 314
71. Construction of large reflecting telescope 322
72. Reflecting telescope with a single mirror 332
73. Huygenian eye-piece. 336
74. Ramsden’s eye-piece. 339
75, 76. Combination of lenses for achromatic eye-pieces. 340
77, 78. Diagonal eye-pieces. 344, 345
79. Terrestrial eye-piece with four lenses. 349
80. Eye-piece of an old Dutch achromatic telescope. 356
81. Pancratic eye-piece. 359
82. Manner of exhibiting the solar spots. 384
84. Mode of measuring distances from one station. 430
85. Cavallo’s micrometer. 446
86. The equatorial telescope, or portable observatory. 455
87. Figure to illustrate the principle of the quadrant. 491
88. The astronomical quadrant. 493
89. The astronomical circle. 496
90. The transit instrument. 502
91. Plan of a private observatory. 511
92. Rotatory dome for an observatory. 513
93. Wheel-work of a planetarium. 521
94. Perspective view of a planetarium. 522
95. Apparatus for exhibiting the retrograde motions of the
planets. 525
96. Section of the wheel-work of Dr. Henderson’s planetarium. 528
97. Thirteen views of the supposed form of Saturn’s ring. 539
98. Earl of Rosse’s Great Telescope. 559
99. Section of the machinery connected with the telescope. 560
100. Perspective view of the author’s observatory--to front the title.
THE
PRACTICAL ASTRONOMER.
PART I.
ON LIGHT.
INTRODUCTION.
Light is that invisible etherial matter which renders objects
perceptible by the visual organs. It appears to be distributed
throughout the immensity of the universe, and is essentially requisite
to the enjoyment of every rank of perceptive existence. It is by
the agency of this mysterious substance, that we become acquainted
with the beauties and sublimities of the universe, and the wonderful
operations of the Almighty Creator. Without its universal influence,
an impenetrable veil would be thrown over the distant scenes of
creation; the sun, the moon, the planets, and the starry orbs, would
be shrouded in the deepest darkness, and the variegated surface of
the globe on which we dwell, would be almost unnoticed and unknown.
Creation would disappear, a mysterious gloom would surround the mind
of every intelligence, all around would appear a dismal waste, and an
undistinguished chaos. To whatever quarter we might turn, no form nor
comeliness would be seen, and scarcely a trace of the perfections and
agency of an All Wise and Almighty Being could be perceived throughout
the universal gloom. In short, without the influence of light, no world
could be inhabited, no animated being could subsist in the manner it
now does, no knowledge could be acquired of the works of God, and
happiness, even in the lowest degree, could scarcely be enjoyed by any
organized intelligence.
We have never yet known what it is to live in a world deprived of this
delightful visitant; for in the darkest night we enjoy a share of its
beneficial agency, and even in the deepest dungeon its influence is
not altogether unfelt.[1] The blind, indeed, do not directly enjoy
the advantages of light, but its influence is reflected upon them,
and their knowledge is promoted through the medium of those who enjoy
the use of their visual organs. Were all the inhabitants of the world
deprived of their eye-sight, neither knowledge nor happiness, such as
we now possess, could possibly be enjoyed.
There is nothing which so strikingly displays the beneficial and
enlivening effects of light, as the dawn of a mild morning after a
night of darkness and tempest. All appears gloom and desolation, in
our terrestrial abode, till a faint light begins to whiten the eastern
horizon. Every succeeding moment brings along with it something new
and enlivening. The crescent of light towards the east, now expands its
dimensions and rises upwards towards the cope of heaven; and objects,
which a little before were immersed in the deepest gloom, begin to be
clearly distinguished. At length the sun arises, and all nature is
animated by his appearance; the magnificent scene of creation, which
a little before was involved in obscurity, opens gradually to view,
and every object around excites sentiments of wonder, delight, and
adoration. The radiance which emanates from this luminary, displays
before us a world strewed with blessings and embellished with the most
beautiful attire. It unveils the lofty mountains and the forests with
which they are crowned--the fruitful fields with the crops that cover
them--the meadows, with the rivers which water and refresh them--the
plains adorned with verdure, the placid lake and the expansive ocean.
It removes the curtain of darkness from the abodes of men, and shows us
the cities, towns and villages, the lofty domes, the glittering spires,
and the palaces and temples with which the landscape is adorned. The
flowers expand their buds and put forth their colours, the birds awake
to melody, man goes forth to his labour, the sounds of human voices are
heard, and all appears life and activity, as if a new world had emerged
from the darkness of Chaos.
The whole of this splendid scene, which light produces, may be
considered as a new creation, no less grand and beneficent than the
first creation, when the command was issued, “Let there be light, and
light was.” The aurora and the rising sun cause the earth and all
the objects which adorn its surface, to arise out of that profound
darkness and apparent desolation which deprived us of the view of them,
as if they had been no more. It may be affirmed, in full accordance
with truth, that the efflux of light in the dawn of the morning, after
a dark and cloudy night, is even more magnificent and exhilarating
than at the first moment of its creation. At that period, there
were no spectators on earth to admire its glorious effects; and no
objects, such as we now behold, to be embellished with its radiance.
The earth was a shapeless chaos, where no beauty or order could be
perceived; the mountains had not reared their heads; the seas were not
collected into their channels; no rivers rolled through the valleys, no
verdure adorned the plains; the atmosphere was not raised on high to
reflect the radiance, and no animated beings existed to diversify and
enliven the scene. But now, when the dawning of the morning scatters
the darkness of the night, it opens to view a scene of beauty and
magnificence. The heavens are adorned with azure, the clouds are tinged
with the most lively colours, the mountains and plains are clothed with
verdure, and the whole of this lower creation stands forth arrayed with
diversified scenes of beneficence and grandeur, while the contemplative
eye looks round and wonders.
Such, then, are the important and beneficent effects of that _light_
which every moment diffuses its blessings around us. It may justly
be considered as one of the most essential substances connected with
the system of the material universe, and which gives efficiency to
all the other principles and arrangements of nature. Hence we are
informed, in the sacred history, that light was the first production
of the Almighty Creator, and the first born of created beings; for
without it the universe would have presented nothing but an immense
blank to all sentient existences. Hence, likewise, the Divine Being
is metaphorically represented under the idea of light, as being the
source of knowledge and felicity to all subordinate intelligences: “God
is _light_, and in Him is no darkness at all;” and he is exhibited as
“dwelling in light unapproachable and full of glory, whom no man hath
seen or can see.” In allusion to these circumstances, Milton, in his
Paradise Lost, introduces the following beautiful apostrophe:--
‘Hail holy light! offspring of heaven first born,
Or of the eternal co-eternal beam!
May I express thee unblam’d? since God is light,
And never but in unapproached light
Dwelt from eternity; dwelt then in thee,
Bright effluence of bright essence increate.
----Before the sun
Before the heavens thou wert, and at the voice
Of God, as with a mantle, did’st invest
The rising world of waters dark and deep
Won from the void and formless infinite.’
As light is an element of so much importance and utility in the
system of nature, so we find that arrangements have been made for its
universal diffusion throughout all the worlds in the universe. The sun
is one of the principal sources of light to this earth on which we
dwell, and to all the other planetary bodies. And, in order that it may
be _equally_ distributed over every portion of the surfaces of these
globes, to suit the exigencies of their inhabitants, they are endowed
with a motion of rotation, by which every part of their surfaces is
alternately turned towards the source of light; and when one hemisphere
is deprived of the direct influence of the solar rays, its inhabitants
derive a portion of light from luminaries in more distant regions, and
have their views directed to other suns and systems dispersed, in
countless numbers, throughout the remote spaces of the universe. Around
several of the planets, satellites, or moons, have been arranged for
the purpose of throwing light on their surfaces in the absence of the
sun, while at the same time the primary planets themselves reflect an
effulgence of light upon their satellites. All the stars which our
unassisted vision can discern in the midnight sky, and the millions
more which the telescope alone enables us to descry, must be considered
as so many fountains of light, not merely to illuminate the voids of
immensity, but to irradiate with their beams surrounding worlds with
which they are more immediately connected, and to diffuse a general
lustre throughout the amplitudes of infinite space. And, therefore,
we have every reason to believe, that, could we fly, for thousands of
years, with the swiftness of a seraph, through the spaces of immensity,
we should never approach a region of absolute darkness, but should find
ourselves, every moment encompassed with the emanations of light, and
cheered with its benign influences. That Almighty Being who inhabiteth
immensity and “dwells in light inaccessible,” evidently appears to
have diffused light over the remotest spaces of his creation, and to
have thrown a radiance upon all the provinces of his wide and eternal
empire, so that every intellectual being, wherever existing, may feel
its beneficent effects, and be enabled, through its agency, to trace
his wonderful operations, and the glorious attributes with which he is
invested.
As the science of astronomy depends solely on the influence of light
upon the organ of vision, which is the most noble and extensive of
all our senses; and as the construction of telescopes and other
astronomical instruments is founded upon our knowledge of the
nature of light and the laws by which it operates--it is essentially
requisite, before proceeding to a description of such instruments, to
take a cursory view of its nature and properties, in so far as they
have been ascertained, and the effects it produces when obstructed by
certain bodies, or when passing through different mediums.
CHAPTER I.
GENERAL PROPERTIES OF LIGHT.
It is not my intention to discuss the subject of light in minute
detail--a subject which is of considerable extent, and which would
require a separate treatise to illustrate it in all its aspects and
bearings. All that I propose is to offer a few illustrations of
its general properties, and the laws by which it is refracted and
reflected, so as to prepare the way for explaining the nature and
construction of telescopes, and other optical instruments.
There is no branch of natural science more deserving of our study and
investigation than that which relates to light--whether we consider its
beautiful and extensive effects--the magnificence and grandeur of the
objects it unfolds to view--the numerous and diversified phenomena it
exhibits--the optical instruments which a knowledge of its properties
has enabled us to construct--or the daily advantages we derive, as
social beings, from its universal diffusion. If air, which serves as
the medium of sound, and the vehicle of speech, enables us to carry on
an interchange of thought and affection with our fellow-men; how much
more extensively is that intercourse increased by light, which presents
the images of our friends and other objects as it were immediately
before us, in all their interesting forms and aspects--the speaking
eye--the rosy cheeks--the benevolent smile, and the intellectual
forehead! The eye, more susceptible of multifarious impressions than
the other senses, ‘takes in at once the landscape of the world,’ and
enables us to distinguish, in a moment, the shapes and forms of all
its objects, their relative positions, the colours that adorn them,
their diversified aspect, and the motions by which they are transported
from one portion of space to another. Light, through the medium of the
eye, not only unfolds to us the persons of others, in all their minute
modifications and peculiarities, but exhibits us to ourselves. It
presents to our own vision a faithful portrait of our peculiar features
behind reflecting substances, without which property we should remain
entirely ignorant of those traits of countenance which characterize us
in the eyes of others.
But, what is the nature of this substance we call _light_, which thus
unfolds to us the scenes of creation? On this subject two leading
opinions have prevailed in the philosophical world. One of those
opinions is, that the whole sphere of the universe is filled with a
subtle matter, which receives from luminous bodies an agitation which
is incessantly continued, and which, by its vibratory motion, enables
us to perceive luminous bodies. According to this opinion, light may
be considered as analogous to sound, which is conveyed to the ear by
the vibratory motions of the air. This was the hypothesis of Descartes,
which was adopted, with some modifications, by the celebrated Euler,
Huygens, Franklin, and other philosophers, and has been admitted by
several scientific gentlemen of the present day. The other opinion is,
that light consists of the emission or emanation of the particles of
luminous bodies, thrown out incessantly on all sides, in consequence
of the continued agitation it experiences. This is the hypothesis of
the illustrious Newton, and has been most generally adopted by British
philosophers.
To the first hypothesis, it is objected that, if true, ‘light would
not only spread itself in a direct line, but its motion would be
transmitted in every direction like that of sound, and would convey
the impression of luminous bodies in the regions of space beyond the
obstacles that intervene to stop its progress.’ No wall or other opaque
body could obstruct its course, if it undulated in every direction like
sound; and it would be a necessary consequence, that we should have no
night, nor any such phenomena as eclipses of the sun or moon, or of
the satellites of Jupiter and Saturn. This objection has never been
very satisfactorily answered. On the other hand, Euler brings forward
the following objections against the Newtonian doctrine of emanation.
1. That, were the sun emitting continually, and in all directions,
such floods of luminous matter with a velocity so prodigious, he must
speedily be exhausted, or at least, some alteration must, after the
lapse of so many ages, be perceptible. 2. That the sun is not the only
body that emits rays, but that all the stars have the same quality; and
as every where the rays of the sun must be crossing the rays of the
stars, their collision must be violent in the extreme, and that their
direction must be changed by such a collision.[2]
To the first of these objections it is answered--that so vast is the
tenuity of light, that it utterly exceeds the power of conception:
the most delicate instrument having never been certainly put in
motion by the impulse of the accumulated sun-beams. It has been
calculated that in the space of 385,130,000 Egyptian years, (of 360
days,) the sun would lose only the 1/1,217,420th of his bulk from the
continual efflux of his light. And, therefore, if in 385 _millions_ of
years the sun’s diminution would be so extremely small, it would be
altogether insensible during the comparatively short period of five
or six thousand years. To the second objection it is replied--that
the particles of light are so extremely rare that their distance from
one another is incomparably greater than their diameters--that all
objections of this kind vanish when we attend to the continuation of
the impression upon the retina, and to the small number of luminous
particles which are on that account necessary for producing constant
vision. For it appears, from the accurate experiments of M. D’Arcy,
that the impression of light upon the retina continues _eight thirds_,
and as a particle of light would move through 26,000 miles in that
time, constant vision would be maintained by a succession of luminous
particles twenty-six thousand miles distant from each other.
Without attempting to decide on the merits of these two hypotheses, I
shall leave the reader to adopt that opinion which he may judge to be
attended with the fewest difficulties, and proceed to illustrate some
of the _properties of light_:--and in the discussion of this subject, I
shall generally adhere to the terms employed by those who have adopted
the hypothesis of the _emanation_ of light.
1. _Light emanates or radiates from luminous bodies in a straight
line._ This property is proved by the impossibility of seeing light
through bent tubes, or small holes pierced in metallic plates placed
one behind another, except the holes be placed in a straight line.
If we endeavour to look at the sun or a candle through the bore of a
bended pipe, we cannot perceive the object, nor any light proceeding
from it, but through a straight pipe the object may be perceived. This
is likewise evident from the form of the rays of light that penetrate
a dark room, which proceed straight forward in lines proceeding from
the luminous body; and from the form of the _shadows_ which bodies
project, which are bounded by right lines passing from the luminous
body, and meeting the lines which terminate the interposing body. This
property may be demonstrated to the eye, by causing light to pass
through small holes into a dark room filled with smoke or dust. It is
to be understood, however, that in this case, the rays of light are
considered as passing through the same medium; for when they pass from
air into water, glass, or other media, they are bent at the point where
they enter a different medium, as we shall afterwards have occasion to
explain.
2. _Light moves with amazing velocity._ The ancients believed
that it was propagated from the sun and other luminous bodies
_instantaneously_; but the observations of modern astronomers have
demonstrated that this is an erroneous hypothesis, and that light, like
other projectiles, occupies a certain time in passing from one part of
space to another. Its velocity, however, is prodigious, and exceeds
that of any other body with which we are acquainted. It flies across
the earth’s orbit--a space 190 millions of miles in extent, in the
course of sixteen and a half minutes, which is at the rate of 192,000
miles every second, and more than a million of times swifter than a
cannon ball flying with its greatest velocity. It appears from the
discoveries of Dr. Bradley, respecting the aberration of the stars,
that light flies from those bodies, with a velocity similar, if not
exactly the same; so that the light of the sun, the planets, the stars,
and every luminous body in the universe is propagated with _uniform_
velocity.[3] But, if the velocity of light be so very great, it may be
asked, how does it not strike against all objects with a force equal
to its velocity? If the finest sand were thrown against our bodies
with the hundredth part of this velocity, each grain would pierce us
as certainly as the sharpest and swiftest arrows from a bow. It is a
principle in mechanics that the force with which all bodies strike,
is in proportion to the size of these bodies, or the quantity of
matter they contain, multiplied by the velocity with which they move.
Therefore if the particles of light were not almost infinitely small,
they would, of necessity prove destructive in the highest degree. If a
particle of light were equal in size to the twelve hundred thousandth
part of a small grain of sand,--supposing light to be material--we
should be no more able to withstand its force than we should that of
sand shot point blank from the mouth of a cannon. Every object would
be battered and perforated by such celestial artillery, till our world
were laid in ruins, and every living being destroyed. And herein
are the wisdom and benevolence of the Creator displayed in making
the particles of light so extremely small as to render them in some
degree proportionate to the greatness of the force with which they are
impelled; otherwise, all nature would have been thrown into ruin and
confusion, and the great globes of the universe shattered to atoms.
We have many proofs, besides the above, that the particles of light are
next to infinitely small. We find that they penetrate with facility the
hardest substances, such as crystal, glass, various kinds of precious
stones, and even the diamond itself, though among the hardest of
stones; for such bodies could not be transparent, unless light found
an easy passage through their pores. When a candle is lighted in an
elevated situation, in the space of a second or two, it will fill a
cubical space (if there be no interruption) of two miles around it, in
every direction, with luminous particles, before the least sensible
part of its substance is lost by the candle:--that is, it will in a
short instant, fill a sphere four miles in diameter, twelve and a half
miles in circumference, and containing thirty-three and a half cubical
miles with particles of light; for an eye placed in any part of this
cubical space would perceive the light emitted by the candle. It has
been calculated that the number of particles of light contained in
such a space cannot be less than _four hundred septillions_--a number
which is _six billions_ of times greater than the number of grains of
sand which could be contained in the whole earth considered as a solid
globe, and supposing each cubic inch of it to contain ten hundred
thousand grains. Such is the inconceivable tenuity of that substance
which emanates from all luminous bodies, and which gives beauty and
splendour to the universe! This may also be evinced by the following
experiment. Make a small pin-hole in a piece of black paper, and hold
the paper upright facing a row of candles placed near each other, and
at a little distance behind the black paper, place a piece of white
pasteboard. On this pasteboard the rays which flow from all the candles
through the small hole in the black paper, will form as many specks
of light as there are candles, each speck being as clear and distinct
as if there were only one speck from a single candle. This experiment
shows that the streams of light from the different candles pass through
the small hole without confusion, and consequently, that the particles
of light are exceedingly small. For the same reason we can easily see
through a small hole not more than 1/100th of an inch in diameter, the
sky, the trees, houses, and nearly all the objects in an extensive
landscape, occupying nearly an entire hemisphere, the light of all
which may pass through this small aperture.
3. _Light is sent forth in all directions from every visible point
of luminous bodies._ If we hold a sheet of paper before a candle,
or the sun, or any other source of light, we shall find that the
paper is illuminated in whatever position we hold it, provided the
light is not obstructed by its edge or by any other body. Hence,
wherever a spectator is placed with regard to a luminous body,
every point of that part of its surface which is toward him will
be visible, when no intervening object intercepts the passage of
the light. Hence, likewise, it follows, that the sun illuminates,
not only an immense plane extending along the paths of the planets,
from the one side of the orbit of Uranus to the other, but the whole
of that sphere, or solid space, of which the distance of Uranus
is the radius. The diameter of this sphere is three thousand six
hundred _millions_ of miles, and it, consequently, contains about
24,000,000,000,000,000,000,000,000,000, or twenty-four thousand
_quartillions_ of cubical miles,--every point of which immense space
is filled with the solar beams. Not only so, but the whole cubical
space which intervenes between the sun and the nearest fixed stars
is more or less illuminated by his rays. For, at the distance of
Sirius, or any other of the nearest stars, the sun would be visible,
though only as a small twinkling orb; and consequently, his rays
must be diffused, however faint, throughout the most distant spaces
whence he is visible. The diameter of this immense sphere of light
cannot be less than _forty billions_ of miles, and its solid contents
33,500,000,000,000,000,000,000,000,000,000,000,000,000 or, thirty-three
thousand, five hundred _sextillions_ of cubical miles. All this
immense, and incomprehensible space is filled with the radiations of
the solar orb; for were an eye placed in any one point of it, where
no extraneous body interposed, the sun would be visible either as a
large luminous orb, or as a small twinkling star. But he can be visible
only by the rays he emits, and which enter the organs of vision. How
inconceivably immense, then, must be the quantity of rays which are
thrown off in all directions from that luminary which is the source of
our day! Every star must likewise be considered as emitting innumerable
streams of radiance over a space equally extensive, so that no point in
the universe can be conceived where absolute darkness prevails, unless
in the _interior_ regions of planetary bodies.
4. _The effect of light upon the eye is not instantaneous, but
continues for a short space of time._ This may be proved and
illustrated by the following examples:--If a stick--or a ball
connected with a string--be whirled round in a circle, and a certain
degree of velocity given it, the object will appear to fill the whole
circle it describes. If a lighted firebrand be whirled round in the
same rapid manner, a complete circle of light will be exhibited.
This experiment obviously shows that the impression made on the
eye by the light from the ball or the firebrand--when in any given
point of the circle--is sufficiently lasting to remain till it has
described the whole circle, and again renews its effect, as often as
the circular motion is continued. The same is proved by the following
considerations:--We are continually shutting our eyes, _or winking_;
and, during the time our eyes are shut, on such occasions, we should
lose the view of surrounding objects, if the impression of light did
not continue a certain time while the eye-lid covers the pupil; but
experience proves that during such vibrations of the eye-lids, the
light from surrounding objects is not sensibly intercepted. If we look
for some time steadily at the light of a candle, and particularly, if
we look directly at the sun, without any interposing medium, or if we
look for any considerable time at this luminary, through a telescope
with a coloured glass interposed--in all these cases, if we shut our
eyes immediately after viewing such objects, we shall still perceive a
faint image of the object, by the impression which its light has made
upon our eyes.
‘With respect to the _duration_ of the impression of light, it has been
observed that the teeth of a cog-wheel in a clock were still visible
in succession, when the velocity of rotation brought 246 teeth through
a given fixed point in a second. In this case it is clear that if the
impression made on the eye by the light reflected from any tooth, had
lasted without sensible diminution for the 246th part of a second, the
teeth would have formed one unbroken line, because a new tooth would
have continually arrived in the place of the interior one before its
image could have disappeared. If a live coal be whirled round, it is
observed that the luminous circle is complete, when the rotation is
performed in the (8-1/2)/60th of a second. In this instance we see
that the impression was much more durable than the former. Lastly, if
an observer sitting in a room direct his sight through a window, to
any particular object out of doors, for about half a minute, and then
shut his eyes and cover them with his hands, he will still continue to
see the window, together with the outline of the terrestrial objects
bordering on the sky. This appearance will remain for near a minute,
though occasionally vanishing and changing colour in a manner that
brevity forbids our minutely describing. From these facts we are
authorized to conclude, that all impressions of light on the eye, last
a considerable time, that the brightest objects make the most lasting
impressions; and that, if the object be very bright, or the eye weak,
the impression may remain for a time so strong, as to mix with and
confuse the subsequent impressions made by other objects. In the last
case the eye is said to be _dazzled_ by the light.’[4]
The following experiment has likewise been suggested as a proof of the
impression which light makes upon the eye. If a card, on both sides of
which a figure is drawn, for example, a bird and a cage, be made to
revolve rapidly on the straight line which divides it symmetrically,
the eye will perceive both figures at the same time, provided they
return successively to the same place. M. D’Arcy found by various
experiments, that, in general, the impression which light produces on
the eye, lasts about _the eighth of a second_. M. Plateau, of Brussels,
found that the impression of different colours lasted the following
periods; the numbers here stated being the decimal parts of a _second_.
_Flame_, 0.242. or nearly one fourth of a second; _Burning coal_,
0.229; _White_, 0.182, or, a little more than one sixth of a second;
_Blue_, 0.186; _Yellow_, 0.173; _Red_, 0.184.
5. _Light_, though extremely minute, _is supposed to have a certain
degree of force or momentum_. In order to prove this, the late
ingenious Mr. Mitchell contrived the following experiment. He
constructed a small vane in the form of a common weather-cock, of a
_very thin_ plate of copper, about an inch square, and attached to
one of the finest harpsicord wires, about ten inches long, and nicely
balanced at the other end of the wire, by a grain of very small shot.
The instrument had also fixed to it in the middle, at right angles to
the length of the wire, and in an horizontal direction, a small bit
of a very slender sewing needle, about half an inch long, which was
made magnetical. In this state the whole instrument might weigh about
ten grains. The vane was supported in the manner of the needle in the
mariner’s compass, so that it could turn with the greatest ease; and
to prevent its being affected by the vibrations of the air, it was
enclosed in a glass case or box. The rays of the sun were then thrown
upon the broad part of the vane or copper plate, from a concave mirror
of about two feet diameter, which, passing through the front glass of
the box, were collected into the focus of the mirror upon the copper
plate. In consequence of this the plate began to move with a slow
motion of about an inch in a second of time, till it had moved through
a space of about two inches and a half, when it struck against the
back of the box. The mirror being removed, the instrument returned to
its former situation, and the rays of the sun being again thrown upon
it, it again began to move, and struck against the back of the box as
before. This was repeated three or four times with the same success.
On the above experiment, the following calculation has been founded:
If we impute the motion produced in this experiment to the impulse of
the rays of light, and suppose that the instrument weighed ten grains,
and acquired a velocity of one inch in a second, we shall find that the
quantity of matter contained in the rays falling upon the instrument in
that time amounted to no more than one twelve hundred-millionth part
of a grain, the velocity of light exceeding the velocity of one inch
in a second in the proportion of about 12,000,000,000 to 1. The light
in this experiment was collected from a surface of about three square
feet, which reflecting only about half what falls upon it, the quantity
of matter contained in the rays of the sun incident upon a foot and a
half of surface in one second of time, ought to be no more than the
twelve hundred-millionth part of a grain. But the density of the rays
of light at the surface of the sun is greater than that at the earth
in the proportion of 45,000 to 1; there ought therefore to issue from
one square foot of the sun’s surface in one second of time, in order to
supply the waste by light 1/45,000th part of a grain of matter, that
is, a little more than two grains a day, or about 4,752,000 grains, or
670 pounds avoirdupoise, nearly, in 6,000 years, a quantity which would
have shortened the sun’s diameter no more than about ten feet, if it
were formed of the density of water only.
If the above experiment be considered as having been accurately
performed, and if the calculations founded upon it be correct, it
appears that there can be no grounds for apprehension that the sun can
ever be sensibly diminished by the immense and incessant radiations
proceeding from his body on the supposition that light is a material
emanation. For the diameter of the sun is no less than 880,000 miles;
and, before this diameter could be shortened, by the emission of
light, one English mile, it would require three millions, one hundred
and sixty-eight thousand years, at the rate now stated; and, before
it could be shortened ten miles, it would require a period of above
thirty-one millions of years. And although the sun were thus actually
diminished, it would produce no sensible effect or derangement
throughout the planetary system. We have no reason to believe that
the system, _in its present state and arrangements_, was intended to
endure for ever, and before that luminary could be so far reduced,
during the revolutions of eternity, as to produce any irregularities
in the system, new arrangements and modifications might be introduced
by the hand of the All Wise and Omnipotent Creator. Besides, it is not
improbable that a system of means is established by which the sun and
all the luminaries in the universe receive back again a portion of the
light which they are continually emitting, either from the planets from
whose surfaces it is reflected, or from the millions of stars whose
rays are continually traversing the immense spaces of creation, or
from some other sources to us unknown.
6. _The intensity of light is diminished in proportion to the square
of the distance from the luminous body._ Thus, a person at two feet
distance from a candle, has only the fourth part of the light he would
have at one foot, at three feet distance the ninth part, at four feet
the sixteenth part, at five feet the twenty fifth part, and so on for
other distances. Hence the light received by the planets of the Solar
system decreases in proportion to the squares of the distances of these
bodies from the sun. This may be illustrated by the following figure,
[Illustration: _Figure 1._]
Suppose the light which flows from a point A, and passes through a
square hole B, is received upon a plane C, parallel to the plane of the
hole--or, let the figure C be considered as the shadow of the plane B.
When the distance of C is double of B, the length and breadth of the
shadow C will be each double of the length and breadth of the plane B,
and treble when AD is treble of AB, and so on, which may be easily
examined by the light of a candle placed at A. Therefore the surface of
the shadow C, at the distance AC--double of AB, is divisible into four
squares, and at a treble distance, into nine squares, severally equal
to the square B, as represented in the figure. The light, then, which
falls upon the plane B being suffered to pass to double that distance,
will be uniformly spread over four times the space, and consequently
will be four times thinner in every part of that space. And at a treble
distance it will be nine times thinner, and at a quadruple distance
sixteen times thinner than it was at first. Consequently the quantities
of this rarified light received upon a surface of any given size and
shape when removed successively to these several distances, will be but
one-fourth, one-ninth, one-sixteenth, of the whole quantity received by
it at the first distance AB.
In conformity with this law, the relative quantities of light on the
surfaces of the planets may be easily determined, when their distances
from the sun are known. Thus, the distance of Uranus from the sun is
1,800,000,000 miles, which is about nineteen times greater than the
distance of the earth from the same luminary. The square of 19 is 361;
consequently the earth enjoys 361 times the intensity of light when
compared with that of Uranus; in other words, this distant planet
enjoys only the 1/361 part of the quantity of light which falls upon
the earth. This quantity, however, is equivalent to the light we should
enjoy from the combined effulgence of 348 full moons; and if the
pupils of the eyes of the inhabitants of this planet be much larger
than ours, and the _retina_ of the eye be endued with a much greater
degree of nervous sensibility, they may perceive objects with as great
a degree of splendour as we perceive on the objects which surround
us in this world. Following out the same principle, we find that the
quantity of light enjoyed by the planet Mercury is nearly _seven_ times
greater than that of the Earth, and that of Venus nearly _double_ of
what we enjoy--that Mars has less than the one half--Jupiter the _one
twenty-seventh_ part--and Saturn only the _one ninetieth_ part of the
light which falls upon the Earth. That the light of these distant
planets, however, is not so weak as we might at first imagine appears
from the brilliancy they exhibit, when viewed in our nocturnal sky,
either with the telescope or with the unassisted eye--and likewise from
the circumstance that a very small portion of the Sun--such as the one
fortieth or one fiftieth part diffuses a quantity of light sufficient
for most of the purposes of life, as is found in the case of total
eclipses of the Sun, when his western limb begins to be visible, only
like a fine luminous thread, for his light is then sufficient to render
distinctly visible all the parts of the surrounding landscape.
7. _It is by light reflected from opake bodies that most of the objects
around us are rendered visible._ When a lighted candle is brought
into a dark room, not only the candle but all other bodies in the
room become visible. Rays of the sun passing into a dark room render
luminous a sheet of paper on which they fall, and this sheet in its
turn enlightens, to a certain extent, the whole apartment, and renders
objects in it visible, so long as it receives the rays of the sun. In
like manner, the moon and the planets are opake bodies, but the light
of the sun falling upon them, and being reflected from their surfaces,
renders them visible. Were no light to fall on them from the sun, or
were they not endued with a power of reflecting it, they would be
altogether invisible to our sight. When the moon comes between us and
the sun, as in a total eclipse of that luminary, as no solar light is
reflected from the surface next the earth, she is invisible--only the
curve or outline of her figure being distinguished by her shadow. In
this case, however, there is a certain portion of reflected light on
the lunar hemisphere next the earth, though not distinguishable during
a solar eclipse. The earth is enlightened by the sun, and a portion of
the rays which fall upon it is reflected upon the dark hemisphere of
the moon which is then towards the earth. This reflected light from the
earth is distinctly perceptible, when the moon appears as a slender
crescent, two or three days after new moon--when the earth reflects its
light back on the moon, in the same manner as the full moon reflects
her light on the earth. Hence, even at this period of the moon, her
whole face becomes visible to us, but its light is not uniform or of
equal intensity. The thin crescent on which the full blaze of the solar
light falls, is very brilliant and distinctly seen, while the other
part, on which falls only a comparatively feeble light from the earth,
appears very faint, and is little more than visible to the naked eye,
but with a telescope of moderate power,--if the atmosphere be very
clear--it appears beautifully distinct, so that the relative positions
of many of the lunar spots may be distinguished.
The intensity of reflected light is very small, when compared with
that which proceeds directly from luminous bodies. M. Bouguer, a
French philosopher, who made a variety of experiments to ascertain the
proportion of light emitted by the heavenly bodies, concluded from
these experiments, that the light transmitted from the sun to the
earth is at least 300,000 times as great as that which descends to us
from the full moon--and that, of 300,000 rays which the moon receives,
from 170,000 to 200,000 are absorbed. Hence we find that, however
brilliant the moon may appear at night--in the day time she appears as
obscure as a small portion of dusky cloud to which she happens to be
adjacent, and reflects no more light than a portion of whitish cloud of
the same size. And as the full moon fills only the ninety thousandth
part of the sky, it would require at least ninety thousand moons to
produce as much light as we enjoy in the day-time under a cloudy sky.
As the moon and the planets are rendered visible to us only by light
reflected from their surfaces, so it is in the same way that the images
of most of the objects around us are conveyed to our organs of vision.
We behold all the objects which compose an extensive landscape,--the
hills and vales, the woods and lawns, the lakes and rivers, and the
habitations of man--in consequence of the capacity with which they
are endued of sending forth reflected rays to the eye, from every
point of their surfaces and in all directions. In connection with the
reflection of light, the following curious observation may be stated.
Baron Funk, visiting some silver mines in Sweden, observed, that,
‘in a clear day, it was as dark as pitch underground in the eye of a
pit, at sixty or seventy fathoms deep; whereas, in a cloudy or rainy
day, he could see to read even at 106 fathoms deep. Enquiring of the
miners, he was informed that this is always the case; and reflecting
upon it, he imagined it arose from this circumstance, that when the
atmosphere is full of clouds, light is reflected from them into the
pit in all directions, and that thereby a considerable proportion of
the rays are reflected perpendicularly upon the earth: whereas when the
atmosphere is clear, there are no opaque bodies to reflect the light in
this manner, at least in a sufficient quantity; and rays from the sun
himself can never fall perpendicularly in that country.’--The reason
here assigned is, in all probability, the true cause of the phenomenon
now described.
8. It is supposed by some philosophers that _light is subject to the
same laws of attraction that govern all other material substances_--and
that _it is imbibed and forms a constituent part of certain bodies_.
This has been inferred from the phenomena of the _Bolognian stone_, and
what are generally called the _solar phosphori_. The Bolognian stone
was first discovered about the year 1680, by Leascariolo, a shoe-maker
of Bologna. Having collected together some stones of a shining
appearance at the bottom of Monte Paterno, and being in quest of some
alchemical secret, he put them into a crucible to calcine them--that
is, to reduce them to the state of cinders. Having taken them out of
the crucible, and exposed them to the light of the sun, he afterwards
happened to carry them into a dark place, when to his surprise, he
observed that they possessed a self-illuminating power, and continued
to emit faint rays of light for some hours afterwards. In consequence
of this discovery, the Bolognian spar came into considerable demand
among natural philosophers and the curious in general; and the best
way of preparing it seems to have been hit upon by the family of
Zagoni, who supplied all Europe with Bolognian phosphorus, till the
discovery of more powerful phosphoric substances put an end to their
monopoly.--In the year 1677, Baldwin, a native of Misnia, observed
that chalk dissolved in aqua-fortis exactly resembled the Bolognian
stone in its property of imbibing light, and emitting it after it was
brought into the dark ; and hence it has obtained the name of Baldwin’s
phosphorus.
In 1730 M. du Fay directed his attention to this subject, and observed
that all earthy substances susceptible of calcination, either by
mere fire, or when assisted by the previous action of nitrous acid,
possessed the property of becoming more or less luminous, when calcined
and exposed for a short time in the light--that the most perfect of
these phosphori were limestones, and other kinds of carbonated lime,
gypsum, and particularly the topaz, and that some diamonds were also
observed to be luminous by simple exposure to the sun’s rays. Sometime
afterwards, Beccaria discovered that a great variety of other bodies
were convertible into phosphori by exposure to the mere light of the
sun, such as, organic animal remains, most compound salts, nitre and
borax--all the farinaceous and oily seeds of vegetable substances,
all the gums and several of the resins--the white woods and vegetable
fibre, either in the form of paper or linen; also starch and loaf-sugar
proved to be good phosphori, after being made thoroughly dry, and
exposed to the direct rays of the sun. Certain animal substances by
a similar treatment were also converted into phosphori; particularly
bone, sinew, glue, hair, horn, hoof, feathers, and fish-shells. The
same property was communicated to rock crystal and some other of
the gems, by rubbing them against each other so as to roughen their
surfaces, and then placing them for some minutes in the focus of a
lens, by which the rays of light were concentrated upon them, at the
same time that they were also moderately heated.
In the year 1768 Mr. Canton contributed some important facts in
relation to solar phosphori, and communicated a method of preparing
a very powerful one, which, after the inventor, is usually called
_Canton’s phosphorus_. He affirms that his phosphorus, enclosed in a
glass flask, and hermetically sealed, retains its property of becoming
luminous for at least four years, without any apparent decrease of
activity. It has also been found that, if a common box smoothing-iron,
heated in the usual manner, be placed for half a minute on a sheet
of dry, white paper, and the paper be then exposed to the light, and
afterwards examined in a dark closet, it will be found that the whole
paper will be luminous, that part, however, on which the iron had stood
being much more shining than the rest.
From the above facts it would seem that certain bodies have the power
of imbibing light and again emitting it, in certain circumstances,
and that this power may remain for a considerable length of time. It
is observed that the light which such bodies emit bears an analogy to
that which they have imbibed. In general, the illuminated phosphorus is
reddish; but when a weak light only has been admitted to it, or when
it has been received through pieces of white paper, the emitted light
is pale or whitish.--Mr. Morgan, in the seventy-fifth volume of the
Philosophical Transactions, treats the subject of light at considerable
length; and as a foundation for his reasoning, he assumes the following
data:--1. That light is a body, and like all others, subject to the
laws of attraction. 2. That light is a heterogeneous body; and that
the same attractive power operates with different degrees of force
on its different parts. To the principle of attraction, likewise,
Sir Isaac Newton has referred the most extraordinary phenomena of
light, Refraction and Inflection. He has also endeavoured to show that
light is not only subject to the law of attraction but of repulsion
also, since it is repelled or reflected from certain bodies. If
such principles be admitted, then, it is highly probable that the
phosphorescent bodies to which we have adverted have a power of
attracting or imbibing the substance of light, and of retaining or
giving it out under certain circumstances, and that the matter of light
is incorporated at least with the surface of such bodies. But on this
subject, as on many others, there is a difference of opinion among
philosophers.[5]
9. _Light is found to produce a remarkable effect on Plants and
Flowers, and other vegetable productions._ Of all the phenomena which
living vegetables exhibit there are few that appear more extraordinary
than the energy and constancy with which their stems incline toward the
light. Most of the discous flowers follow the sun in his course. They
attend him to his evening retreat, and meet his rising lustre in the
morning with the same unerring law. They unfold their flowers on the
approach of this luminary; they follow his course by turning on their
stems, and close them as soon as he disappears. If a plant, also, is
shut up in a dark room, and a small hole afterwards opened by which the
light of the sun may enter, the plant will turn towards that hole, and
even alter its own shape in order to get near it; so that though it
was straight before, it will in time become crooked, that it may get
near the light. Vegetables placed in rooms where they receive light
only in one direction, always extend themselves in that direction. If
they receive light in two directions, they direct their course towards
that which is strongest. It is not the _heat_ but the _light_ of the
sun which the plant thus covets; for, though a fire be kept in the
room, capable of giving a much stronger heat than the sun, the plant
will turn away from the fire in order to enjoy the solar light. Trees
growing in thick forests, where they only receive light from above,
direct their shoots almost invariably upwards, and therefore become
much taller and less spreading than such as stand single.
The _green_ colour of plants is likewise found to depend on the sun’s
light being allowed to shine on them; for without the influence of
the solar light, they are always of a _white_ colour. It is found by
experiment that, if a plant which has been reared in darkness be
exposed to the light of day, in two or three days it will acquire a
green colour perceptibly similar to that of plants which have grown
in open day-light. If we expose to the light one part of the plant,
whether leaf or branch, this part alone will become green. If we cover
any part of a leaf with an opake substance, this place will remain
white, while the rest becomes green. The whiteness of the inner leaves
of cabbages is a partial effect of the same cause, and many other
examples of the same kind might easily be produced. M. Decandolle,
who seems to have paid particular attention to this subject, has the
following remarks: ‘It is certain, that between the white state of
plants vegetating in darkness, and complete verdure, every possible
intermediate degree exists, determined by the intensity of the light.
Of this any one may easily satisfy himself by attending to the colour
of a plant exposed to the full day-light; it exhibits in succession
all the degrees of verdure. I had already seen the same phenomenon,
in a particular manner, by exposing plants reared in darkness to the
light of lamps. In these experiments, I not only saw the colour come on
gradually, according to the continuance of the exposure to light; but
I satisfied myself, that a certain intensity of permanent light never
gives to a plant more than a certain degree of colour. The same fact
readily shows itself in nature, when we examine the plants that grow
under shelter or in forests, or when we examine in succession the state
of the leaves that form the heads of cabbages.’[6]
It is likewise found that the _perspiration_ of vegetables is increased
or diminished, in a certain measure by the degree of _light_ which
falls upon them. The experiments of Mr. P. Miller and others, prove
that plants uniformly perspire most in the forenoon, though the
temperature of the air in which they are placed should be unvaried.
M. Guettard likewise informs us that a plant exposed to the rays of
the sun, has its perspiration increased to a much greater degree than
if it had been exposed to the same heat under the shade. Vegetables
are likewise found to be indebted to light for their smell, taste,
combustibility, maturity, and the resinous principle, which equally
depend upon this fluid. The aromatic substances, resins, and volatile
oil are the productions of southern climates, where the light is more
pure, constant, and intense. In fine, another remarkable property of
light on the vegetable kingdom is that, when vegetables are exposed to
open day-light, or to the sun’s rays, they emit oxygen gas or vital
air. It has been proved that, in the production of this effect, the
sun does not act as a body that heats. The emission of the gas is
determined by the light: pure air is therefore separated by the action
of light, and the operation is stronger as the light is more vivid.
By this continual emission of vital air, the Almighty incessantly
purifies the atmosphere, and repairs the loss of pure air occasioned by
respiration, combustion, fermentation, putrefaction, and numerous other
processes which have a tendency to contaminate this fluid so essential
to the vigor and comfort of animal life; so that, in this way, by the
agency of light, a due equilibrium is always maintained between the
constituent parts of the atmosphere.
In connection with this subject the following curious phenomenon may
be stated, as related by M. Haggern, a Lecturer on Natural History in
Sweden. One evening he perceived a faint flash of light repeatedly
dart from a marigold. Surprised at such an uncommon appearance, he
resolved to examine it with attention; and, to be assured it was no
deception of the eye, he placed a man near him, with orders to make
a signal at the moment when he observed the light. They both saw
it constantly at the same moment. The light was most brilliant on
marigolds of an orange or flame colour, but scarcely visible on pale
ones. The flash was frequently seen on the same flower two or three
times in quick succession; but more commonly at intervals of several
minutes; and when several flowers in the same place emitted their
light together, it could be observed at a considerable distance. The
phenomenon was remarked in the months of July and August at sun-set,
and for half an hour when the atmosphere was clear; but after a
rainy day, or when the air was loaded with vapours, nothing of it
was seen. The following flowers emitted flashes more or less vivid,
in this order:--1. The Marigold, 2. Monk’s hood, 3. The Orange Lily,
4. The Indian Pink. As to the _cause_ of this phenomenon, different
opinions may be entertained. From the rapidity of the flash and other
circumstances, it may be conjectured that electricity is concerned in
producing this appearance. M. Haggern, after having observed the flash
from the orange lily, the antheræ of which are at considerable distance
from the petals, found that the light proceeded from the petals only;
whence he concludes, that this electrical light is caused by the pollen
which, in flying off, is scattered on the petals. But, perhaps, the
true cause of it still remains to be ascertained.
10. _Light has been supposed to produce a certain degree of influence
on the_ PROPAGATION OF SOUND?--M. Parolette, in a long
paper in the ‘Journal de Physique,’ vol. 68, which is copied into
‘Nicholson’s Philosophical Journal,’ vol. 25, pp. 28-39,--has offered
a variety of remarks, and detailed a number of experiments on this
subject. The author states the following circumstances as having
suggested the connection between light and sound. ‘In 1803, I lived
in Paris, and being accustomed to rise before day to finish a work on
which I had long been employed, I found myself frequently disturbed
by the sound of carriages, as my windows looked into one of the most
frequented streets in that city. This circumstance which disturbed me
in my studies every morning, led me to remark, that the appearance of
day-break peculiarly affected the propagation of the sound: from dull
and deep, which it was before day, it seemed to me to acquire a more
sonorous sharpness in the period that succeeded the dissipation of
darkness. The rolling of the wheels seemed to announce the friction
of some substances grown more elastic; and my ear on attending to it
perceived this difference diminish, in proportion as the sound of
wheels was confounded with those excited by the tumult of objects
quitting their nocturnal silence. Struck with this observation, I
attempted to discover whether any particular causes had deceived my
ears. I rose several times before day for this purpose alone, and was
every time confirmed in my suspicion, that light must have a peculiar
influence on the propagation of sound. This variation, however, in
the manner in which the air gave sounds might be the effect of the
agitation of the atmosphere produced by the rarefaction the presence
of the sun occasioned; but the situation of my windows, and the usual
direction of the morning breeze, militated against this argument.’
The author then proceeds to give a description of a very delicate
instrument, and various apparatus for measuring the propagation and
intensity of sound, and the various experiments both in the dark, and
in day-light, and likewise under different changes of the atmosphere,
which were made with his apparatus--all of which tended to prove
that light had a sensible influence in the propagation of sound. But
the detail of these experiments and their several results would be
too tedious to be here transcribed.--The night has generally been
considered as more favourable than the day for the transmission of
sound. ‘That this is the case (says Parolette) with respect to our ears
cannot be doubted; but this argues nothing against my opinion. We hear
further by night on account of the silence, and this always contributes
to it, while the noise of a wind favourable to the propagation of a
sound, may prevent the sound from being heard.’ In reference to the
cause which produces the effect now stated, he proposes the following
queries. ‘Is the atmospheric air more dense on the appearance of light
than in darkness? Is this greater density of the air or of the elastic
fluid that is subservient to the propagation of sound, the effect of
aeriform substances kept in this state through the medium of light?’
He is disposed, on the whole, to conclude, that the effect in question
is owing to the action of light upon the oxygen of the atmosphere,
since oxygen gas is found by experiment to be best adapted to the
transmission of sound.
Our author concludes his communication with the following
remarks:--‘Light has a velocity 900,000 times as rapid as that of
sound. Whether it emanate from the sun and reach to our earth, or
act by means of vibrations agitating the particles of a fluid of a
peculiar nature--the particles of this fluid must be extremely light,
elastic and active. Nor does it appear to me unreasonable, to ascribe
to the mechanical action of these particles set in motion by the sun,
the effects its presence occasions in the vibrations that proceed from
sonorous bodies. The more deeply we investigate the theory of light,
the more we must perceive, that the powers by which the universe is
moved reside in the imperceptible particles of bodies; and that the
grand results of nature are but an assemblage of an order of actions
that take place in its infinitely small parts; consequently, we cannot
institute a series of experiments more interesting than those which
tend to develope the properties of light. Our organs of sense are so
immediately connected with the fluid that enlightens us, that the
notion of having acquired an idea of the mode of action of this fluid
presents itself to our minds, as the hope of a striking advance in
the knowledge of what composes the organic mechanism of our life, and
of that of beings which closely follow the rank assigned to the human
species.’
* * * * *
Such is a brief description of some of the leading properties of light.
Of all the objects that present themselves to the philosophic and
contemplative mind, light is one of the noblest and most interesting.
The action it exerts on all the combinations of matter, its extreme
divisibility, the rapidity of its propagation, the sublime wonders it
reveals, and the office it performs in what constitutes the life of
organic beings, lead us to consider it as a substance acting the first
part in the economy of nature. The magic power which this emanation
from the heavens exerts on our organs of vision, in exhibiting to our
view the sublime spectacle of the universe, cannot be sufficiently
admired. Nor is its power confined to the organs of sight; all our
senses are, in a greater or less degree, subjected to the action of
light, and all the objects in this lower creation--whether in the
animal, the vegetable, or the mineral kingdoms--are, to a certain
extent, susceptible of its influence. Our globe appears to be little
more than an accumulation of terrestrial materials introduced into the
boundless ocean of the _solar light_, as a theatre on which it may
display its exhaustless power and energy, and give animation, beauty
and sublimity to every surrounding scene--and to regulate all the
powers of nature, and render them subservient to the purposes for which
they were ordained. This elementary substance appears to be universal
in its movements, and in its influence. It descends to us from the
solar orb. It wings its way through the voids of space, along a course
of ninety-five millions of miles, till it arrives at the outskirts of
our globe; it passes freely through the surrounding atmosphere, it
strikes upon the clouds and is reflected by them; it irradiates the
mountains, the vales, the forests, the rivers, the seas, and all the
productions of the vegetable kingdom, and adorns them with a countless
assemblage of colours. It scatters and disperses its rays from one end
of creation to another, diffusing itself throughout every sphere of the
universe. It flies without intermission from star to star, and from
suns to planets, throughout the boundless sphere of immensity, forming
a connecting chain and a medium of communication among all the worlds
and beings within the wide empire of Omnipotence.
When the sun is said “to rule over the day,” it is intimated that he
acts as the vicegerent of the Almighty, who has invested him with a
mechanical power of giving light, life and motion to all the beings
susceptible of receiving impressions from his radiance. As the servant
of his creator he distributes blessings without number among all the
tribes of sentient and intelligent existence. When his rays illumine
the eastern sky in the morning, all nature is enlivened with his
presence. When he sinks beneath the western horizon, the flowers droop,
the birds retire to their nests, and a mantle of darkness is spread
over the landscape of the world. When he approaches the equinox in
spring, the animal and vegetable tribes revive, and nature puts on a
new and a smiling aspect. When he declines towards the winter solstice,
dreariness and desolation ensue, and a temporary death takes place
among the tribes of the vegetable world.--This splendid luminary,
whose light embellishes the whole of this lower creation, forms the
most lively representation of Him who is the source and the centre of
all beauty and perfection. “God is a sun,” the sun of the moral and
spiritual universe, from whom all the emanations of knowledge, love and
felicity descend. “He covereth himself with light as with a garment.”
and “dwells in light inaccessible and full of glory.” The felicity
and enjoyments of the future world are adumbrated under the ideas of
_light_ and _glory_. “The glory of God enlightens the celestial city,”
its inhabitants are represented as “the saints _in light_,” it is
declared that “their _sun_ shall no more go down,” and that “the Lord
God is their _everlasting light_.” So that light not only cheers and
enlivens all beings throughout the material creation, but is the emblem
of the Eternal Mind, and of all that is delightful and transporting in
the scenes of a blessed immortality.
In the formation of light, and the beneficent effects it produces,
the wisdom and goodness of the Almighty are conspicuously displayed.
Without the beams of the sun and the influence of light, what were all
the realms of this world, but an undistinguished chaos and so many
dungeons of darkness? In vain should we roll our eyes around to behold,
amidst the universal gloom, the flowery fields, the verdant plains, the
flowing streams, the expansive ocean, the moon walking in brightness,
the planets in their courses, or the innumerable host of stars. All
would be lost to the eye of man, and the “blackness of darkness” would
surround him for ever. And with how much wisdom has every thing been
arranged in relation to the motion and minuteness of light? Were it
capable of being transformed into a solid substance, and retain its
present velocity, it would form the most dreadful and appalling element
in nature, and produce universal terror and destruction throughout the
universe. That this is not impossible, and could easily be effected by
the hand of Omnipotence, appears from such substances as _phosphorus_,
where light is supposed to be concentrated in a solid state. But in all
its operations and effects, as it is now directed by unerring wisdom
and beneficence, it exhibits itself as the most benign and delightful
element connected with the constitution of the material system,
diffusing splendour and felicity wherever its influence extends.
CHAPTER II.
ON THE REFRACTION OF LIGHT.
Refraction is the turning or bending of the rays of light out of their
natural course.
Light, when proceeding from a luminous body--without being reflected
from any opake substance or inflected by passing near one--is
invariably found to proceed in straight lines without the least
deviation. But if it happens to pass obliquely from one medium to
another, it always leaves the direction it had before and assumes a
new one. This change of direction, or _bending_ of the rays of light,
is what is called _Refraction_--a term which probably had its origin
from the broken appearance which a staff or a long pole exhibits, when
a portion of it is immersed in water--the word, derived from the Latin
_frango_, literally signifying _breaking_ or bending.
When light is thus refracted, or has taken a new direction, it then
proceeds invariably in a straight line till it meets with a different
medium,[7] when it is again turned out of its course. It must be
observed, however, that though we may by this means cause the rays of
light to make any number of angles in their course, it is impossible
for us to make them describe a curve, except in one single case,
namely, where they pass through a medium, the density of which either
uniformly increases or diminishes. This is the case with the light of
the celestial bodies, which passes downwards through our atmosphere,
and likewise with that which is reflected upwards through it by
terrestrial objects. In both these cases it describes a curve of the
hyperbolic kind; but at all other times, it proceeds in straight lines,
or in what may be taken for straight lines without any sensible error.
There are two circumstances essential to refraction. 1. That the rays
of light shall pass out of one medium into another of a different
density, or of a greater or less degree of resistance. 2. That they
pass in an _oblique_ direction. The denser the refracting medium, or
that into which the ray enters, the greater will be its refracting
power; and of two refracting mediums of the same density, that which
is of an oily or inflammable nature will have a greater refracting
power than the other. The nature of refraction may be more particularly
explained and illustrated by the following figure and description.
Let ADHI fig. 2, be a body of water, AD its surface, C a point in which
a ray of light BC enters from the air into the water. This ray, by the
greater density of the water, instead of passing straight forward in
its first direction to K, will be bent at the point C, and pass along
in the direction CE, which is called the _refracted_ ray. Let the line
FG be drawn perpendicular to the surface of the water in C, then it
is evident that the ray BC, in passing out of air, a _rare_ medium,
into a _dense_ medium, as water, is refracted into a ray CE which is
_nearer_ to the perpendicular CG than the incident ray BC, and on the
contrary, the ray EC passing out of a denser medium into a rarer will
be refracted into CB, which is _farther_ from the perpendicular.
[Illustration: _figure 2._]
The same thing may be otherwise illustrated as follows:--suppose a hole
made in one of the sides of the vessel as at _a_, and a lighted candle
placed within two or three feet of it, when empty, so that its flame
may be at L, a ray of light proceeding from it will pass through the
hole _a_ in a straight line LBCK till it reach the bottom of the vessel
at K, where it will form a small circle of light. Having put a mark at
the point K, pour water into the vessel till it rise to the height AD,
and the round spot that was formerly at K, will appear at E; that is,
the ray which went straight forward, when the vessel was empty, to K,
has been bent at the point C, where it falls into the the water, into
the line CE. In this experiment it is necessary that the front of the
vessel should be of glass, in order that the course of the ray may be
seen; and if a little soap be mixed with the water so as to give it a
little mistiness, the ray CE will be distinctly perceived. If, in place
of fresh water we pour in salt water, it will be found that the ray BC
is more bent at C. In like manner alcohol will refract the ray BC more
than salt water, and oil more than alcohol, and a piece of solid glass,
of the shape of the water, would refract the light still more than the
oil.
The angle of refraction depends on the obliquity of the rays falling on
the refracting surface being always such, that the sine of the incident
angle is to the sine of the refracted angle, in a given proportion. The
_incident_ angle is the angle made by a ray of light and a line drawn
perpendicular to the refracting surface, at the point where the light
enters the surface. The _refracted_ angle is the angle made by the ray
in the refracting medium with the same perpendicular produced. The
_sine_ of the angle is a line which serves to measure the angle, being
drawn from a point in one leg perpendicular to the other. The following
figure (fig. 3.) will tend to illustrate these definitions.
[Illustration: _figure 3._]
In this figure BC is the incident ray, CE the refracted ray, DG the
perpendicular, AD the sine of the angle of incidence ACD, and HR the
sine of the angle of refraction GCE. Now, it is a proposition in optics
that,--the sine AD of the angle of incidence BCD is either accurately
or very nearly in a given proportion to the sine HR of the angle of
refraction GCE. This ratio of the sines is as four to three, when the
refraction is made out of air into water, that is AD is to HR as four
to three. When the refraction is out of air into glass, the proportion
is about as thirty-one to twenty, or nearly as three to two. If the
refraction be out of air into diamond it is as five to two, that is
AD : HR :: 5 : 2. The denser the medium is, the less is the angle and
sine of refraction. If a ray of light MC, were to pass from air into
water, or from empty space into air, in the direction MC perpendicular
to the plane NO which separates the two mediums, it would suffer no
refraction, because one of the essentials to that effect is wanting,
namely, the _obliquity_ of the incidence.
It may be also proper to remark, that a ray of light cannot pass out
of a denser medium into a rarer, if the angle of incidence exceed a
certain limit. Thus a ray of light will not pass out of glass into air,
if the angle of incidence exceed 40° 11´; or out of glass into water,
if the angle of incidence exceed 59° 20´. In such cases refraction will
be changed into reflection.
The following common experiments, which are easily performed, will
illustrate the doctrine of refraction. Put a shilling or any other
small object which is easily distinguished, into a bason or any other
similar vessel, and then retire to such a distance as that the edge
of the vessel shall just hide it from your sight. If then you cause
another person to fill the vessel with water, you will then find that
the shilling is rendered perfectly visible, although you have not in
the slightest degree changed your position. The reason of this is,
that the rays of light, by which it is rendered visible, _are bent
out of their course_. Thus, suppose the shilling to have been placed
in the bottom of the bason at E, (fig. 2.) the ray of light BC which
passes obliquely from the air into water at C, instead of continuing
its course to K, takes the direction CE, and consequently an object
at E would be rendered visible by rays proceeding in that direction,
when they would not have touched it had they proceeded in their direct
course.
The same principle is illustrated by the following experiment. Place
a bason or square box on a table, and a candle at a small distance
from it; lay a small rod or stick across the sides of the bason, and
mark the place where the extremity of the shadow falls, by placing a
shilling or other object at the point; then let water be poured into
the bason, and the shadow will then fall much nearer to the side next
the candle than before. This experiment may likewise be performed by
simply observing the change produced on the shadow of the side of
the bason itself. Again, put a long stick obliquely into deep water,
and the stick will seem to be broken at the point where it appears
at the surface of the water--the part which is immersed in the water
appearing to be bent upwards. Hence every one must have observed that,
in rowing a boat, the ends of the oars appear bent or broken every time
they are immersed in the water, and their appearance at such times is
a representation of the course of the refracted rays. Again, fill a
pretty deep jar with water, and you will observe the bottom of the jar
considerably elevated, so that it appears much shallower than it did
before the water was poured in, in the proportion of nearly a third of
its depth, which is owing to the same cause as that which makes the
end of a stick immersed in water appear more elevated than it would do
if there were no refraction. Another experiment may be just mentioned.
Put a sixpence in a wine-glass, and pour upon it a little water.
When viewed in a certain position, two sixpences will appear in the
glass--one image of the sixpence from below, which comes directly to
the eye, and another which appears considerably raised above the other,
in consequence of the rays of light rising through the water, and being
refracted. In this experiment the wine-glass should not be more than
half filled with water.
The refraction of light explains the causes of many curious and
interesting phenomena both in the heavens and on the earth. When we
stand on the banks of a river, and look obliquely through the waters
to its bottom, we are apt to think it is much shallower than it really
is. If it be eight feet deep in reality, it will appear from the bank
to be only six feet; if it be five feet and a half deep, it will appear
only about four feet. This is owing to the effects of refraction, by
which the bottom of the river is apparently raised by the refraction
of the light passing through the water into air, so as to make the
bottom appear higher than it really is, as in the experiment with the
jar of water. This is a circumstance of some importance to be known
and attended to in order to personal safety. For many school-boys and
other young persons have lost their lives by attempting to ford a
river, the bottom of which appeared to be within their reach, when they
viewed it from its banks: and even adult travellers on horseback have
sometimes fallen victims to this optical deception; and this is not
the only case in which a knowledge of the laws of nature may be useful
in guarding us against dangers and fatal accidents.
It is likewise owing to this refractive power in water, that a skilful
marksman who wishes to shoot fish under water, is obliged to take aim
considerably _below_ the fish as it appears, because it seems much
nearer the top of the water than it really is. An acquaintance with
this property of light is particularly useful to divers, for, in any
of their movements or operations, should they aim directly at the
object, they would arrive at a point considerably beyond it; whereas,
by having some idea of the depth of the water, and the angle which a
line drawn from the eye to the object makes with its surface, the point
at the bottom of the water, between the eye and the object at which the
aim is to be taken, may be easily determined. For the same reason, a
person below water does not see objects distinctly. For, as the aqueous
humour of the eye has the same refractive power as water, the rays of
light from any object under water will undergo no refraction in passing
through the cornea, and aqueous humour, and will therefore meet in a
point far behind the retina. But if any person accustomed to go below
water should use a pair of spectacles, consisting of two convex lenses,
the radius of whose surface is three tenths of an inch--which is nearly
the radius of the convexity of the cornea--he will see objects as
distinctly below water as above it.
It is owing to refraction, that we cannot judge so accurately of
magnitudes and distances in water as in air. A fish looks considerably
larger in water than when taken out of it. An object plunged
_vertically_ into water always appears contracted, and the more so as
its upper extremity approaches nearer the surface of the water. Every
thing remaining in the same situation, if we take the object gradually
out of the water, and it be of a slender form, we shall see it become
larger and larger, by a rapid developement, as it were, of all its
parts. The distortion of objects, seen through a crooked pane of glass
in a window, likewise arises from its unequal refraction of the rays
that pass through it. It has been calculated that in looking through
the common glass of a window, objects appear about the one thirtieth of
an inch out of their real place, by means of the refraction.
Refraction likewise produces an effect upon the _heavenly bodies_, so
that their apparent positions are generally different from their real.
By the refractive power of the atmosphere, the sun is seen before he
comes to the horizon in the morning, and after he sinks beneath it in
the evening; and hence this luminary is never seen in the place in
which it really is, except when it passes the zenith at noon, to places
within the torrid zone. The sun is visible, when actually thirty-two
minutes of a degree below the horizon, and when the opake rotundity of
the earth is interposed between our eye and that orb, just on the same
principle as, in the experiment with the shilling and basin of water,
the shilling was seen when the edge of the basin interposed between it
and the sight. The refractive power of the atmosphere has been found
to be much greater, in certain cases, than what has been now stated.
In the year 1595 a company of Dutch sailors having been wrecked on
the shores of Nova Zembla, and having been obliged to remain in that
desolate region during a night of more than three months--beheld the
sun make his appearance in the horizon about sixteen days before the
time in which he should have risen according to calculation, and when
his body was actually more than four degrees below the horizon; which
circumstance has been attributed to the great refractive power of the
atmosphere in those intensely cold regions. This refraction of the
atmosphere, which renders the apparent rising and setting of the sun
both earlier and later than the real, produces at least one important
beneficial effect. It procures for us the benefit of a much longer day,
at all seasons of the year, than we should enjoy, did not this property
of the atmosphere produce this effect. It is owing to the same cause
that the disks of the sun and moon appear elliptical or oval, when seen
in the horizon, their horizontal diameters appearing longer than their
vertical--which is caused by the greater refraction of the rays coming
from the lower limb, which is immersed in the densest part of the
atmosphere.
The illumination of the heavens which precedes the rising of the sun,
and continues sometime after he is set--or, what is commonly called
the morning and evening _twilight_--is likewise produced by the
atmospherical refraction--which circumstance forms a very pleasing and
beneficial arrangement in the system of nature. It not only prolongs
to us the influence of the solar light, and adds nearly two hours to
the length of our day, but prevents us from being transported all at
once from the darkness of midnight to the splendour of noon-day, and
from the effulgence of day to the gloom and horrors of the night--which
would bewilder the traveller and navigator in their journeys by sea or
land, and strike the living world with terror and amazement.
The following figure will illustrate the position now stated, and
the manner in which the refraction of the atmosphere produces these
effects. Let A _a_ C, fig. 4, represent one half of our globe, and the
dark space between that curve and B _r_ D, the atmosphere. A person
standing on the earth’s surface at _a_ would see the sun rise at _b_,
when that luminary was in reality only at _c_--more than half a degree
below the horizon. When the rays of the sun, after having proceeded
in a straight line through empty space, strike the upper part of the
atmosphere at the point _d_, they are bent out of their right-lined
course, by the refraction of the atmosphere, into the direction _d a_,
so that the body of the sun, though actually intercepted by the curve
of the earth’s convexity consisting of a dense mass of land or water,
is actually beheld by the spectator at _a_. The refractive power of the
atmosphere gradually diminishes from the horizon to the zenith, and
increases from the zenith to the horizon, in proportion to the density
of its different strata, being densest at its lower extremity next the
earth, and more rare towards its higher regions. If a person at _a_
had the sun, _e_, in his zenith, he would see him where he really
is; for his rays coming perpendicularly through the atmosphere, would
be equally attracted in all directions, and would therefore suffer no
inflection. But, about two in the afternoon, he would see the sun at
_i_, though, in reality, he was at _k_, thirty-three seconds lower than
his apparent situation. At about four in the afternoon he would see him
at _m_, when he is at _n_, one minute and thirty-eight seconds from his
apparent situation. But at six o’clock, when we shall suppose he sets,
he will be seen at _o_, though he is at that time at _p_, more than
thirty-two minutes below the horizon. These phenomena arise from the
different refractive powers of the atmosphere at different elevations,
and from the obliquity with which the rays of light fall upon it; for
we see every object along that line in which the rays from it are
directed by the last medium through which they passed.
[Illustration: _figure 4._]
The same phenomena happen in relation to the moon, the planets, the
comets, the stars, and every other celestial body, all of which appear
more elevated, especially when near the horizon, than their true
places. The variable and increasing refraction from the zenith to the
horizon, is a source of considerable trouble and difficulty in making
astronomical observations, and in nautical calculations. For, in order
to determine the real altitudes of the heavenly bodies, the exact
degree of refraction, at the observed elevation, must be taken into
account. To the same cause we are to ascribe a phenomenon that has
sometimes occurred--namely, that the moon has been seen rising totally
eclipsed, while the sun was still visible in the opposite quarter of
the horizon. At the middle of a total eclipse of the moon, the sun
and moon are in opposition, or 180 degrees asunder; and, therefore,
were no atmosphere surrounding the earth, these luminaries, in such a
position, could never be seen above the horizon at the same time. But,
by the refraction of the atmosphere near the horizon, the bodies of the
sun and moon are raised more than 32 minutes above their true places,
which is equal, and sometimes more than equal to the apparent diameters
of these bodies.
_Extraordinary cases of refraction in relation to terrestrial objects._
In consequence of the accidental condensation of certain strata of
the atmosphere, some very singular effects have been produced in the
apparent elevation of terrestrial objects to a position much beyond
that in which they usually appear. The following instance is worthy of
notice. It is taken from the Philosophical Transactions of London for
1798, and was communicated by W. Latham, Esq., F.R.S., who observed
the phenomenon from Hastings, on the south coast of England:--‘On July
26, 1797, about five o’clock in the afternoon, as I was sitting in my
dining-room in this place, which is situated upon the Parade, close
to the sea-shore, nearly fronting the south, my attention was excited
by a number of people running down to the sea-side. Upon inquiring
the reason, I was informed, that the coast of France was plainly to
be distinguished by the naked eye. I immediately went down to the
shore, and was surprised to find that, even without the assistance
of a telescope, I could very plainly see the cliffs on the opposite
coast, which, at the nearest part, are between forty and fifty miles
distant, and are not to be discerned from that low situation by the
aid of the best glasses. They appeared to be only a few miles off,
and seemed to extend for some leagues along the coast. I pursued my
walk along the shore eastward, close to the water’s edge, conversing
with the sailors and fishermen upon the subject. They at first would
not be persuaded of the reality of the appearance; but they soon
became so thoroughly convinced by the cliffs gradually appearing more
elevated, and approaching nearer, as it were, that they pointed out and
named to me the different places they had been accustomed to visit,
such as the Bay, the Old Head, or Man, the Windmill, &c. at Boulogne,
St. Vallery, and other places on the coast of Picardy, which they
afterwards confirmed, when they viewed them through their telescopes.
Their observations were, that the places appeared as near as if they
were sailing, at a small distance, into the harbours. The day on which
this phenomenon was seen was extremely hot; it was high water at
Hastings about two o’clock, P.M., and not a breath of wind was
stirring the whole day.’ From the summit of an adjacent hill, a most
beautiful scene is said to have presented itself. At one glance the
spectators could see Dungeness, Dover Cliffs, and the French coast, all
along from Calais to St. Vallery, and, as some affirmed, as far to the
westward as Dieppe, which could not be much less than eighty or ninety
miles. By the telescope, the French fishing-boats were plainly seen at
anchor, and the different colours of the land on the heights, with the
buildings, were perfectly discernible.
This singular phenomenon was doubtless occasioned by an extraordinary
refraction produced either by an unusual expansion, or condensation
of the lower strata of the atmosphere, arising from circumstances
connected with the extreme heat of the season. The objects seem to
have been apparently raised far above their natural positions; for,
from the beach at Hastings, a straight line drawn across towards the
French coast, would have been intercepted by the curve of the waters.
They seem also to have been magnified by the refraction, and brought
apparently four or five times nearer the eye than in the ordinary state
of the atmosphere.
The following are likewise instances of unusual refraction:--When
Captain Colby was ranging over the coast of Caithness, with the
telescope of his great Theodolite, on the 21st of June, 1819, at
eight o’clock, P.M. from Corryhabbie Hill, near Mortlich, in
Banffshire, he observed a brig over the land of Caithness, sailing to
the westward in the Pentland Frith, between the Dunnet and Duncansby
heads. Having satisfied himself as to the fact, he requested his
assistants, Lieutenants Robe and Dawson, to look through the telescope,
which they immediately did, and observed the brig likewise. It was
very distinctly visible for several minutes, while the party continued
to look at it, and to satisfy themselves as to its position. The brig
could not have been less than from ninety to one hundred miles distant;
and, as the station on Corryhabbie is not above 850 yards above the
sea, the phenomenon is interesting. The thermometer was at 44°. The
night and day preceding the sight of the brig had been continually
rainy and misty, and it was not till 7 o’clock of the evening of the
21st that the clouds cleared off the hill.[8]
Captain Scoresby relates a singular phenomenon of this kind, which
occurred while he was traversing the Polar seas. His ship had been
separated by the ice from that of his father for a considerable time,
and he was looking out for her every day, with great anxiety. At
length, one evening, to his utter astonishment, he saw her suspended in
the air, in an inverted position, traced on the horizon in the clearest
colours, and with the most distinct and perfect representation. He
sailed in the direction in which he saw this visionary phenomenon, and
actually found his father’s vessel by its indication. He was divided
from him by immense masses of icebergs, and at such a distance, that
it was quite impossible to have seen the ship in her actual situation,
or to have seen her at all, if her spectrum had not been thus raised
several degrees above the horizon into the sky by this extraordinary
refraction. She was reckoned to be seventeen miles beyond the visible
horizon, and thirty miles distant.
Mrs. Somerville states, that a friend of her’s, while standing on
the plains of Hindostan, saw the whole upper chain of the Himalaya
mountains start into view, from a sudden change in the density of the
air, occasioned by a heavy shower, after a long course of dry and hot
weather. In looking at distant objects through a telescope, over the
top of a ridge of hills, about two miles distant, I have several times
observed, that some of the more distant objects which are sometimes
hid by the interposition of a ridge of hills, are, at other times,
distinctly visible above them. I have sometimes observed, that objects
near the middle of the field of view of a telescope, which was in a
fixed position, have suddenly appeared to descend to the lower part,
or ascend to the upper part of the field, while the telescope remained
unaltered. I have likewise seen, with a powerful telescope, the Bell
Rock Lighthouse, at the distance of about twenty miles, to appear as if
contracted to less than two-thirds of its usual apparent height, while
every part of it was quite distinct and well-defined, and in the course
of an hour or less, it appeared to shoot up to its usual apparent
elevation--all which phenomena are evidently produced by the same cause
to which we have been adverting.
Such are some of the striking effects produced by the refraction of
light. It enables us to see objects in a direction where they are not;
it raises, apparently, the bottoms of lakes and rivers: it magnifies
objects when their light passes through dense mediums: it makes the
sun appear above the horizon, when he is actually below it, and thus
increases the length of our day: it produces the Aurora and the evening
twilight, which forms, in many instances, the most delightful part of a
summer day: it prevents us from being involved in total darkness, the
moment after the sun has descended beneath the horizon: it modifies the
appearances of the celestial bodies, and the directions in which they
are beheld: it tinges the sun, moon, and stars, as well as the clouds,
with a ruddy hue, when near the horizon: it elevates the appearance of
terrestrial objects, and, in certain extraordinary cases, brings them
nearer to our view, and enables us to behold them when beyond the line
of our visible horizon. In combination with the power of reflection,
it creates visionary landscapes, and a variety of grotesque and
extraordinary appearances, which delight and astonish, and sometimes
appal the beholders. In short,--as we shall afterwards see more
particularly--the refraction of light through glasses of different
figures, forms the principle on which telescopes and microscopes
are constructed, by which both the remote and the minute wonders of
creation have been disclosed to view. So that had there been no bodies
capable of refracting the rays of light, we should have remained for
ever ignorant of many sublime and august objects in the remote regions
of the universe, and of the admirable mechanism and the countless
variety of minute objects which lie beyond the range of the unassisted
eye in our lower creation, all of which are calculated to direct our
views, and to enlarge our conceptions of the Almighty Creator.
In the operation of the law of refraction in these and numerous other
instances, we have a specimen of the diversified and beneficent effects
which the Almighty can produce by the agency of a single principle in
nature. By the influence of the simple law of gravitation, the planets
are retained in their orbits, the moon directed in her course around
the earth, and the whole of the bodies connected with the sun preserved
in one harmonious system. By the same law the mountains of our globe
rest on a solid basis, the rivers flow through the plains toward the
seas, the ocean is confined to its prescribed boundaries, and the
inhabitants of the earth are retained to its surface and prevented
from flying upwards through the voids of space. In like manner the law
by which light is refracted produces a variety of beneficial effects
essential to the present constitution of our world and the comfort
of its inhabitants. When a ray of light enters obliquely into the
atmosphere, instead of passing directly through, it bends a little
downwards, so that the greater portion of the rays which thus enter the
atmospheric mass, descend by inflection to the earth. We then enjoy
the benefit of that light which would otherwise have been totally
lost. We perceive the light of day an hour before the solar orb makes
its appearance, and a portion of its light is still retained when
it has descended nearly eighteen degrees below our horizon. We thus
enjoy, throughout the year, seven hundred and thirty hours of light
which would have been lost, had it not been refracted down upon us
from the upper regions of the atmosphere. To the inhabitants of the
polar regions this effect is still more interesting and beneficial.
Were it not for their twilight, they would be involved, for a much
longer period than they now are, in perpetual darkness; but by the
powerful refraction of light which takes place in the frigid zones, the
day sooner makes its appearance towards spring, and their long winter
nights are, in certain cases, shortened by a period of thirty days.
Under the poles, where the darkness of night would continue six months
without intermission, if there were no refraction, total darkness does
not prevail during the one half of this period. When the sun sets, at
the North pole about the 23rd of September, the inhabitants (if any)
enjoy a perpetual aurora, till he has descended 18 degrees below the
horizon. In his course through the ecliptic the sun is two months
before he can reach this point, during which time there is a perpetual
twilight. In two months more he arrives again at the same point, namely
18 degrees below the horizon, when a new twilight commences, which is
continually increasing in brilliancy, for other two months, at the end
of which the body of this luminary is seen rising in all its glory.
So that, in this region, the light of day is enjoyed, in a greater or
less degree, for ten months without interruption, by the effects of
atmospheric refraction; and, during the two months when the influence
of the solar light is entirely withdrawn, the moon is shining above the
horizon for two half months without intermission; and thus it happens,
that no more than two separate fortnights are passed in absolute
darkness; and this darkness is alleviated by the light of the stars and
the frequent coruscations of the Aurora Borealis. Hence, it appears,
that there are no portions of our globe that enjoy, throughout the
year, so large a portion of the solar light, as these northern regions,
which is chiefly owing to the refraction of the atmosphere.
The refraction of light by the atmosphere, combined with its power
of reflecting it, is likewise the cause of that universal light and
splendour which appears on all the objects around us. Were the earth
disrobed of its atmosphere, and exposed naked to the solar beams--in
this case, we might see the sun without having day, strictly so called.
His rising would not be preceded by any twilight as it now is. The most
intense darkness would cover us till the very moment of his rising;
he would then suddenly break out from under the horizon with the same
splendour he would exhibit at the highest part of his course, and would
not change his brightness till the very moment of his setting, when
in an instant all would be black as the darkest night. At noon day we
should see the sun like an intensely brilliant globe shining in a sky
as black as ebony, like a clear fire in the night seen in the midst of
an extensive field, and his rays would show us the adjacent objects
immediately around us; but the rays which fall on the objects remote
from us would be for ever lost in the expanse of the heavens. Instead
of the beautiful azure of the sky, and the colours which distinguish
the face of nature by day, we should see nothing but an abyss of
darkness, and the stars shining from a vault as dark as chaos. Thus
there would be no day, such as we now enjoy, without the atmosphere:
since it is by the refraction and reflections connected with this
aerial fluid that light is so modified and directed, as to produce all
that beauty, splendour and harmony, which appear on the concave of the
sky, and on the objects which diversify our terrestrial abode.
The effect of refraction, in respect to _terrestrial_ objects, is
likewise of a beneficial nature. The quantity of this refraction
is estimated by Dr. Maskelyne at one-tenth of the distance of the
object observed, expressed in degrees of a great circle. Hence, if
the distance be 10,000 fathoms, its tenth part 1000 fathoms, is the
sixtieth part of a degree, or one minute, which is the refraction
in altitude. Le Gendre estimates it at one fourteenth; De Lambre at
one eleventh; and others at a twelfth of the distance; but it must
be supposed to vary at different times and places according to the
varying state of the atmosphere. This refraction, as it makes objects
appear to be raised higher than they really are, enlarges the extent
of our landscapes, and enables us to perceive distant objects which
would otherwise have been invisible. It is particularly useful to
the navigator at sea. It is one important object of the mariner when
traversing his course, to look out for capes and headlands, rocks and
islands, so as to descry them as soon as they are within the reach
of his eye. Now, by means of refraction, the tops of hills and the
elevated parts of coasts, are apparently raised into the air, so that
they may be discovered several leagues further off on the sea than
they would be, did no such refractive power exist. This circumstance
is therefore a considerable benefit to the science of navigation, in
enabling the mariner to steer his course aright, and to give him the
most early warning of the track he ought to take, or of the dangers to
which he may be exposed.
In short, the effects produced by the refraction and reflection of
light on the scenery connected with our globe, teach us that these
principles, in the hand of the Almighty, might be so modified and
directed, as to produce the most picturesque, the most glorious and
wonderful phenomena, such as mortal eyes have never yet seen, and of
which human imagination can form no conception; and in other worlds,
more resplendent and magnificent than ours, such scenes may be fully
realized, in combination with the operation of physical principles and
agents, with which we are at present unacquainted. From what we already
know of the effects of the reflection and the refraction of light, it
is not beyond the bounds of probability to suppose, that in certain
regions of the universe, light may be reflected and refracted through
different mediums, in such a manner, as to present to the view of
their inhabitants the prominent scenes connected with distant systems
and worlds, and to an extent, as shall infinitely surpass the effects
produced by our most powerful telescopes.
CHAPTER III.
ON THE REFRACTION OF LIGHT THROUGH SPHERICAL TRANSPARENT SUBSTANCES, OR
LENSES.
It is to the refraction of light that we are indebted for the use
of lenses or artificial glasses to aid the powers of vision. It
lays the foundation of telescopes, microscopes, camera obscuras,
phantasmagorias, and other optical instruments, by which so many
beautiful, useful, and wonderful effects have been produced. In order
therefore to illustrate the principles on which such instruments are
constructed, it is necessary to explain the manner in which the rays
of light are refracted and modified, when passing through spherical
mediums of different forms. I do not intend however to enter into
the minutiæ of this subject, nor into any abstract mathematical
demonstrations, but shall simply offer a few explanations of general
principles, and several experimental illustrations, which may enable
the general reader to understand the construction of the optical
instruments to be afterwards described.
A lens is a transparent substance of a different density from the
surrounding medium, and terminating in two surfaces, either both
spherical, or one spherical and the other plain. It is usually made
of _glass_, but may also be formed of any other transparent substance,
as ice, crystal, diamond, pebbles, or by fluids of different densities
and refractive powers, enclosed between concave glasses. Lenses are
ground into various forms, according to the purpose they are intended
to serve. They may be generally distinguished as being either _convex_
or _concave_. A convex glass is thickest in the middle, and thinner
towards the edges. A concave glass is thin in the middle, and thicker
towards the extremities. Of these there are various forms, which are
represented in fig. 5. A, is a _plano-convex_ lens, which has one side
plane, and the other spherical or convex. B, is a _plano-concave_,
which is plane on the one side and concave on the other. C, is a
_double-convex_, or one which is spherical on both sides. D, a
_double-concave_, or concave on both sides. E, is called a _meniscus_,
which is convex on one side and concave on the other. F, is a
_concavo-convex_, the convex side of which is of a smaller sphere than
the concave. In regard to the _degree_ of convexity or concavity in
lenses, it is evident that there may be almost an infinite variety. For
every convex surface is to be considered as the segment of a circle,
the diameter and radius of which may vary to almost any extent. Hence,
lenses have been formed by opticians, varying from one-fiftieth of an
inch in radius, to two hundred feet. When we speak of the length of
the radius of a lens,--as for instance, when we say that a lens is two
inches or forty inches radius, we mean, that the convex surface of the
glass is the part of a circle the radius of which, or half the diameter
is two inches or forty inches; or in other words, were the portion of
the sphere on which it is ground formed into a globe of corresponding
convexity, it would be four inches or eighty inches in diameter.
[Illustration: _figure 5._]
[Illustration: _figure 6._]
[Illustration: _figure 7._]
[Illustration: _figure 8._]
The _axis_ of a lens is a straight line drawn through the center of its
spherical surface; and as the spherical sides of every lens are arches
of circles the axis of the lens would pass through the centre of that
circle of which its sides are segments. _Rays_ are those emanations
of light which proceed from a luminous body, or from a body that is
illuminated. The _Radiant_ is that body or object which emits the rays
of light--whether it be a self-luminous body, or one that only reflects
the rays of light. Rays may proceed from a Radiant in different
directions. They may be either parallel, converging, or diverging.
_Parallel_ rays are those which proceed _equally distant_ from each
other through their whole course. Rays proceeding from the sun, the
planets, the stars, and distant terrestrial objects are considered as
parallel, as in fig. 6. _Converging_ rays are such as, proceeding from
a body, approach nearer and nearer in their progress, tending to a
certain point where they all unite. Thus, the rays proceeding from the
object AB, (fig. 7.) to the point F, are said to converge towards that
point. All convex glasses cause parallel rays, which fall upon them
to converge in a greater or less degree; and they render converging
rays still more convergent. If AB, fig. 7. represent a convex lens,
and H G I parallel rays falling upon it, they will be refracted and
converge towards the point F, which is called the _focus_, or burning
point; because, when the sun’s rays are thus converged to a point by a
large lens, they set on fire combustible substances. In this point the
rays meet and intersect each other. _Diverging rays_ are those which,
proceeding from any point as A, fig. 8, continually recede from each
other as they pass along in their course towards BC. All the rays which
proceed from near objects as a window in a room, or an adjacent house
or garden are more or less divergent. The following figures show the
effects of parallel, converging and diverging rays in passing through
a double convex lens.
[Illustration: _figure 11._]
[Illustration: _figure 9._]
[Illustration: _figure 10._]
Fig. 9, shows the effects of parallel rays, KA, DE, LB, falling on a
convex glass AB. The rays which fall near the extremities at A and B,
are bent or refracted towards CF, the focus, and centre of convexity.
It will be observed, that they are less refracted as they approach
the center of the lens, and the central ray DEC, which is called the
_axis_ of the lens, and which passes through its center, suffers
no refraction. Fig. 10, exhibits the course of _converging_ rays,
when passing through a similar lens. In this case the rays converge
to a focus _nearer_ to the lens than the center; for a convex lens
uniformly increases the convergence of converging rays. The converging
rays here represented, may be conceived as having been refracted by
another convex lens of a longer focus, and, passing on towards a point
of convergence, were intercepted by the lens AB. The point D is the
place where the rays would have converged to a focus, had they not been
thus intercepted. Fig. 11, represents the course of diverging rays
when falling on a double convex glass. In this case the rays D B, D A,
&c., after passing through the lens, converge to a focus at a point
considerably farther from the lens than its centre, as at F. Such rays
must be considered as proceeding from near objects, and the fact may be
illustrated by the following experiment. Take a common reading-glass,
and hold it in the rays of the sun, opposite a sheet of writing-paper
or a white wall, and observe _at what distance_ from the glass the rays
on the paper converge to a small distinct white spot. This distance
gives the focal length of the lens by parallel rays. If now, we hold
the glass within a few feet of a window, or a burning candle, and
receive its image on the paper, the focal distance of the image from
the glass will be found to be longer. If, in the former case, the focal
distance was twelve inches,--in the latter case it will be thirteen,
fifteen, or sixteen inches, according to the distance of the window or
the candle from the glass.
If the lens A B, fig. 9, on which parallel rays are represented as
falling, were a _plano-convex_, as represented at A, fig, 5, the rays
would converge to a point P, at double the radius, or the whole
diameter of the sphere of which it is a segment. If the thickness of a
plano-convex be considered, and if it be exposed on its convex side to
parallel rays, as those of the sun, the focus will be at the distance
of _twice the radius, wanting two-thirds of the thickness of the lens_.
But if the same lens be exposed with its plane side to parallel rays,
the focus will then be precisely at the distance of twice the radius
from the glass.
The effects of _concave_ lenses are directly opposite to those of
convex. Parallel rays, striking one of those glasses, instead of
converging towards a point, are made to _diverge_. Rays already
divergent are rendered more so, and convergent rays are made less
convergent. Hence objects seen through concave glasses appear
considerably smaller and more distant than they really are. The
following diagram, fig. 12, represents the course of parallel rays
through a double concave lens, where the parallel rays T A, D E, I B,
&c., when passing through the concave glass A B, diverge into the rays
G L, E C, H P, &c., as if they proceeded from F, a point before the
lens, which is the principal focus of the lens.
[Illustration: _figure 12._]
The principal focal distance E F, is the same as in convex lenses.
Concave glasses are used to correct the imperfect vision of
short-sighted persons. As the form of the eye of such persons is too
convex, the rays are made to converge before they reach the optic
nerve; and therefore a concave glass, causing a little divergency,
assists this defect of vision, by diminishing the effect produced
by the too great convexity of the eye, and lengthening its focus.
These glasses are seldom used, in modern times, in the construction
of optical instruments, except as eye-glasses for small pocket
perspectives, and opera glasses.
_To find the focal distance of a concave glass._ Take a piece of
paste-board or card paper, and cut a round hole in it, not larger
than the diameter of the lens; and, on another piece of paste-board,
describe a circle whose diameter is just double the diameter of the
hole. Then apply the piece with the hole in it to the lens, and hold
them in the sun-beams, with the other piece at such a distance behind,
that the light proceeding from the hole may spread or diverge so as
precisely to fill the circle; then the distance of the circle from the
lens is equal to its virtual focus, or to its radius, if it be a double
concave, and to its diameter, if a plano-concave. Let _d, e_, (fig.
12,) represent the diameter of the hole, and _g, i_, the diameter of
the circle, then the distance C, I, is the virtual focus of the lens.[9]
The _meniscus_ represented at E, fig. 5, is like the crystal of a
common watch, and as the convexity is the same as the concavity,
it neither magnifies nor diminishes. Sometimes, however, it is
made in the form of a crescent, as at F, fig. 5, and is called a
_concavo-convex_ lens; and, when the convexity is greater than the
concavity, or, when it is thickest in the middle, it acts nearly in the
same way as a double or plano-convex lens of the same focal distance.
_Of the_ IMAGES _formed by convex lenses._
It is a remarkable circumstance, and which would naturally excite
admiration, were it not so common and well known, that _when the rays
of light from any object are refracted through a convex lens, they
paint a distinct and accurate picture of the object before it, in all
its colours, shades, and proportions_. Previous to experience, we
could have had no conception that light, when passing through such
substances, and converging to a point, could have produced so admirable
an effect,--an effect on which the construction and utility of all our
optical instruments depend. The following figure will illustrate this
position. Let L, N, represent a double convex lens, A, C, _a_, its
axis, and OB, an object perpendicular to it. A ray passing from the
extremity of the object at O, after being refracted by the lens at F,
will pass on in the direction FI, and form an image of that part of the
object at I. This ray will be the axis of all the rays which fall on
the lens from the point O, and I will be the focus where they will all
be collected. In like manner BCM, is the axis of that parcel of rays
which proceed from the extremity of the object B, and their focus will
be at M; and since all the points in the object between O, and B, must
necessarily have their foci between I and M, a complete picture of the
points from which they come will be depicted, and consequently an image
of the whole object OB.
[Illustration: _figure 13._]
It is obvious, from the figure, that the image of the object is
formed in the focus of the lens, in an _inverted position_. It must
necessarily be in this position, as the rays cross at C, the centre of
the lens; and as it is impossible that the rays from the upper part
of the object O, can be carried by refraction to the upper end of
the image at M. This is a universal principle in relation to convex
lenses of every description, and requires to be attended to in the
construction and use of all kinds of telescopes and microscopes. It
is easily illustrated by experiment. Take a convex lens of eight,
twelve, or fifteen inches focal distance, such as a reading glass, or
the glass belonging to a pair of spectacles, and holding it, at its
focal distance from a white wall, in a line with a burning candle, the
flame of the candle will be seen depicted on the wall in an inverted
position, or turned upside down. The same experiment may be performed
with a window-sash, or any other bright object. But, the most beautiful
exhibition of the images of objects formed by convex lenses, is made
by darkening a room, and placing a convex lens of a long focal distance
in a hole cut out of the window-shutter; when a beautiful inverted
landscape, or picture of all the objects before the window, will be
painted on a white paper or screen placed in the focus of the glass.
The image thus formed exhibits not only the proportions and colours,
but also the motions of all the objects opposite the lens, forming as
it were a living landscape. This property of lenses lays the foundation
of the camera obscura, an instrument to be afterwards described.
The following principles in relation to images formed by convex lenses
may be stated. 1. That _the image subtends the same angle at the centre
of the glass as the object itself does_. Were an eye placed at C, the
centre of the lens LN, fig. 13, it would see the object OB, and the
image IM under the same optical angle, or, in other words, they would
appear equally large. For, whenever right lines intersect each other,
as OI and BM, the opposite angles are always equal, that is, the angle
MCI is equal to the angle OCB. 2. _The length of the image formed by
a convex lens, is to the length of the object, as the distance of the
image is to the distance of the object from the lens_: that is, MI is
to OB :: as CA to CA. Suppose the distance of the object CA
from the lens, to be forty-eight inches, the length of the object OB
= sixteen inches, and the distance of the image from the lens, six
inches, then the length of the image will be found by the following
proportion, 48 : 16 :: 6 : 2, that is, the length of the image, in such
a case, is two inches. 3. _If the object be at an infinite distance,
the image will be formed exactly in the focus._ 4. _If the object be
at the same distance from the lens as its focus, the image is removed
to an infinite distance on the opposite side_; in other words, the
rays will proceed in a _parallel_ direction. On this principle, lamps
on the streets are sometimes directed to throw a bright light along a
foot-path where it is wanted, when a large convex glass is placed at
its focal distance from the burner; and on the same principle, light
is thrown to a great distance from lighthouses, either by a very large
convex lens of a short focal distance, or by a concave reflector. 5.
_If the object be at double the distance of the focus from the glass,
the image will also be at double the distance of the focus from the
glass._ Thus, if a lens of six inches focal distance be held at twelve
inches distance from a candle, the image of the candle will be formed
at twelve inches from the glass on the other side. 6. _If the object
be a little further from the lens than its focal distance, an image
will be formed, at a distance from the object, which will be greater
or smaller in proportion to the distance._ For example, if a lens
five inches focus, be held at a little more than five inches from a
candle, and a wall or screen at five feet six inches distant, receive
the image, a large and inverted image of the candle will be depicted,
which will be magnified in proportion as the distance of the wall
from the candle exceeds the distance of the lens from the candle.
Suppose the distance of the lens to be five and a half inches, then
the distance of the wall where the image is formed, being twelve times
greater, the image of the candle will be magnified twelve times. If
MI (fig. 13.) be considered as the object, then OB will represent the
magnified image on the wall. On this principle the image of the object
is formed by the small object glass of a compound microscope. On the
same principle the large pictures are formed by the Magic Lantern and
the Phantasmagoria; and in the same way small objects are represented
in a magnified form, on a sheet or wall by the Solar microscope. 7.
_All convex lenses magnify the objects seen through them, in a greater
or less degree._ The shorter the focal distance of the lens, the
greater is the magnifying power. A lens four inches focal distance,
will magnify objects placed in the focus, two times in length and
breadth; a lens two inches focus will magnify four times, a lens one
inch focus eight times; a lens half an inch focus sixteen times, &c.
supposing eight inches to be the least distance at which we see near
objects distinctly. In viewing objects with small lenses, the object
to be magnified should be placed exactly at the focal distance of the
lens, and the eye at about the same distance on the other side of
the lens. When we speak of magnifying power, as, for example, that a
lens one inch focal distance magnifies objects eight times, it is to
be understood of the _lineal_ dimensions of the object. But as every
object at which we look has breadth as well as length, the _surface_
of the object is in reality magnified sixty-four times, or the square
of its lineal dimensions; and for the same reason a lens half an inch
focal distance magnifies the _surfaces_ of objects 256 times.
_Reflections deduced from the preceding subject._
Such are some of the leading principles which require to be recognised
in the construction of refracting telescopes, microscopes, and
other dioptric instruments whose performance chiefly depends on the
_refraction_ of light.--It is worthy of particular notice that all the
phenomena of optical lenses now described, depend upon that peculiar
property which the Creator has impressed upon the rays of light, that,
_when they are refracted to a focus by a convex transparent substance,
they depict an accurate image of the objects whence they proceed_.
This, however common, and however much overlooked by the bulk of
mankind, is indeed a very wonderful property with which light has been
endued. Previous to experience we could have had no conception that
such an effect would be produced; and, in the first instance, we could
not possibly have traced it to all its consequences. All the objects
in creation might have been illuminated as they now are, for aught
we know, without sending forth either direct or reflected rays _with
the property of forming exact representations of the objects whence
they proceeded_. But this we find to be a universal law in regard to
light of every description, whether as emanating directly from the
sun, or as reflected from the objects he illuminates, or as proceeding
from bodies artificially enlightened. It is a law or a property of
light not only in our own system, but throughout all the systems of
the universe to which mortal eyes have yet penetrated. The rays from
the most distant star which astronomers have descried, are endued
with this property, otherwise they could never have been perceived by
means of our optical instruments; for it is by the pictures or images
formed in these instruments that such distant objects are brought to
view. Without this property of light, therefore, we should have had
no telescopes, and consequently we could not have surveyed, as we can
now do, the hills and vales, the deep caverns, the extensive plains,
the circular ranges of mountains, and many other novel scenes which
diversify the surface of our moon. We should have known nothing of the
stupendous spots which appear on the surface of the sun--of the phases
of Venus--of the satellites and belts of Jupiter--of the majestic rings
of Saturn--of the existence of Uranus and his six moons,--or of the
planets Vesta, Juno, Ceres, and Pallas, nor could the exact bulks of
any of these bodies have been accurately determined. But, above all,
we should have been entirely ignorant of the wonderful phenomena of
double stars--which demonstrate that suns revolve around suns--of the
thousands and millions of stars which crowd the profundities of the
Milky Way and other regions of the heavens--of the thousands of Nebulæ
or starry systems which are dispersed throughout the immensity of the
firmament, and many other objects of sublimity and grandeur, which fill
the contemplative mind with admiration and awe, and raise its faculties
to higher conceptions than it could otherwise have formed of the
omnipotence and grandeur of the Almighty Creator.
Without this property of the rays of light we should likewise have
wanted the use of the microscope--an instrument which has disclosed a
world invisible to common eyes, and has opened to our view the most
astonishing exhibitions of Divine mechanism, and of the wisdom and
intelligence of the Eternal Mind. We should have been ignorant of
those tribes of living beings, invisible to the unassisted eye, which
are found in water, vinegar, and many other fluids--many of which are
twenty thousand times smaller than the least visible point, and yet
display the same admirable skill and contrivance in their construction,
as are manifested in the formation of the larger animals. We should
never have beheld the purple tide of life, and even the globules of the
blood rolling with swiftness through veins and arteries smaller than
the finest hair; or had the least conception that numberless species
of animated beings, so minute that a million of them are less than
a grain of sand, could have been rendered visible to human eyes, or
that such a number of vessels, fluids, movements, diversified organs
of sensation, and such a profusion of the richest ornaments and the
gayest colours could have been concentrated in a single point. We
should never have conceived that even the atmosphere is replenished
with invisible animation, that the waters abound with countless myriads
of sensitive existence, that the whole earth is full of life, and
that there is scarcely a tree, plant, or flower, but affords food and
shelter to a species of inhabitants peculiar to itself, which enjoy
the pleasures of existence and share in the bounty of the Creator. We
could have formed no conception of the beauties and the varieties of
mechanism which are displayed in the scenery of that invisible world
to which the microscope introduces us--beauties and varieties, in
point of ornament and delicate contrivance, which even surpass what is
beheld in the visible operations and aspect of nature around us. We
find joints, muscles, a heart, stomach, entrails, veins, arteries, a
variety of motions, a diversity of forms, and a multiplicity of parts
and functions--in breathing atoms. We behold in a small fibre of a
peacock’s feather, not more than one-eighth of an inch in length, a
profusion of beauties no less admirable than is presented by the whole
feather to the naked eye--a stem sending out multitudes of lateral
branches, each of which emits numbers of little sprigs, which consist
of a multitude of bright shining globular parts, adorned with a rich
variety of colours. In the sections of plants, we see thousands and
ten thousands of tubes and pores, and other vessels for the conveyance
of air and juices for the sustenance of the plant; in some instances,
more than ten hundred thousand of these being compressed within the
space of a quarter of an inch in diameter, and presenting to the eye
the most beautiful configurations. There is not a weed, nor a moss, nor
the most insignificant vegetable, which does not show a multiplicity
of vessels disposed in the most curious manner for the circulation of
sap for its nourishment, and which is not adorned with innumerable
graces for its embellishment. All these and ten thousands of other
wonders which lie beyond the limits of natural vision, in this new and
unexplored region of the universe, would have been for ever concealed
from our view, had not the Creator endued the rays of light with the
power of _depicting the images of objects_, when refracted by convex
transparent substances.
In this instance, as well as in many others, we behold a specimen of
the admirable and diversified effects, which the Creator can produce
from the agency of a single principle in nature. By means of optical
instruments, we are now enabled to take a more minute and expansive
view of the amazing operations of nature, both in heaven and on earth,
than former generations could have surmised. These views tend to raise
our conceptions of the attributes of that Almighty Being, who presides
over all the arrangements of the material system, and to present them
to our contemplation in a new, a more elevated, and expansive point of
view. There is, therefore, a connection which may be traced between the
apparently accidental principle of the rays of light forming images of
objects, and the comprehensive views we are now enabled to take of the
character and perfections of the Divinity. Without the existence of the
law or principle alluded to, we could not, in the present state, have
formed precisely the same conceptions either of the Omnipotence, or of
the wisdom and intelligence of the Almighty. Had no microscope ever
been invented, the idea never could have entered into the mind of man,
that worlds of living beings exist beyond the range of natural vision,
that organized beings possessed of animation exist, whose whole bulk
is less than the ten hundred thousandth part of the smallest grain of
sand; that, descending from a visible point to thousands of degrees
beyond it, an invisible world exists, peopled with tribes of every form
and size, the extent of which, and how far it verges towards infinity
downwards, mortals have never yet explored, and perhaps will never be
able to comprehend. This circumstance alone presents before us the
perfections of the divinity in a new aspect, and plainly intimates that
it is the will and the intention of the Deity, that we should explore
his works, and investigate the laws by which the material world is
regulated, that we may acquire more expansive views of his character
and operations. The inventions of man in relation to art and science,
are not therefore to be considered as mere accidental occurrences, but
as special arrangements in the divine government, for the purpose of
carrying forward the human mind to more clear and ample views of the
scenes of the universe, and of the attributes and the agency of Him
“who is wonderful in counsel and excellent in working.”
CHAPTER IV.
ON THE REFLECTION OF LIGHT.
The _reflection_ of the rays of light is that property by which--after
approaching the surfaces of bodies, they are thrown back, or repelled.
It is in consequence of this property that all the objects around
us, and all the diversified landscapes on our globe, are rendered
visible. It is by light reflected from their surfaces that we perceive
the planetary bodies and their satellites, the belts of Jupiter, the
rings of Saturn, the various objects which diversify the surface of
the Moon, and all the bodies in the universe which have no light of
their own. When the rays of light fall upon rough and uneven surfaces,
they are reflected very irregularly and scattered in all directions,
in consequence of which thousands of eyes, at the same time, may
perceive the same objects, in all their peculiar colours, aspects,
and relations. But, when they fall upon certain smooth and polished
surfaces, they are reflected with regularity, and according to certain
laws. Such surfaces, when highly polished, are called _Mirrors_ or
_Speculums_; and it is to the reflection of light from such surfaces,
and the effects it produces, that I am now to direct the attention of
the reader.
Mirrors or Specula, may be distinguished into three kinds, _plane_,
_concave_, and _convex_, according as they are bounded by plane or
spherical surfaces. These are made either of _metal_ or of _glass_,
and have their surfaces highly polished for the purpose of reflecting
the greatest number of rays. Those made of glass are foliated or
quicksilvered on one side; and the metallic specula are generally
formed of a composition of different metallic substances, which,
when accurately polished, is found to reflect the greatest quantity
of light. I shall, in the first place, illustrate the phenomena of
reflection produced by _plane-mirrors_.
When light impinges, or falls, upon a polished flat surface,
rather more than the half of it is reflected, or thrown back in a
direction similar to that of its approach; that is to say, if it fall
_perpendicularly_ on the polished surface, it will be perpendicularly
reflected; but if it fall _obliquely_, it will be reflected with
the same obliquity. Hence, the following fundamental law, regarding
the reflection of light, has been deduced both from experiment and
mathematical demonstration, namely, that _the angle of reflection is,
in all cases, exactly equal to the angle of incidence_. This is a law
which is universal in all cases of reflection, whether it be from
plane or spherical surfaces, or whether these surfaces be concave or
convex, and which requires to be recognized in the construction of all
instruments which depend on the reflection of the rays of light. The
following figure (fig. 14) will illustrate the position now stated.
Let AB represent a plane mirror, and CD a line or ray of light
perpendicular to it. Let FD represent the _incident_ ray from any
object, then DE will be the reflected ray, thrown back in the
direction from D to E, and it will make with the perpendicular CD the
same angle which the incident ray FD did with the same perpendicular,
that is, the angle FDC will be equal to the angle EDC, in all cases of
obliquity. The incident ray of light may be considered as rebounding
from the mirror, like a tennis ball from a marble pavement, or the wall
of a court.
[Illustration: _figure 14._]
In viewing objects by reflection we see them in a different direction
from that in which they really are, namely, along the line in which the
rays come to us last. Thus, if AB (fig. 15) represent a plane mirror,
the image of an object C appears to the eye at E behind the mirror, in
the direction EG, and always in the intersection G of the perpendicular
CG, and the reflected ray EG--and consequently at G as far behind
the mirror, as the object C is before it. We therefore see the image
in the line EG, the direction in which the reflected rays proceed. A
plane mirror does not alter the figure or size of objects; but the
whole image is equal and similar to the whole object, and has a like
situation with respect to one side of the plane, that the object has
with respect to the other.
[Illustration: _figure 15._]
Mr. Walker illustrates the manner in which we see our faces in a mirror
by the following figure (16). AB represents a mirror, and
OC, a person looking into it. If we conceive a ray proceeding from
the forehead CE, it will be sent to the eye at O,
agreeably to the angle of incidence and reflection. But the mind puts
CEO into one line, and the forehead is seen at H,
as if the lines CEO had turned on a hinge at E.--It
seems a wonderful faculty of the mind to put the two oblique lines
CE and OE into one straight line OH, yet
it is seen every time we look at a mirror. For the ray has really
travelled from C to E, and from E to
O, and it is that journey which determines the distance of the
object; and hence we see ourselves as far beyond the mirror as we stand
from it. Though a ray is here taken only from one part of the face, it
may be easily conceived that rays from every other part of the face
must produce a similar effect.
[Illustration: _figure 16._]
In every plain mirror, the image is always equal to the object, at what
distance soever it may be placed; and as the mirror is only at half
the distance of the image from the eye, it will completely receive an
image of _twice_ its own length. Hence a man six feet high may view
himself completely in a looking glass of three feet in length, and
half his own breadth; and this will be the case at whatever distance
he may stand from the glass. Thus, the man AC (fig. 17) will see the
whole of his own image in the glass AB, which is but one half
as large as himself. The rays from the head pass to the mirror in the
line AA, perpendicular to the mirror, and are returned to the
eye in the same line; consequently, having travelled twice the length
AA, the man must see his head at B. From his feet C rays
will be sent to the bottom of the mirror at B; these will be
reflected at an equal angle to the eye in the direction BA, as
if they had proceeded in the direction DBA, so that the man
will see his foot at D, and consequently his whole figure at BD.
[Illustration: _figure 17._]
A person when looking into a mirror, will always see his own image as
far beyond the mirror as he is before it, and as he moves to or from
it, the image will, at the same time, move towards or from him on the
other side; but apparently with a double velocity, because the two
motions are equal and contrary. In like manner, if while the spectator
is at rest, an object be in motion, its image behind the mirror will
be seen to move at the same time. And if the spectator moves, the
images of objects that are at rest will appear to approach, or recede
from him, after the same manner as when he moves towards real objects;
plane mirrors reflecting not only the object, but the distance also,
and that exactly in its natural dimensions--The following principle is
sufficient for explaining most of the phenomena seen in a plane mirror,
namely;--_That the image of an object seen in a plane mirror, is always
in a perpendicular to the mirror joining the object and the image, and
that the image is as much on one side the mirror, as the object is on
the other_.
_Reflection by Convex and Concave Mirrors._
Both convex and concave mirrors are formed of portions of a sphere.
A convex speculum is ground and polished in a _concave_ dish or tool
which is a portion of a sphere, and a concave speculum is ground upon
a convex tool. The inner surface of a sphere brings parallel rays
to a focus at _one fourth_ of its diameter, as represented in the
following figure, where C is the centre of the sphere on which the
concave speculum AB is formed, and F the focus where parallel rays from
a distant object would be united, after reflection, that is, at one
half the radius, or one fourth of the diameter from the surface of the
speculum. Were a speculum of this kind presented to the sun, F would
be the point where the reflected rays would be converged to a focus,
and set fire to combustible substances if the speculum be of a large
diameter, and of a short focal distance. Were a candle placed in that
focus, its light would be reflected parallel as represented in the
figure. These are properties of concave specula which require to be
particularly attended to in the construction of reflecting telescopes.
It follows, from what has been now stated, that if we intend to form
a speculum of a certain focal distance,--for example, two feet, it is
necessary that _it should be ground upon a tool whose radius is double
that distance_, or four feet.
[Illustration: _figure 18._]
_Properties of Convex Mirrors._
From a convex surface, parallel rays when reflected are made to
diverge; convergent rays are reflected less convergent; and divergent
rays are rendered more divergent. It is the nature of all convex
mirrors and surfaces to scatter or _disperse_ the rays of light, and in
every instance to impede their convergence. The following figure shows
the course of parallel rays as reflected from a convex mirror. AEB is
the convex surface of the mirror; and KA, IE, LB, parallel rays falling
upon it. These rays, when they strike the mirror, are made to diverge
in the direction AG, BH, &c. and both the parallel and divergent rays
are here represented as they appear in a dark chamber, when a convex
mirror is presented to the solar rays. The dotted lines denote only the
course or tendency of the reflected rays, towards the _virtual_ focus
F, were they not intercepted by the mirror. This virtual focus is just
equal to half the radius CE.
[Illustration: _figure 19._]
The following are some of the properties of convex mirrors: 1. The
image appears always erect, and behind the reflecting surface. 2. _The
image is always smaller than the object_, and the diminution is greater
in proportion as the object is further from the mirror, but if the
object touch the mirror, the image at the point of contact is of the
same size as the object. 3. The image does not appear so far behind the
reflecting surface as in a plain mirror. 4. The image of a straight
object, placed either parallel or oblique to the mirror is seen
_curved_ in the mirror; because the different points of the object are
not all at an equal distance from the surface of the mirror. 5. Concave
mirrors have a _real_ focus where an image is actually formed; but
convex specula have only a _virtual_ focus, and this focus is behind
the mirror; no image of any object being formed before it.
The following are some of the purposes to which convex mirrors are
applied. They are frequently employed by painters for reducing the
proportions of the objects they wish to represent, as the images of
objects diminish in proportion to the smallness of the radius of
convexity, and to the distances of objects from the surface of the
mirror. They form a fashionable part of modern furniture, as they
exhibit a large company assembled in a room, with all the furniture it
contains, in a very small compass, so that a large hall with all its
objects, and even an extensive landscape, being reduced in size, may
be seen from one point of view. They are likewise used as the small
specula of those reflecting telescopes which are fitted up on the
_Cassegrainian_ plan, and in the construction of Smith’s Reflecting
Microscope. But on the whole, they are very little used in the
construction of optical instruments.
_Properties of Concave speculums._
Concave specula have properties very different from those which are
convex; they are of more importance in the construction of reflecting
telescopes and other optical instruments; and therefore require more
minute description and illustration. Concave mirrors cause parallel
rays to converge; they increase the convergence of rays that are
already converging; they diminish the divergence of diverging rays;
and, in some cases, render them parallel and even convergent; which
effects are all in proportion to the concavity of the mirror. The
following figures show the course of diverging and parallel rays as
reflected from concave mirrors.
Fig. 20 represents the course of _parallel_ rays, and AB, the concave
mirror on which they fall. In this case, they are reflected so as to
unite at F, which point is distant from its surface _one fourth_ of the
diameter of the sphere of the mirror. This point is called the focus of
parallel rays, or _the true focus of the mirror_. And, since the sun
beams are parallel among themselves, if they are received on a concave
mirror, they will all be reflected to that point, and there burn in
proportion to the quantity of rays collected by the mirror. Fig. 21.
shows the direction of _diverging_ rays, or those which proceed from
a near object. These rays proceeding from an object further from
the mirror than the true focal point, as from D to A and to B, are
reflected converging and meet at a point F, _further from the mirror_
than the focal point of parallel rays. If the distance of the radiant,
or object D, be equal to the radius CE, then will the focal distance
be likewise equal to the radius: That is, if an object be placed in
the center of a concave speculum, the image will be reflected upon the
object, or they will seem to meet and embrace each other in the centre.
If the distance of the radiant be equal to half the radius, its image
will be reflected to an infinite distance, for the rays will then be
parallel. If, therefore, a luminous body be placed at half the radius
from a concave speculum, it will enlighten places directly before it
at great distances. Hence their use when placed behind a candle in a
common lantern; hence their utility in throwing light upon objects in
the Magic Lantern and Phantasmagoria, and hence the vast importance
of very large mirrors of this description, as now used in most of our
Light Houses, for throwing a brilliant light to great distances at sea
to guide the mariner when directing his course under the cloud of night.
[Illustration: _figure 20._]
[Illustration: _figure 21._]
When _converging_ rays fall upon a concave mirror, they are reflected
more converging and unite at a point between the focus of parallel rays
and the mirror; that is, nearer the mirror than one half the radius;
and their precise degree of convergency will be greater than that
wherein they converged before reflection.
_Of the images formed by Concave Mirrors._
If rays proceeding from a distant object fall upon a concave speculum,
they will paint an image or representation of the object on its focus
_before_ the mirror. This image will be inverted, because the rays
cross at the points where the image is formed. We have already seen
that a convex glass forms an image of an object _behind_ it; the rays
of light from objects _pass through_ the glass, and the picture is
formed on the side farthest from the object. But in concave mirrors
the images of distant objects--and of all objects that are farther
from its surface than its principal focus--are formed _before_ the
mirror, or on the same side as the object. In almost every other
respect, however, the effect of a concave mirror is the same as that
of a convex lens, in regard to the formation of images, and the course
pursued by the rays of light, except that the effect is produced in the
one case by refraction, and in the other by reflection. The following
figure represents the manner in which images are formed by concave
mirrors. GF represents the reflecting surface of the mirror; OAB, the
object; and IAM, the image formed by the mirror. The rays
proceeding from O, will be carried to the mirror, in the direction OG,
and according to the law that the angle of incidence is equal to the
angle of reflection, will be reflected to I, in the direction GI. In
like manner the rays from B, will be reflected from F to M, the rays
from A, will be reflected to a, and so of all the intermediate rays, so
that an inverted image of the object OB, will be formed at IM. If the
rays proceeded from objects at a very great distance the image would be
formed in the real focus of the mirror, or at one-fourth the diameter
of the sphere from its surface; but near objects, which send forth
diverging rays, will have their images formed a little farther from the
surface of the mirror.
[Illustration: _figure 22._]
If we suppose a real object placed at IM, then OB will represent its
magnified image, which will be larger than the object, in proportion to
its distance from the mirror. This may be experimentally illustrated
by a concave mirror and a candle. Suppose a concave mirror whose focal
distance is five inches, and that a candle is placed before it, at
a little beyond its focus, (as at IM)--suppose at five and a half
inches,--and that a wall or white screen receives the image, at the
distance of five feet six inches from the mirror, an image of the
candle will be formed on the wall which will be twelve times longer
and broader than the candle itself. In this way concave mirrors may be
made to magnify the images of objects to an indefinite extent. This
experiment is an exact counterpart of what is effected in similar
circumstances by a convex lens, as described p. 74; the mirror
performing the same thing by reflection, as the lens did by refraction.
From what has been stated in relation to concave mirrors it will be
easily understood how they make such powerful burning-glasses. Suppose
the focal distance of a concave mirror to be twelve inches, and its
diameter or breadth twelve inches. When the sun’s rays fall on such
a mirror, they form an image of the sun at the focal point whose
diameter is found to be about one-tenth of an inch. All the rays
which fall upon the mirror are converged into this small point; and
consequently their intensity is in proportion as the square of the
surface of the mirror is to the square of the image. The squares of
these diameters are as 14,400 to 1; and consequently the density of the
sun’s rays, in the focus, is to their density on the surface of the
mirror as 14,400 to 1. That is, the heat of the solar rays in the focus
of such a mirror will be fourteen thousand four hundred times greater
than before--a heat which is capable of producing very powerful effects
in melting and setting fire to substances of almost every description.
Were we desirous of forming an image by a concave speculum which shall
be exactly equal to the object, the object must be placed exactly in
the centre; and, by an experiment of this kind, the centre of the
concavity of a mirror may be found.
In the cases now stated, the images of objects are all formed in the
front of the mirror, or between it and the object. But there is a case
in which the image is formed behind the mirror. This happens when the
object is placed between the mirror and the focus of parallel rays,
and then the image is larger than the object. In fig. 23, GF is a
concave mirror, whose focus of parallel rays is at E. If an object OB
be placed a little within this focus, as at A, a large image IM will
be seen _behind_ the mirror, somewhat curved and erect, which will be
seen by an eye looking directly into the front of the mirror. Here
the image appears at a greater distance behind the mirror than the
object is before it, and the object appears magnified in proportion
to its distance from the focus and the mirror. If the mirror be one
inch focal distance, and the object be placed eight-tenths of an inch
from its surface, the image would be five times as large as the object
in length and breadth, and consequently twenty-five times larger in
surface. In this way small objects may be magnified by reflection, as
such objects are magnified by refraction, in the case of deep convex
lenses. When such mirrors are large, for example six inches diameter,
and eight or ten inches focal distance, they exhibit the human face
as of an enormous bulk. This is illustrated by the following figure.
Let C N, Fig. 24, represent the surface of a concave mirror,
and A a human face looking into it, the face will appear magnified
as represented by the image behind the mirror D Q. Suppose
a ray A C proceeding from the forehead, and another M
N from the chin; these rays are reflected to the person’s eye
at O, which consequently sees the image in the lines of
reflection O D, O Q, and in the angle D O
Q, and consequently magnified much beyond the natural size, and at
a small distance behind the mirror.
[Illustration: _figure 23._]
[Illustration: _figure 24._]
If we suppose the side T U to represent a _convex_ mirror,
and the figure D Q a head of an ordinary size, then the
figure A will represent the diminished appearance which a person’s
face exhibits, when viewed in such a mirror. It will not only appear
reduced, but somewhat distorted; because from the form of the mirror,
one part of the object is nearer to it than another, and consequently
will be reflected under a different angle.
The effect we have now mentioned as produced by _concave_ mirrors,
will only take place when the eye is nearer the mirror than its
principal focus. If the spectator retire beyond this focus--suppose to
the distance of five or six feet, he will not see the image _behind_
the mirror; but he will see his image in a diminished form, hanging
upside down, and suspended in the air, in a line between his eye and
the mirror. In this case, his image is formed _before_ the mirror as
represented at IM fig. 22. In this situation, if you hold out your hand
towards the mirror, the hand of the image will come out towards your
hand, and, when at the centre of concavity, it will be of an equal size
with it, and you may shake hands with this aerial image. If you move
your hand farther, you will find the hand of the image pass by your
hand, and come between it and your body. If you move your hand towards
either side, the hand of the image will move towards the other side;
the image moving always in a contrary direction to the object. All
this while the by-standers, if any, see nothing of the image, because
none of the reflected rays that form it can enter their eyes.--The
following figure represents a phenomenon produced in the same manner.
A B is a concave mirror of a large size; C represents
a hand presented before the mirror, at a point farther distant than
its focus. In this case, an inverted image of the hand is formed which
is seen hanging in the air at M. The rays C and
D go diverging from the two opposite points of the object, and
by the action of the mirror, they are again made to converge to points
at O and S where they cross, form an image, and again
proceed divergent to the eye.[10]
[Illustration: _figure 25._]
In consequence of the properties of concave mirrors, now described,
many curious experiments and optical deceptions have been exhibited.
The appearance of images in the air, suspended between the mirror and
the object, have sometimes been displayed with such dexterity and an
air of mystery, as to have struck with astonishment those who were
ignorant of the cause. In this way birds, flying angels, spectres and
other objects have been exhibited, and when the hand attempts to lay
hold on them, it finds them to be nothing, and they seem to vanish
into air. An apple or a beautiful flower is presented, and when a
spectator attempts to touch it, it instantly vanishes, and a death’s
head immediately appears, and seems to snap at his fingers. A person
with a drawn sword appears before him, in an attitude as if about to
run him through, or one terrific phantom starts up after another, or
sometimes the resemblances of deceased persons are made to appear, as
if, by the art of conjuration, they had been forced to return from the
world of spirits. In all such exhibitions, a very large concave mirror
is requisite, a brilliant light must be thrown upon the objects, and
every arrangement is made, by means of partitions, &c., to prevent
either the light, the mirror, or the object from being seen by the
spectators. The following representation (fig. 26.) shows one of the
methods by which this is effected: A is a large concave
mirror, either of metal or of glass, placed on the back part of a dark
box, D is the performer, concealed from the spectators by
the cross partition C; E is a strong light, which
is likewise concealed by the partition I, which is thrown
upon the actor D, or upon any thing he may hold in his hand.
If he hold a book, as represented in the figure, the light reflected
from it will pass between the partitions C and I to
the mirror, and will be reflected from thence to Z, where
the image of the book will appear so distinct and tangible, that a
spectator looking through the opening at X, will imagine that it is in
his power to take hold of it. In like manner, the person situated at
D, may exhibit his own head or body--a portrait, a painting,
a spectre, a landscape, or any object or device which he can strongly
illuminate.
[Illustration: _figure 26._]
[Illustration: _figure 27._]
[Illustration: _figure 28._]
There is another experiment, made with a concave mirror, which has
somewhat puzzled philosophers to account for the phenomena. Take a
glass bottle AC, (fig. 27) and fill it with water to the point B;
leave the upper part BC empty, and cork it in the common manner. Place
this bottle opposite a concave mirror, and beyond its focus, that
it may appear reversed, and, before the mirror place yourself still
further distant from the bottle, and it will appear in the situation
A B C. Now, it is remarkable in this apparent bottle, that
the _water_, which, according to the laws of catoptrics, should
appear at A B, appears on the contrary at B C, and
consequently, the part A B appears empty. If the bottle be
inverted and placed before the mirror, its image will appear in its
natural erect position, and the water which is in reality at BC (fig.
28) is seen at A B. If while the bottle is inverted, it be
uncorked, and the water run gently out, it will appear, that, while the
part BC is _emptying_, that of A B in the image is filling,
and, what is remarkable, as soon as the bottle is empty, the illusion
ceases, the image also appearing entirely empty.--The remarkable
circumstances in this experiment are, first, not only to see the object
where it is not, but also where its _image_ is not; and secondly, that
of two objects which are really in the same place, as the surface of
the bottle and the water it contains, the one is seen at one place, and
the other at another; and to see the bottle in the place of its image,
and the water where neither it nor its image are.
The following experiments are stated by Mr. Ferguson in his “Lectures
on select Subjects,” &c. “If a fire be made in a large room, and a
smooth mahogany table be placed at a good distance near the wall,
before a large concave mirror, so placed that the light of the fire may
be reflected from the mirror to its focus upon the table; if a person
stand by the table, he will see nothing upon it but a longish beam of
light: but if he stand at a distance toward the fire, not directly
between the fire and mirror, he will see an image of the fire upon the
table, large and erect. And if another person who knows nothing of
the matter beforehand should chance to come into the room, and should
look from the fire toward the table, he would be startled at the
appearance; for the table would seem to be on fire, and by being near
the wainscot, to endanger the whole house. In this experiment there
should be no light in the room but what proceeds from the fire; and the
mirror ought to be at least fifteen inches in diameter. If the fire be
darkened by a screen, and a large candle be placed at the back of the
screen, a person standing by the candle will see the appearance of a
very fine large star, or rather planet, upon the table, as bright as
Venus or Jupiter. And if a small wax taper--whose flame is much less
than the flame of the candle--be placed near the candle, a satellite to
the planet will appear on the table; and if the taper be moved round
the candle, the satellite will go round the planet.”
Many other illustrations of the effects of concave specula might have
been given, but I shall conclude this department by briefly stating
some of the _general properties of speculums_.
1. There is a great resemblance between the properties of _convex_
lenses and _concave_ mirrors. They both form an inverted focal image
of any remote object, by the convergence of the pencil of rays. In
those instruments whose performances are the effects of reflection, as
reflecting telescopes, the concave mirror is substituted in the place
of the convex lens. The whole effect of these instruments, in bringing
to view remote objects in heaven and on earth, entirely depends on the
property of a concave mirror in forming _images_ of objects in its
focus. 2. The image of an object placed beyond the centre, is less
than the object; if the object be placed between the principal focus
and the centre, the image is greater than the object. In both cases
the image is inverted. 3. When the object is placed between the focus
and the mirror, the image situated _behind_ the mirror is greater
than the object, and it has the same direction: in proportion as the
object approaches the focus, the image becomes larger and more distant.
These and similar results are proved by placing a lighted candle at
different distances from a concave mirror. 4. An eye cannot see an
image in the air except it be placed in the diverging rays; but if the
image be received on a piece of white paper, it may be seen in any
position of the eye, as the rays are then reflected in every direction.
5. If a picture drawn according to the rules of perspective, be placed
before a large concave speculum, a little nearer than its principal
focus, the image of the picture will appear extremely natural, and
very nearly like the real objects whence it was taken. Not only are
the objects considerably magnified, so as to approach to their natural
size, but they have also different apparent distances, as in nature, so
that the view of the inside of a church appears very like what it is
in reality, and representations of landscapes appear very nearly, as
they do from the spot whence they were taken. In this respect a large
concave speculum may be made to serve nearly the same purpose, as the
Optical Diagonal Machine, in viewing perspective prints. 6. The concave
speculum is that alone which is used as the great mirror which forms
the first image in reflecting telescopes; and it is likewise the only
kind of speculum used as the small mirror, in that construction of the
instrument called the _Gregorian Reflector_.
_Quantity of light reflected by polished surfaces._
As this is a circumstance connected with the construction of reflecting
telescopes, it may not be improper, in this place, to state some
of the results of the accurate experiments of M. Bonguer on this
subject. This philosopher ascertained that of the light reflected from
mercury, or quicksilver, more than _one-fourth_ is lost, though it is
probable that no substances reflect more light than this. The rays
were received at an angle of eleven and a half degrees of incidence,
measured from the surface of the reflecting body, and not from the
perpendicular. The reflection from _water_ was found to be almost
as great as that of quicksilver; so that in very small angles it
reflects nearly three-fourths of the direct light. This is the reason
why so strong a reflection appears on water, when one walks, in still
weather, on the brink of a lake opposite to the sun. The direct light
of the sun diminishes gradually as it approaches the horizon, while
the reflected light at the same time grows stronger; so that there
is a certain elevation of the sun in which the united force of the
direct and reflected light will be the greatest possible, and this is
when he is twelve or thirteen degrees in altitude. On the other hand,
light reflected from water at _great angles_ of incidence is extremely
small. When the light was perpendicular, it reflected no more than the
thirty-seventh part which mercury does in the same circumstances, and
only the fifty-fifth part of what fell upon it in this case.
Using a smooth piece of glass, one line in thickness, he found that,
when it was placed at an angle of fifteen degrees with the incident
rays, it reflected 628 parts of 1000 which fell upon it; at the same
time, a metallic mirror which he tried in the same circumstances,
reflected only 561 of them. At a less angle of incidence much more
light was reflected; so that at an angle of three degrees, the glass
reflected 700 parts, and the metal something less, as in the former
case. The most striking observations made by this experimenter relate
to the very great difference in the quantity of light reflected at
different angles of incidence. He found that for 1000 incident rays,
the reflected rays, at different angles of incidence, were as follows.
Angles of Rays reflected Rays reflected
incidence by water by glass
5° 501 549
10 333 412
15 211 299
30 65 112
50 22 34
70 18 25
90 18 25
With regard to such mirrors as the specula of reflecting telescopes, it
will be found, in general, that they reflect little more than the _one
half_ of the rays which fall upon them.
_Uncommon appearances in nature produced by the combined influences of
Reflection and Refraction._
The reflection and refraction of the rays of light frequently produce
phenomena which astonish the beholders, and which have been regarded
by the ignorant and the superstitious, as the effects of supernatural
agency. Of these phenomena I shall state a few examples.
One of the most striking appearances of this kind is what has been
termed the _Fata Morgana_, or optical appearances of figures in the
sea and the air, as seen in the Faro of Messina. The following account
is translated from a work of Minasi, who witnessed the phenomenon, and
wrote a dissertation on the subject. “When the rising sun shines from
that point whence its incident ray forms an angle of about forty-five
degrees to the sea of Riggio, and the bright surface of the water
in the bay is not disturbed either by the wind or the current, the
spectator being placed on an eminence of the city, with his back to the
sun and his face to the sea;--on a sudden there appear on the water,
as in a catoptric theatre, various multiplied objects, that is to
say, numberless series of pilasters, arches, castles well delineated,
regular columns, lofty towers, superb palaces, with balconies and
windows, extended alleys of trees, delightful plains with herds and
flocks, armies of men on foot and horseback, and many other strange
images, in their natural colours and proper actions, passing rapidly in
succession along the surface of the sea, during the whole of the short
period of time, while the above mentioned causes remain.--But, if in
addition to the circumstances now described, the atmosphere be highly
impregnated with vapour and dense exhalations, not previously dispersed
by the winds or the sun, it then happens that, in this vapour, as in
a curtain extended along the channel, at the height of about thirty
palms, and nearly down to the sea, the observer will behold the scene
of the same objects, not only reflected from the surface of the sea,
but likewise in the air, though not so distant or well defined, as the
former objects from the sea.--Lastly, if the air be slightly hazy or
opake, and at the same time dewy and adapted to form the iris, the then
above-mentioned objects will appear only at the surface of the sea, as
in the first case, but all vividly coloured or fringed with red, green,
blue and other prismatic colours.”[11]
It is somewhat difficult to account for all the appearances here
described; but, in all probability, they are produced by a calm sea,
and one or more strata of superincumbent air differing in refractive
and consequently in reflective power. At any rate reflection and
refraction are some of the essential causes which operate in the
production of the phenomena.
The _Mirage_, seen in the deserts of Africa, is a phenomenon, in all
probability produced by a similar cause. M. Monge, who accompanied
the French army to Egypt, relates that, when in the desert between
Alexandria and Cairo, the mirage of the blue sky was inverted, and
so mingled with the sand below, as to give to the desolate and arid
wilderness an appearance of the most rich and beautiful country. They
saw, in all directions, green islands, surrounded with extensive lakes
of pure, transparent water. Nothing could be conceived more lovely and
picturesque than the landscape. In the tranquil surface of the lakes,
the trees and houses with which the islands were covered, were strongly
reflected with vivid and varied hues, and the party hastened forward to
enjoy the cool refreshments of shade and stream which these populous
villages proffered to them. When they arrived, the lake on whose bosom
they floated, the trees among whose foliage they were embowered, and
the people who stood on the shore inviting their approach, had all
vanished, and nothing remained but an uniform and irksome desert of
sand and sky, with a few naked huts and ragged Arabs. Had they not been
undeceived by their nearer approach, there was not a man in the French
army who would not have sworn that the visionary trees and lakes had a
real existence in the midst of the desert.
Dr. Clark observed precisely the same appearances at Rosetta. The city
seemed surrounded with a beautiful sheet of water; and so certain was
his Greek interpreter--who was unacquainted with the country--of this
fact, that he was quite indignant at an Arab who attempted to explain
to him that it was a mere optical delusion. At length they reached
Rosetta in about two hours, without meeting with any water; and on
looking back on the sand they had just crossed, it seemed to them as if
they had waded through a vast blue lake.
[Illustration: _figure 29._]
On the 1st of August, 1798, Dr. Vince observed at Ramsgate a ship which
appeared as at A, (fig. 29.) the topmast being the only part
of it that was seen above the horizon. An inverted image of it was seen
at B, immediately above the real ship A, and an erect
image at C, both of them being complete and well defined.
The sea was distinctly seen between them, as at V W. As the
ship rose to the horizon the image C gradually disappeared,
and while this was going on, the image B descended, but the
mainmast of B did not meet the mainmast of A. The
two images BC were perfectly visible when the whole ship was
actually below the horizon. Dr. Vince then directed his telescope to
another ship whose hull was just in the horizon, and he observed a
complete inverted image of it, the mainmast of which just touched the
mainmast of the ship itself. He saw at the same time several other
ships whose images appeared in nearly a similar manner, in one of
which the two images were visible when the whole ship was beneath the
horizon. These phenomena must have been produced by the same causes
which operated in the case formerly mentioned, in relation to Captain
Scoresby, when he saw the figure of his father’s ship inverted in the
distant horizon. Such cases are, perhaps not uncommon, especially in
calm and sultry weather, but they are seldom observed, except when a
person’s attention is accidentally directed to the phenomenon, and,
unless he use a telescope, it will not be so distinctly perceived.
The following phenomenon, of a description nearly related to the above,
has been supposed to be chiefly owing to _reflection_. On the 18th
of November, 1804, Dr. Buchan, when watching the rising sun, about
a mile to the east of Brighton, just as the solar disk emerged from
the surface of the water, saw the face of the cliff on which he was
standing, a windmill, his own figure and the figure of his friend,
distinctly represented, precisely opposite, at some distance from the
ocean. This appearance lasted about ten minutes, till the sun had
risen nearly his own diameter above the sea. The whole then seemed to
be elevated into the air and successively disappeared. The surface of
the sun was covered with a dense fog of many yards in height, which
gradually receded from the rays of the sun as he ascended from the
horizon.
The following appearance most probably arose chiefly from the
_refraction_ of the atmosphere. It was beheld at Ramsgate, by Dr.
Vince of Cambridge and another gentleman. It is well known that
the four turrets of Dover castle are seen at Ramsgate, over a hill
which intervenes between a full prospect of the whole. On the 2nd of
August, 1806, not only were the four turrets visible, but the castle
itself appeared as though situated on that side of the hill nearest
Ramsgate, and so striking was the appearance, that for a long time the
Doctor thought it an illusion; but at last, by accurate observation,
was convinced that it was an actual image of the castle. He, with
another individual, observed it attentively for twenty minutes, but
were prevented by rain from making further observations. Between the
observers and the land from which the hill rises, there were about six
miles of sea, and from thence to the top of the hill there was about
the same distance, their own height above the surface of the water was
about seventy feet.--The cause of this phenomenon was, undoubtedly,
_unequal refraction_. The air being more dense near the ground and
above the sea than at greater heights, reached the eye of the observer,
not in straight but in curvilinear lines. If the rays from the castle
had in their path struck an eye at a much greater distance than
Ramsgate, the probability is, that the image of the castle would have
been inverted in the air; but in the present case,the rays from the
turret and the base of the castle had not crossed each other.
To similar causes as those now alluded to are to be attributed such
phenomena as the following:
_The Spectre of the Brocken._ This is a wonderful and, at first sight,
a terrific phenomenon, which is sometimes seen from the summit of one
of the Hartz mountains in Hanover, which is about 3,300 feet above the
level of the sea, and overlooks all the country fifteen miles round.
From this mountain the most gigantic and terrific spectres have been
seen, which have terrified the credulous, and gratified the curious, in
a very high degree. M. Hawé who witnessed this phenomenon, says, the
sun rose about four o’clock, after he had ascended to the summit, in a
serene sky, free of clouds; and about a quarter past five, when looking
round to see if the sky continued clear, he suddenly beheld at a little
distance, a human figure of _a monstrous size_ turned towards him, and
glaring at him. While gazing on this gigantic spectre, with a mixture
of awe and apprehension, a sudden gust of wind nearly carried off his
hat, and he clapt his hand to his head to detain it, when to his great
delight, the colossal spectre did the same. He changed his body into a
variety of attitudes,all which the spectre exactly imitated, and then
suddenly vanished without any apparent cause, and, in a short time as
suddenly appeared. Being joined by another spectator, after the first
visions had disappeared, they kept steadily looking for the aërial
spectres, when two gigantic monsters suddenly appeared. These spectres
had been long considered as preternatural, by the inhabitants of the
adjacent districts, and the whole country had been filled with awe
and terror. Some of the lakes of Ireland are found to be susceptible
of producing illusions, particularly the lake of _Killarney_. This
romantic sheet of water is bounded on one side, by a semicircle of
rugged mountains, and on the other by a flat morass; and the vapours
generated in the marsh, and broken by the mountains, continually
represent the most fantastic objects. Frequently men riding along
the shore are seen as if they were moving across the lake, which is
supposed to have given rise to the legend of O’Donougho, a magician who
is said to be visible on the lake every May morning.
There can be little doubt that most of those visionary appearances
which have been frequently seen in the sky and in mountainous regions,
are phantoms produced by the cause to which I am adverting, such as
armies of footmen and horsemen, which some have asserted to have been
seen in the air near the horizon. A well authenticated instance of this
kind occurred in the Highlands of Scotland:--Mr. Wren of Wetton Hall,
and D. Stricket his servant, in the year 1744, were sitting at the
door of the house in a summer evening, when they were surprised to see
opposite to them on the side of Sonterfell hill--a place so extremely
steep, that scarce a horse could _walk_ slowly along it--the figure of
a man with a dog pursuing several horses, all running at a most rapid
pace. Onwards they passed till at last they disappeared at the lower
end of the Fell. In expectation of finding the man dashed to pieces by
so tremendous a fall, they went early next morning and made a search,
but no trace of man or horse, or the prints of their feet on the turf
could be found. Sometime afterwards, about seven in the evening, on the
same spot, they beheld a troop of horsemen advancing in close ranks and
at a brisk pace. The inmates of every cottage for a mile round beheld
the wondrous scene, though they had formerly ridiculed the story told
by Mr. Wren and his servant, and were struck with surprise and fear.
The figures were seen for upwards of two hours, till the approach of
darkness rendered them invisible. The various evolutions and changes
through which the troops passed were distinctly visible, and were
marked by all the observers. It is not improbable that these aërial
troopers were produced by the same cause which made the castle of Dover
to appear on the side of the hill next to Ramsgate, and it is supposed
that they were the images of a body of rebels, on the other side of the
hill, exercising themselves previous to the rebellion in 1745.[12]
I shall mention only another instance of this description which lately
occurred in France, and for a time caused a powerful sensation among
all ranks. On Sunday the 17th of December, 1826, the clergy in the
parish of Migné, in the vicinity of Poictiers, were engaged in the
exercises of the Jubilee which preceded the festival of Christmas,
and a number of persons to the amount of 3000 souls assisted in the
service. They had planted as part of the ceremony, a large cross,
twenty-five feet high, and painted red, in the open air beside the
church. While one of the preachers, about five in the evening, was
addressing the multitude, he reminded them of the miraculous cross
which appeared in the sky to Constantine and his army, and the effect
it produced--when suddenly a similar celestial cross appeared in the
heavens just before the porch of the church about 200 feet above
the horizon, and 140 feet in length, and its breadth from three to
four feet, of a bright silver colour tinged with red. The curate
and congregation fixed their wondering gaze upon this extraordinary
phenomenon, and the effect produced on the minds of the assembly
was strong and solemn: they spontaneously threw themselves on their
knees; and many, who had been remiss in their religious duties, humbly
confessed their sins, and made vows of penance and reformation. A
commission was appointed to investigate the truth of this extraordinary
appearance, and a memorial stating the above and other facts was
subscribed by more than forty persons of rank and intelligence, so
that no doubt was entertained as to the reality of the phenomenon.
By many it was considered as strictly miraculous, as having happened
at the time and in the circumstances mentioned. But it is evident,
from what we have already stated, that it may be accounted for on
physical principles. The large cross of wood painted red was doubtless
the real object which produced the magnified image. The state of the
atmosphere, according to the descriptions given in the memorial, must
have been favourable for the production of such images. The spectrum
of the wooden cross must have been cast on the concave surface of
some atmospheric mirror, and so reflected back to the eyes of the
spectators, from an opposite place--retaining exactly the same shape
and proportions, but dilated in size; and what is worthy of attention,
it was tinged with red, the very colour of the object of which it was
the reflected image.
Such phenomena as we have now described, and the causes of them which
science is able to unfold, are worthy of consideration, in order to
divest the mind of superstitious terrors, and enable it clearly to
perceive the laws by which the Almighty directs the movements of the
material system. When any appearance in nature, exactly the reverse
of every thing we could have previously conceived--presents itself
to view, and when we know of no material cause by which it could be
produced, the mind must feel a certain degree of awe and terror, and
will naturally resort to supernatural agency as acting either in
opposition to the established laws of the universe, or beyond the range
to which they are confined. Besides the fears and apprehensions to
which such erroneous conceptions give rise, they tend to convey false
and distorted impressions of the attributes of the Deity and of his
moral government. Science, therefore, performs an invaluable service to
man, by removing the cause of superstitious alarms, by investigating
the laws and principles which operate in the physical system, and by
assigning reasons for those occasional phenomena, which at first sight
appeared beyond the range of the operation of natural causes.
The late ingenious Dr. Wollaston illustrated the causes of some of
the phenomena we have described, in the following manner. He looked
along the side of a red hot poker at a word or object ten or twelve
feet distant; and at a distance less than three eights of an inch from
the line of the poker, an _inverted_ image was seen, and within and
without that image, an _erect_ image, in consequence of the change
produced, by the heat of the poker, in the density of the air. He also
suggested the following experiment as another illustration of the
same principle, namely, viewing an object through a stratum of spirit
of wine lying above water, or a stratum of water laid above one of
syrup. He poured into a _square_ phial a small quantity of clear syrup,
and above this he poured an equal quantity of _water_ which gradually
combined with the syrup, as seen at A. fig. 30. The word ‘Syrup,’ on
a card held behind the bottle, appeared erect when seen through the
pure spirit, but inverted, when seen through the mixture of water and
syrup. He afterwards put nearly the same quantity of rectified spirits
of wine above the water, as seen at B, and he saw the appearance as
represented, namely, the true place of the word ‘Spirit,’ and the
inverted and erect images below. These substances, by their gradual
incorporation, produce refracting power, diminishing from the _spirit
of wine_ to the _water_, or from the _syrup_ to the _water_; so that by
looking through the mixed stratum, an inverted image of any object is
seen behind the bottle. These experiments show that the _mirage_ and
several other atmospherical phenomena may be produced by variations in
the refractive power of different strata of the atmosphere.
[Illustration: _figure 30._]
It is not unlikely that phenomena of a new and different description
from any we have hitherto observed, may be produced from the same
causes to which we have adverted. A certain optical writer remarks--‘If
the variation of the refractive power of the air takes place in a
horizontal line perpendicular to the line of vision, that is, from
right to left, then we may have a _lateral_ Mirage, that is, an image
of a ship may be seen on the right or left hand of the real ship, or
on both, if the variation of refractive power is the same on each side
of the line of vision, and a fact of this kind was once observed on
the Lake of Geneva. If there should happen at the same time, both a
vertical and a lateral variation of refractive power in the air, and
if the variation should be such as to expand or elongate the object in
both directions, then the object would be magnified as if seen through
a telescope, and might be seen and recognized at a distance at which it
would not otherwise have been visible. If the refracting power, on the
contrary, varied, so as to construct the object in both directions, the
image of it would be diminished as if seen through a concave lens.’
_Remarks and Reflections, in reference to the phenomena described
above._
Such, then, are some of the striking and interesting effects
produced by the refraction and the reflection of the rays of light.
As the formation of the _images_ of objects by convex lenses, lays
the foundation of the construction of refracting telescopes and
microscopes, and of all the discoveries they have brought to light, so
the property of _concave specula_, in forming similar images, is that
on which the construction of _Reflecting_ telescopes entirely depends.
To this circumstance Herschel was indebted for the powerful telescopes
he was enabled to construct--which were all formed on the principle of
reflection--and for all the discoveries they enabled him to make in
the planetary system, and in the sidereal heavens. The same principles
which operate in optical instruments, under the agency of man, we have
reason to believe, frequently act on a more expansive scale in various
parts of the system of nature. The magnificent _Cross_ which astonished
the preacher and the immense congregation assembled at Migné, was, in
all probability, formed by a vast atmospherical speculum formed by the
hand of nature, and representing its objects on a scale far superior to
that of human art; and probably, to the same cause is to be attributed
the singular phenomenon of the coast of France having been made to
appear within two or three miles of the town of Hastings, as formerly
described, (see p. 53.) Many other phenomena which we have never
witnessed, and of which we can form no conception, may be produced by
the same cause operating in an infinity of modes.
The facts we have stated above, and the variety of modes by which light
may be refracted and reflected by different substances in nature,
lead us to form some conceptions of the magnificent and diversified
scenes which light may produce in other systems and worlds, under
the arrangements of the all-wise and Beneficent Creator. Light, in
all its modifications and varieties of colour and reflection, may be
considered as the beauty and glory of the universe, and the source
of unnumbered enjoyments to all its inhabitants. It is a symbol of
the Divinity himself; for “GOD IS LIGHT, and in Him is no
darkness at all.” It is a representative of Him who is exhibited in
the Sacred oracles, as “The SUN of Righteousness,” and “the
LIGHT of the world.” It is an emblem of the glories and
felicities of that future world, where knowledge shall be perfected,
and happiness complete; for its inhabitants are designated “the saints
in _light_;” and it is declared in Sacred history, to have been the
first born of created beings. In our lower world, its effects on the
objects which surround us, and its influences upon all sensitive
beings, are multifarious and highly admirable. While passing from
infinitude to infinitude, it reveals the depth and immensity of
the heavens, the glory of the sun, the beauty of the stars, the
arrangements of the planets, the rainbow encompassing the sky with
its glorious circle, the embroidery of flowers, the rich clothing of
the meadows, the valleys standing thick with corn, “the cattle on a
thousand hills,” the rivers rolling through the plains, and the wide
expanse of the ocean. But in other worlds the scenes it creates may
be far more resplendent and magnificent. This may depend upon the
refractive and reflective powers with which the Creator has endowed
the atmospheres of other planets, and the peculiar constitution of the
various objects with which they are connected. It is evident, from
what we already know of the reflection of light, that very slight
modifications of certain physical principles, and very slight additions
to the arrangements of our terrestrial system, might produce scenes of
beauty, magnificence and splendour of which, at present, we can form
no conception. And, it is not unlikely that by such diversities of
arrangement, in other worlds, _an infinite variety_ of natural scenery
is produced throughout the universe.
In the arrangements connected with the planet Saturn, and the immense
rings with which it is encompassed, and in the various positions which
its satellites daily assume with regard to one another, to the planet
itself, and to these rings--there is, in all probability, a combination
of refractions, reflections, light, and shadows, which produce scenes
wonderfully diversified, and surpassing in grandeur what we can now
distinctly conceive. In the remote regions of the heavens, there are
certain bodies composed of immense masses of luminous matter, not yet
formed into any regular system, and which are known by the name of
_Nebulæ_. What should hinder us from supposing that certain exterior
portions of those masses form speculums of enormous size, as some
parts of our atmosphere are sometimes found to do? Such specula may
be conceived to be hundreds and even thousands of miles in diameter,
and that they may form images of the most distant objects in the
heavens, on a scale of immense magnitude and extent, and which may
be reflected, in all their grandeur, to the eyes of intelligences at
a vast distance. And, if the organs of vision of such beings, be far
superior to ours in acuteness and penetrating power, they may thus be
enabled to take a survey of an immense sphere of vision, and to descry
magnificent objects at distances the most remote from the sphere they
occupy. Whatever grounds there may be for such suppositions, it must be
admitted, that all the knowledge we have hitherto acquired respecting
the operation of light, and the splendid effects it is capable of
producing, is small indeed, and limited to a narrow circle, compared
with the immensity of its range, the infinite modifications it may
undergo, and the wondrous scenes it may create in regions of creation
to which human eyes have never yet penetrated,--and which may present
to view objects of brilliancy and magnificence such as, “Eye hath not
yet seen, nor ear heard, nor hath it entered into the heart of man to
conceive.”
CHAPTER V.
SECT. I.--ON THE COLOURS OF LIGHT.
We have hitherto considered light chiefly as a simple homogeneous
substance, as if all its rays were white, and as if they were all
refracted in the same manner by the different lenses on which they
fall. Investigations however, into the nature of this wonderful
fluid, have demonstrated that this is not the case, and that it is
possessed of certain additional properties, of the utmost importance
in the system of nature. Had every ray of light been a pure white, and
incapable of being separated into any other colours, the scene of the
universe would have exhibited a very different aspect from what we
now behold. One uniform hue would have appeared over the whole face
of nature, and one object could scarcely have been distinguished from
another. The different shades of verdure which now diversify every
landscape, the brilliant colouring of the flowery fields, and almost
all the beauties and sublimities which adorn this lower creation would
have been withdrawn. But it is now ascertained that every ray of white
light is composed of an assemblage of colours, whence proceed that
infinite variety of shade and colour with which the whole of our
terrestrial habitation is arrayed. Those colours are found not to be
in the objects themselves, but in the rays of light which fall upon
them, without which they would either be invisible, or wear an uniform
aspect. In reference to this point, Goldsmith has well observed: ‘The
blushing beauties of the rose, the modest blue of the violet, are not
in the flowers themselves, but in the light that adorns them. Odour,
softness, and beauty of figure are their own; but it is light alone
that dresses them up in those robes which shame the monarch’s glory.’
Many strange opinions and hypotheses were entertained respecting
colours, by the ancients, and even by many modern writers, prior to
the time of Sir Isaac Newton. The Pythagoreans called colour the
_superficies_ of bodies; Plato said that it was a flame issuing from
them. According to Zeno it is the first configuration of matter,
and according to Aristotle, it is that which moves bodies actually
transparent. Among the moderns, Des Cartes imagined that the difference
of colour proceeds from the prevalence of the direct or rotatory
motions of the particles of light. Grimaldi, Dechales, and others,
thought the differences of colour depended upon the quick or slow
vibrations of a certain elastic medium filling the whole universe.
Rohault imagined that the different colours were made by the rays of
light entering the eye at different angles with respect to the optic
axis; and Dr. Hook conceived that colour is caused by the sensation
of the oblique or uneven pulse of light; and this being capable of
no more than two varieties, he concluded that there could be no
more than two primary colours. Such were some of the crude opinions
which prevailed before the era of the illustrious Newton, by whose
enlightened investigations the true theory of colours was at last
discovered. In the year 1666 this philosopher began to investigate the
subject; and finding the coloured image of the sun, formed by a glass
prism, to be of an oblong and not of a circular form, as according to
the laws of refraction it ought to be, he was surprised at the great
disproportion between its length and breadth, the former being _five_
times the length of the latter; and he began to conjecture that light
is not _homogeneal_, but that it consists of rays some of which are
much more refrangible than others. Prior to this period, philosophers
supposed that _all_ light, in passing out of one medium into another
of different density was _equally_ refracted in the same or like
circumstances; but Newton discovered that this is not the fact; but
that there are _different species_ of light, and that each species is
disposed both to suffer a different degree of refrangibility in passing
out of one medium into another,--and to excite in us the idea of a
_different colour_ from the rest; and that bodies appear of that colour
which arises from the peculiar rays they are disposed to reflect. It is
now, therefore, universally acknowledged, that the light of the sun,
which to us seems perfectly homogeneal and white, is composed of no
fewer than _seven_ different colours, namely _Red_, _Orange_, _Yellow_,
_Green_, _Blue_, _Indigo and Violet_. A body which appears of a red
colour has the property of reflecting the red rays more powerfully
than any of the others; a body of a green colour reflects the green
rays more copiously than rays of any other colour, and so of the
orange, yellow, blue, purple and violet. A body which is of a _black_
colour, instead of reflecting--_absorbs_ all, or the greater part of
the rays that fall upon it; and, on the contrary, a body that appears
_white_ reflects the greater part of the rays indiscriminately without
separating the one from the other.
Before proceeding to describe the experiments by which the above
results were obtained, it may be proper to give some idea of the form
and effects of the _Prism_ by which such experiments are made. This
instrument is triangular and straight, and generally about three or
four inches long. It is commonly made of white glass, as free as
possible from veins and bubbles, and other similar defects, and is
solid throughout. Its lateral faces, or sides, should be perfectly
plane and of a fine polish. The angle formed by the two faces, one
receiving the ray of light that is refracted in the instrument, and the
other affording it an issue on its returning into the air, is called
the _refracting angle_ of the prism, as ACB, (fig. 31.) The manner in
which Newton performed his experiments, and established the discovery
to which we have alluded, is as follows.
In the window-shutter EG, (fig. 31.) of a dark room, a hole F, was
made, of about one third of an inch diameter, and behind it was
placed a glass prism ACB, so that the beam of light, SF, proceeding
directly from the sun was made to pass through the prism. Before the
interposition of the prism, the beam proceeded in a straight line
towards T, where it formed a round white spot; but being now bent
out of its course by the prism, it formed an oblong image OP, upon
the white pasteboard, or screen LM, containing the seven colours
marked in the figure--the _red_ being the _least_, and the _violet_
the _most_ refracted from the original direction of the solar beam,
ST. This oblong image is called the _prismatic spectrum_. If the
refracting angle of the prism ACB, be 64 degrees, and the distance
of the pasteboard from the prism about 18 feet, the length of the
image OP will be about 10 inches, and the breadth 2 inches. The sides
of the spectrum are right lines distinctly bounded, and the ends are
semicircular. From this circumstance it is evident that it is still
the image of the sun, but elongated by the refractive power of the
prism. It is evident from the figure, that since some part of the
beam, RO, is refracted much further out of its natural course WT, than
some other part of the beam, as WP, the rays towards RO have a much
greater disposition to be refracted than those toward WP; and that this
disposition arises from the naturally different qualities of those
rays, is evident from this consideration, that the refracting angle or
power of the prism is the same in regard to the superior part of the
beam as to the inferior.
[Illustration: _figure 31._]
By making a hole in the screen LM opposite any one of the colours of
the spectrum, so as to allow that colour alone to pass--and by letting
the colour thus separated fall upon a second prism--Newton found that
the light of each of the colours was alike refrangible, because the
second prism could not separate them into an oblong image, or into
any other colour. Hence he called all the seven colours _simple_
or homogeneous, in opposition to _white_ light, which he called
_compound_ or heterogeneous. With the prism which this philosopher
used he found the lengths of the colours and spaces of the spectrum
to be as follows: Red, 45; Orange, 27; Yellow, 40; Green, 60; Blue,
60; Indigo, 48; Violet, 80: or 360 in all. But these spaces vary a
little with prisms formed of different substances, and as they are
not separated by distinct limits, it is difficult to obtain any thing
like an accurate measure of their relative extents. Newton examined
the ratio between the sines of incidence and refraction of these
decompounded rays (see p. 30,) and found that each of the seven primary
colour-making rays, had certain limits within which they were confined.
Thus let the sine of incidence in glass be divided into 50 equal
parts, the sine of refraction into air of the _least_ refrangible,
and the _most_ refrangible rays will contain respectively 77 and 78
such parts. The sines of refraction of all the degrees of _red_ will
have the intermediate degrees of magnitude, from 77 to 77 one-eighth;
_Orange_ from 77 one-eighth to 77 one-fifth; _Yellow_ from 77 one-fifth
to 77 one-third; _Green_ from 77 one-third to 77 one-half; _Blue_
from 77 one-half to 77 two-thirds; _Indigo_ from 77 two-thirds to 77
seven-ninths; and _Violet_ from 77 seven-ninths to 78.
From what has been now stated, it is evident that, in proportion as any
part of an optic glass bears a resemblance to the form of a prism, the
component rays that pass through it must be necessarily separated, and
will consequently paint or tinge the object with colours. The edges of
every convex lens approach to this form, and it is on this account that
the extremities of objects when viewed through them are found to be
tinged with the prismatic colours. In such a glass, therefore, those
different coloured rays will have _different foci_, and will form their
respective images at different distances from the lens. Thus, suppose
LN (fig. 32.) to represent a double convex-lens, and OB an object at
some distance from it. If the object OB was of a pure red colour, the
rays proceeding from it would form a red image at RR; if the
object was of a violet colour, an image of that colour would be formed
at VV, _nearer_ the lens; and if the object was white or any
other combination of the colour-making rays, those rays would have
their respective foci at different distances from the lens, and form a
succession of images, in the order of the prismatic colours, between
the space RR and VV.
[Illustration: _figure 32._]
[Illustration: _figure 33._]
This may be illustrated by experiment in the following manner. Take
a card or slip of white pasteboard, as ABEF, (fig. 33.) and paint
one half ABCD _red_, the other half CF, _violet_ or indigo; and tying
black threads across it, set it near the flame of a candle G, then
take a lens HI, and holding a sheet of white paper behind it, move it
backwards and forwards upon the edge of a graduated ruler, till you
see the black threads most distinctly in the image, and you will find
the focus of the violet FE, much nearer than that of the red
AC, which plainly shows that bodies of different colours can
never be depicted by convex-lenses, without some degree of confusion.
The quantity of dispersion of the coloured rays in convex lenses
depends upon the focal length of the glass; the space which the
coloured images occupy being about the twenty-eighth part. Thus if
the lens be twenty-eight inches focal distance, the space between
RR and VV (fig 32) will be about one inch; if it
be twenty-eight feet focus, the same space will be about one foot,
and so on in proportion. Now, when such a succession of images formed
by the different coloured rays, is viewed through an eye-glass, it
will seem to form but one image, and consequently very indistinct,
and tinged with various colours, and as the red figure RR is
largest, or seen under the greatest angle--the extreme parts of this
confused image will be red, and a succession of the prismatic colours
will be formed within this red fringe, as is generally found in common
refracting-telescopes, constructed with a single object-glass. It is
owing to this circumstance that the common refracting telescope cannot
be much improved without having recourse to lenses of a very long
focal distance; and hence, about 150 years ago, such telescopes were
constructed of 80, and 100, and 120 feet in length. But still the image
was not formed so distinctly as was desired, and the aperture of the
object-glass was obliged to be limited. This is a defect which was
long regarded as without a remedy; and even Newton himself despaired
of discovering any means by which the defects of refracting telescopes
might be removed and their improvement effected. This, however, was
accomplished by Dollond to an extent far surpassing what could have
been expected, of which a particular account will be given in the
sequel.
It was originally remarked by Newton, and the fact has since
been confirmed by the experiments of Sir W. Herschel, that _the
different-coloured rays have not the same illuminating power_. The
violet rays appear to have the least illuminating effect; the indigo
more, and the effect increases in the order of the colours,--the
_green_ being very great; between the green and the yellow the greatest
of all; the yellow the same as the green; but the red less than the
yellow. Herschel also endeavoured to determine whether the power of the
differently-coloured rays to _heat_ bodies, varied with their power
to illuminate them. He introduced a beam of light into a dark room,
which was decomposed by a prism, and then exposed a very sensible
thermometer to all the rays in succession, and observed the heights
to which it rose in a given time. He found that their heating power
increased from the violet to the red. The mercury in the thermometer
rose higher when its bulb was placed in the Indigo than when it was
placed in the violet, still higher in blue, and highest of all at red.
Upon placing the bulb of the thermometer below the red, quite out of
the spectrum, he was surprised to find that the mercury rose highest
of all; and concluded that _rays proceed from the sun, which have the
power of_ HEATING, _but not of illuminating bodies_. These
rays have been called _invisible_ solar rays. They were about half an
inch from the commencement of the red rays; at a greater distance from
this point the heat began to diminish, but was very perceptible even
at the distance of 1-1/2 inch. He determined that the heating power of
the _red_ to that of the _green_ rays, was 2-3/4 to 1, and 3-1/2 to 1,
in red to _violet_. He afterwards made experiments to collect those
invisible calorific rays, and caused them to act independently of the
light, from which he concluded that they are sufficient to account
for all the effects produced by the solar rays in exciting heat; that
they are capable of passing through glass, and of being refracted and
reflected, after they have been finally detached from the solar beam.
M. Ritter of Jena, Wollaston, Beckman and others, have found
that the rays of the spectrum are possessed of certain _chemical
properties_--that beyond the least brilliant extremity, namely, a
little beyond the _violet_ ray, there are _invisible_ rays which
act chemically, while they have neither the power of heating nor
illuminating bodies. Muriate of silver exposed to the action of the red
rays becomes blackish; a greater effect is produced by the yellow: a
still greater by the violet, and the greatest of all by the _invisible_
rays _beyond_ the violet. When phosphorus is exposed to the action
of the invisible rays beyond the red, it emits white fumes; but the
invisible rays beyond the violet extinguish them. The influence of
these rays is daily seen in the change produced upon vegetable colours,
which fade, when frequently exposed to the direct influence of the sum.
What object they are destined to accomplish in the general economy of
nature, is not yet distinctly known; we cannot however doubt that
they are essentially requisite to various processes going forward in
the material system. And we know that, not only the comfort of all
the tribes of the living world, but the very existence of the animal
and vegetable creation depends upon the unremitting agency of the
_Calorific_ rays.
It has likewise been lately discovered that certain rays of the
spectrum, particularly the _violet, possesses the property of
communicating the magnetic power_. Dr. _Morichini_, of Rome, appears to
have been the first who found that the violet rays of the spectrum had
this property. The result of his experiments, however, was involved in
doubt, till it was established by a series of experiments instituted by
Mrs. _Somerville_, whose name is so well known in the scientific world.
This lady having covered half of a sewing-needle, about an inch long,
with paper, she exposed the other half for two hours, to the violet
rays. The needle had then acquired North polarity. The indigo rays
produced nearly the same effect; and the blue and green rays produced
it in a still less degree. In the yellow, orange, red and invisible
rays, no magnetic influence was exhibited, even though the experiment
was continued for three successive days. The same effects were produced
by enclosing the needle in blue or green glass, or wrapping it in blue
and green ribbands one half of the needle being always covered with
paper.
One of the most curious discoveries of modern times, in reference
to the solar spectrum, is that of Fraunhofer of Munich--one of the
most distinguished artists and opticians on the Continent.[13] He
discovered that the spectrum is covered with dark and coloured lines,
parallel to one another, and perpendicular to the length of the
spectrum; and he counted no less than 590 of these lines. In order
to observe these lines, it is necessary to use prisms of the most
perfect construction, of very pure glass, free of veins--to exclude
all extraneous light, and even to stop those rays which form the
coloured spaces, which we are not examining. It is necessary also to
use a magnifying instrument, and the light must enter and emerge from
the prism at equal angles. One of the important practical results of
this discovery is, that those lines are fixed points in the spectrum,
or rather, that they have always the same position in the coloured
spaces in which they are found. Fraunhofer likewise discovered in the
spectrum produced by the light of Venus, the same streaks, as in the
solar spectrum; in the spectrum of the light of Sirius, he perceived
three large streaks which, according to appearance, had no resemblance
to those of the light of the sun; one of them was in the green, two
in the blue. The stars appear to differ from one another in their
streaks. The electric light differs very much from the light of the sun
and that of a lamp, in regard to the streaks of the spectrum--‘This
experiment may also be made, though in an imperfect manner, by viewing
a narrow slit between two nearly closed window-shutters, through a very
excellent glass prism held close to the eye, with the refracting angle
parallel to the line of light. When the spectrum is formed by the sun’s
rays, either direct or indirect, as from the sky, clouds, rainbow,
moon, or planets, the black bands are always found to be in the same
parts of the spectrum, and under all circumstances to maintain the same
relative position, breadth and intensities.’
From what has been stated in reference to the solar spectrum it will
evidently appear, that white light is nothing else than a compound of
all the prismatic colours; and this may be still farther illustrated
by shewing, that the seven primary colours, when again put together,
recompose white light. This may be rudely proved for the purpose of
illustration, by mixing together seven different powders, having the
colours and proportion of the spectrum; but the best mode, on the
whole, is the following. Let two circles be drawn on a smooth round
board, covered with white paper, as in fig. 34: Let the outermost be
divided into 360 equal parts; then draw seven right lines as A,B,C,
&c., from the center to the outermost circle, making the lines A and
B include 80 degrees of that circle. The lines B and C, 40 degrees; C
and D, 60; D and E, 60; E and F, 48; F and G, 27; G and A, 45. Then
between these two circles paint the space AG red, inclining to orange
near G; GF orange, inclining to yellow near F; FE yellow, inclining to
green near E; ED green, inclining to blue near D; DC blue, inclining to
indigo near C; CB indigo, inclining to violet near B; and BA violet,
inclining to a soft red near A. This done, paint all that part of
the board black which lies within the inner circle; and putting an
axis through the centre of the board, let it be turned swiftly round
that axis, so that the rays proceeding from the above colours, may be
all blended and mixed together in coming to the eye. Then the whole
coloured part will appear like a white ring a little grayish--not
perfectly white, because no art can prepare or lay on perfect colours,
in all their delicate shades, as found in the real spectrum.
[Illustration: _figure 34._]
That all the colours of light, when blended together in their proper
proportions, produce a pure _white_ is rendered certain by the
following experiment. Take a large convex glass, and place it in the
room of the paper or screen on which the solar spectrum was depicted
(LM fig. 31), the glass will unite all the rays which come from the
prism, if a paper is placed to receive them, and you will see a
circular spot of a pure lively white. The rays will cross each other in
the focus of the glass, and, if the paper be removed a little further
from that point, you will see the prismatic colours again displayed,
but in an inverted order, owing to the crossing of the rays.
SECT. 2.--ON THE COLOURS OF NATURAL OBJECTS.
From what has been stated above we may learn the true cause of those
diversified hues exhibited by natural and artificial objects, and the
variegated colouring which appears on the face of nature. It is owing
to the surfaces of bodies being disposed to reflect one colour rather
than another. When this disposition is such that the body reflects
every kind of ray, in the mixed state in which it receives them, that
body appears _white_ to us--which, properly speaking, is no colour,
but rather the assemblage of all colours. If the body has a fitness
to reflect one sort of rays more abundantly than others, by absorbing
all the others, it will appear of the colour belonging to that species
of rays. Thus, the grass is _green_, because it absorbs all the rays
except the green. It is these green rays only which the grass, the
trees, the shrubs, and all the other verdant parts of the landscape
reflect to our sight, and which make them appear green. In the same
manner the different flowers reflect their respective colours; the
rose, the red rays; the violet, the blue; the jonquil, the yellow; the
marigold, the orange, and every object, whether natural or artificial,
appears of that colour which its peculiar texture is fitted to reflect.
A great number of bodies are fitted to reflect at once several kinds
of rays, and of consequence they appear under mixed colours. It may
even happen, that of two bodies which should be green, for example,
one may reflect the pure green of light, and the other the mixture of
yellow and blue. This quality, which varies to infinity, occasions the
different kinds of rays to unite in every possible manner, and every
possible proportion; and hence the inexhaustible variety of shades
and hues which nature has diffused over the landscape of the world.
When a body absorbs nearly all the light which reaches it, that body
appears _black_. It transmits to the eye so few reflected rays that
it is scarcely perceptible in itself, and its presence and form make
no impression upon us, unless as it interrupts the brightness of the
surrounding space. Black is, therefore, the absence of all the coloured
rays.
It is evident, then, that all the various assemblages of colours which
we see in the objects around us, _are not in the bodies themselves_,
but in the light which falls upon them. There is no colour _inherent_
in the grass, the trees, the fruits, and the flowers, nor even in the
most splendid and variegated dress that adorns a lady. All such objects
are as destitute of colour, in themselves, as bodies which are placed
in the centre of the earth, or as the chaotic materials out of which
our globe was formed, before light was created. For where there is no
light, there is no colour. Every object is black, or without colour,
in the dark, and it only appears coloured as soon as light renders it
visible. This is further evident from the following experiment. If we
place a coloured body in one of the colours of the spectrum which is
formed by the prism, it appears of the colour of the rays in which it
is placed. Take, for example, a red rose, and expose it first to the
red rays, and it will appear of a more brilliant ruddy hue. Hold it
in the blue rays, and it appears no longer red, but of a dingy blue
colour, and in like manner its colour will appear different, when
placed in all the other differently coloured rays. This is the reason
why the colours of objects are essentially altered by the nature of
the light in which they are seen. The colours of ribbons and various
pieces of silk or woollen stuff are not the same when viewed by
candle-light as in the day time. In the light of a candle or a lamp,
blue often appears green, and yellow objects assume a whitish aspect.
The reason is that the light of a candle is not so pure a white as that
of the sun, but has a yellowish tinge, and therefore, when refracted
by the prism, the yellowish rays are found to predominate, and the
superabundance of yellow rays gives to blue objects a greenish hue.
The doctrine we are now illustrating is one which a great many persons,
especially among the fair sex, find it difficult to admit. They cannot
conceive it possible that there is no colour really inherent in their
splendid attire, and no tints of beauty in their countenances. ‘What,’
said a certain lady, ‘are there no colours in my shawl, and in the
ribbons that adorn my head-dress--and, are we all as black as negroes
in the dark; I should almost shudder to think of it.’ Such persons,
however, need be in no alarm at the idea; but may console themselves
with the reflection, that, when they are stripped of all their coloured
ornaments in the dark, they are certain that _they will never be seen
by any one_ in that state; and therefore, there is no reason to regret
the temporary loss of those beauties which light creates--when they
themselves and all surrounding objects are _invisible_. But, to give
a still more palpable proof of this position, the following popular
experiments may be stated.
Take a pint of common spirit, and pour it into a soup dish, and then
set it on fire; as it begins to blaze, throw a handful of salt into the
burning spirit, and keep stirring it with a spoon. Several handfuls
may thus be successively thrown in, and then the spectators, standing
around the flame, will see each other frightfully changed, their
colours being altered into a ghastly blackness, in consequence of the
nature of the light which falls upon them--which produces colours very
different from those of the solar light. The following experiment,
as described by Sir D. Brewster, illustrates the same principle.
‘Having obtained the means of illuminating any apartment with _yellow_
light, let the exhibition be made in a room with furniture of various
bright colours, and with oil or water coloured paintings on the wall.
The party which is to witness the experiment should be dressed in a
diversity of the gayest colours; and the brightest coloured flowers,
and highly coloured drawings should be placed on the tables. The room
being at first lighted with ordinary lights, the bright and gay colours
of every thing that it contains will be finely displayed. If the white
lights are now suddenly extinguished, and the yellow lamps lighted,
the most appalling metamorphosis will be exhibited. The astonished
individuals will no longer be able to recognise each other. All the
furniture of the room, and all the objects it contains, will exhibit
only _one_ colour. The flowers will lose their hues; the paintings
and drawings will appear as if they were executed in China ink, and
the gayest dresses, the brightest scarlets, the purest lilacs, the
richest blues and the most vivid greens, will all be converted into one
monotonous yellow. The complexions of the parties, too, will suffer a
corresponding change. One pallid deathlike yellow, will envelope the
young and the old, and the _sallow_ face will alone escape from the
metamorphosis. Each individual derives merriment from the cadaverous
appearance of his neighbour, without being sensible that he is one of
the ghastly assemblage.’
----Like the unnatural hue
Which autumn paints upon the perished leaf,
From such experiments as these we might conclude, that were the solar
rays of a very different description from what they are now found to
be, the colours which embellish the face of nature, and the whole scene
of our sublunary creation would assume a new aspect, and appear very
different from what we now behold around us in every landscape. We find
that the stars display great diversity of colour; which is doubtless
owing to the different kinds of light which are emitted from those
bodies; and hence we may conclude, that the colouring thrown upon the
various objects of the universe is different in every different system,
and that thus, along with other arrangements, an infinite variety of
colouring and of scenery is distributed throughout the immensity of
creation.
The _atmosphere_, in consequence of its different refractive and
reflective powers, is the source of a variety of colours which
frequently embellish and diversify the aspect of our sky. The
air _reflects_ the blue rays most plentifully, and must therefore
_transmit_ the red, orange, and yellow, more copiously than the other
rays. When the sun and other heavenly bodies are at a high elevation,
their light is transmitted without any perceptible change, but when
they are near the horizon, their light must pass through a long and
dense track of air, and must therefore be considerably modified before
it reach the eye of the observer. The momentum of the red rays being
greater than that of the violet, will force their way through the
resisting medium, while the violet rays will be either reflected or
absorbed. If the light of the setting sun, by thus passing through
a long track of air, be divested of the green, blue, indigo, and
violet rays, the remaining rays which are transmitted through the
atmosphere, will illuminate the western clouds, first with an orange
colour; and then, as the sun gradually sinks into the horizon, the
track through which the rays must pass becoming longer, the yellow
and orange are reflected, and the clouds grow more deeply _red_, till
at length the disappearance of the sun leaves them of a leaden hue by
the reflection of the blue light through the air. Similar changes of
colour are sometimes seen on the eastern and western fronts of white
buildings. St. Paul’s Church, in London, is frequently seen at sun-set,
tinged with a very considerable degree of redness; and the same cause
occasions the moon to assume a ruddy colour, by the light transmitted
through the atmosphere. From such atmospherical refractions and
reflections are produced those rich and beautiful hues with which our
sky is gilded by the setting sun, and the glowing red which tinges the
morning and evening clouds, till their ruddy glare is tempered by the
purple of twilight, and the reflected azure of the sky.
When a direct spectrum is thrown on colours darker than itself, it
mixes with them: as the yellow spectrum of the setting sun, thrown on
the green grass, becomes a greener yellow. But when a direct spectrum
is thrown on colours brighter than itself, it becomes instantly changed
into the reverse spectrum, which mixes with those brighter colours.
Thus the yellow spectrum of the setting sun thrown on the luminous
sky, becomes blue, and changes with the colour or brightness of the
clouds on which it appears. The red part of light being capable of
struggling through thick and resisting mediums which intercept all
other colours--is likewise the cause why the sun appears red when seen
through a fog,--why distant light, though transmitted through blue
or green glass, appears red--why lamps at a distance, seen through
the smoke of a long street, are red, while those that are near, are
white. To the same cause it is owing that a diver at the bottom of the
sea is surrounded with the red light which has pierced through the
superincumbent fluid, and that the blue rays are reflected from the
_surface_ of the ocean. Hence, Dr. Halley informs us that, when he was
in a diving bell, at the bottom of the sea, his hand always appeared
red in the water.
The _blue_ rays, as already noticed, being unable to resist the
obstructions they meet with in their course through the atmosphere, are
either reflected or absorbed in their passage. It is to this cause,
that most philosophers ascribe _the blue colour of the sky_, the
faintness and obscurity of distant objects, and the bright azure which
tinges the mountains of a distant landscape.
SECT. 3.--PHENOMENA OF THE RAINBOW.
Since the rays of light are found to be decomposed by refracting
surfaces, and reflected in an infinite variety of modes and shades
of colour, we need not be surprised at the changes produced in any
scene or object by the intervention of another, and by the numerous
modifications of which the primary colours of nature are susceptible.
The vivid colours which gild the rising and the setting sun, must
necessarily differ from those which adorn its noon-day splendour.
Variety of atmospheric scenery will thus necessarily be produced,
greater than the most lively fancy can well imagine. The clouds will
sometimes assume the most fantastic forms, and at other times will be
irradiated with beams of light, or, covered with the darkest hues, will
assume a lowering aspect, prognostive of the thunder’s roar and the
lightning’s flash--all in accordance with the different rays that are
reflected to our eyes, or the quantity absorbed by the vapours which
float in the atmosphere.
Light, which embellishes with so much magnificence a pure and serene
sky, by means of innumerable bright starry orbs which are spread over
it, sometimes, in a dark and cloudy sky, exhibits an ornament which,
by its pomp, splendour and variety of colours, attracts the attention
of every eye that has an opportunity of beholding it. At certain
times, when there is a shower either around us, or at a distance from
us in an opposite quarter to that of the sun, a species of arch or
bow is seen in the sky, adorned with all the seven primary colours of
light. This phenomenon, which is one of the most beautiful meteors in
nature, has obtained the name of the RAINBOW. The rainbow was,
for ages, considered as an inexplicable mystery, and by some nations
it was adored as a deity. Even after the dawn of true philosophy,
it was a considerable time before any discovery of importance was
made, as to the true causes which operate in the production of this
phenomenon. About the year 1571, M. Fletcher of Breslau, made a certain
approximation to the discovery of the true cause, by endeavouring to
account for the colours of the rainbow by means of a double refraction
and one reflection. A nearer approximation was made by Antonio de
Dominis, bishop of Spalatro, about 1601. He maintained that the
double refraction of Fletcher, _with an intervening reflection_,
was sufficient to produce the colours of the bow, and also to bring
the rays that formed them to the eye of the spectator, without any
subsequent reflection. To verify this hypothesis, he procured a small
globe of solid glass, and viewing it when it was exposed to the rays
of the sun--with his back to that luminary--in the same manner as he
had supposed the drops of rain were situated with respect to them, he
observed the same colours which he had seen in the rainbow, and in
the same order. But he could give no good reason _why_ the bow should
be coloured, and much less any satisfactory account of the _order_ in
which the colours appear. It was not till Sir I. Newton discovered
the different refrangibility of the rays of light, that a complete
and satisfactory explanation could be given of all the circumstances
connected with this phenomenon.
As the full elucidation of this subject involves a variety of optical
and mathematical investigations, I shall do little more than explain
the general principle on which the prominent phenomena of the rainbow
may be accounted for, and some of the facts and results which theory
and observation have deduced.
We have just now alluded to an experiment with a glass globe:--If,
then, we take either a solid glass globe, or a hollow globe filled with
water, and suspend it so high in the solar rays above the eye, that
the spectator, with his back to the sun, can see the globe _red_;--if
it be lowered slowly, he will see it orange, then yellow, then green,
then blue, then indigo, and then violet; so that the drop at different
heights, shall present to the eye the seven primitive colours in
succession. In this case, the globe, from its form, will act in some
measure like a prism, and the ray will be separated into its component
parts. The following figure will more particularly illustrate this
point. Suppose A (fig. 35.) to represent a drop of rain--which may be
considered as a globe of glass in miniature, and will produce the same
effect on the rays of light--and let SD represent a ray from
the sun falling upon the upper part of the drop at D. At the
point of entering the drop, it will suffer a refraction, and instead
of going forward to C, it will be bent to N. From
N a part of the light will be reflected to Q--some
part of it will, of course, pass through the drop. By the obliquity
with which it falls on the side of the drop at Q, that part
becomes a kind of prism, and separates the ray into its primitive
colours. It is found by computation that, after a ray has suffered
two refractions and one reflection, as here represented, the least
refrangible part of it, namely the _red_ ray, will make an angle with
the incident solar ray of 42° 2´, as SFQ; and the violet, or
greatest refrangible ray will make with the solar ray, an angle of 40°
17´, as SCQ; and thus all the particles of water within the
difference of those two angles, namely 1° 45´--(supposing the ray to
proceed merely from the centre of the sun)--will exhibit severally the
colours of the prism, and constitute the _interior_ bow of the cloud.
This holds good at whatever height the sun may chance to be in a shower
of rain. If he be at a high altitude, the rainbow will be low; if he be
at a low elevation, the rainbow must be high; and if a shower happen
in a vale, when the spectator is on a mountain, he will sometimes see
the bow in the form of a _complete circle_ below him. We have at
present described the phenomena only of a single drop; but it is to be
considered that in a shower of rain there are drops at all heights and
at all distances; and therefore the eye situated at G will see all the
different colours. All those drops that are in a certain position with
respect to the spectator will reflect the red rays, all those in the
next station the orange, those in the next the green, and so on with
regard to all the other colours.
[Illustration: _figure 35._]
It appears, then, that the first or primary bow is formed by two
refractions and one reflection; but there is frequently a second bow,
on the outside of the other, which is considerably fainter. This is
produced by drops of rain above the drop we have supposed at A. If B
(fig. 35.) represent one of these drops, the ray to be sent to the eye
enters the drop near the bottom, and suffers _two refractions_ and _two
reflections_, by which means the colours become reversed, that is, the
violet is lowest in the _exterior_ bow, and the red is lowest in the
_interior_ one, and the other colours are reversed accordingly. The ray
T is refracted at R: a part of it is reflected from S
to T, and at T it suffers another reflection from
T to U. At the points S and T part
of the ray _passes through_ the drop on account of its transparency,
towards W and X, and therefore we say that _part_
only of the ray is reflected. By these losses and reflections the
exterior bow becomes faint and ill-defined in comparison of the
interior or primary bow. In this case the upper part of the secondary
bow will not be seen when the sun is above 54° 10´ above the horizon;
and the lower part of the bow will not be seen when the sun is 60° 58´
above the horizon.
[Illustration: _figure 36._]
For the further illustrations of this subject, we may introduce the
following section of a bow, (fig. 36.) and, in order to prevent
confusion in attempting to represent all the different colours--let
us suppose only three drops of rain, and three different colours, as
shown in the figure. The spectator O being in the centre of the two
bows, here represented,--the planes of which must be considered as
perpendicular to his view--the drops A,B, and C produce part of the
interior bow by two refractions and one reflection as stated above,
and the drops D,E,F will produce the exterior bow by two refractions
and two reflections, the sun’s rays being represented by 3,3. It is
evident that the angle COP is less than the angle BOP, and that the
angle AOP is the greatest of the three. The largest angle, then, is
formed by the red rays, the middle one consists of the green, and the
smallest the purple or violet. All the drops of rain, therefore, that
happen to be in a certain position with respect to the spectator’s eye,
will reflect the red rays, and form a band or semicircle of red, and so
of the other colours from drops in other positions. If the spectator
alters his station, he will see a bow, but not the same as before; and
if there be many spectators, they will each see a different bow, though
it appears to be the same.
The rainbow assumes a _semicircular_ appearance, because it is only at
certain angles that the refracted rays are visible to our eyes, as is
evident from the experiment of the glass globe formerly alluded to,
which will refract the rays only in a certain position. We have already
stated that the red rays make an angle of 42° 2´, and the violet
an angle of 40° 17´. Now, if a line be drawn horizontally from the
spectator’s eye, it is evident that angles formed with this line, of a
certain dimension, in every direction, will produce a circle, as will
appear by attaching a cord of a given length to a certain point, round
which it may turn as round its axis; and, in every point will describe
an angle with the horizontal line of a certain and determinate extent.
Sometimes it happens that _three_ or more bows are visible, though
with different degrees of distinctness. I have more than once observed
this phenomenon, particularly in Edinburgh, in the month of August,
1825, when three rainbows were distinctly seen in the same quarter
of the sky; and, if I recollect right, a fragment of a fourth made
its appearance. This happens when the rays suffer a third or fourth
reflection; but, on account of the light lost by so many reflections,
such bows are, for the most part, altogether imperceptible.
If there were no ground to intercept the rain and the view of the
observer, the rainbow would form a _complete circle_, the centre
of which is diametrically opposite to the sun. Such circles are
sometimes seen in the spray of the sea or of a cascade, or from the
tops of lofty mountains, when the showers happen in the vales below.
Rainbows of various descriptions are frequently observed rising amidst
the spray and exhalations of waterfalls, and among the waves of the
sea whose tops are blown by the wind into small drops. There is one
regularly seen, when the sun is shining, and the spectator in a proper
position, at the fall of Staubbach, in the bosom of the Alps; one near
Schaffhausen; one at the cascade of Lauffen; and one at the cataract of
Niagara in North America. A still more beautiful one is said to be seen
at Terni, where the whole current of the river Velino, rushing from
a steep precipice of nearly 200 feet high, presents to the spectator
below, a variegated circle, over-arching the fall, and two other bows
suddenly reflected on the right and left. Don Ulloa, in the account
of his journeys in South America, relates that circular rainbows are
frequently seen on the mountains above Quito in Peru. It is said that
a rainbow was once seen near London, caused by the exhalations of
that city, after the sun had been below the horizon more than twenty
minutes.[14] A naval friend, says Mr. Bucke, informed me, that, as he
was one day watching the sun’s effect upon the exhalations near Juan
Fernandez, he saw upwards of five-and-twenty _ires marinæ_ animate
the sea at the same time. In these marine bows the concave sides were
turned upwards, the drops of water rising from below, and not falling
from above, as in the instances of the aerial arches. Rainbows are also
occasionally seen on the grass, in the morning dew, and likewise when
the hoar-frost is descending. Dr. Langwith once saw a bow lying on the
ground, the colours of which were almost as lively as those of a common
rainbow. It was not round but oblong, and was extended several hundred
yards. The colours took up less space, and were much more lively in
those parts of the bow which were near him than in those which were
at a distance. When M. Labillardiere was on Mount Teneriffe, he saw
the contours of his body traced on the clouds beneath him in all the
colours of the solar bow. He had previously witnessed this phenomenon
on the Kesrouan in Asia Minor. The rainbows of Greenland are said to
be frequently of a pale white, fringed with a brownish yellow, arising
from the rays of the sun being reflected from a frozen cloud.
The following is a summary view of the principal facts which have been
ascertained respecting the rainbow:--1. The rainbow can only be seen
when it rains, and in that point of the heavens which is opposite to
the sun. 2. Both the primary and secondary bows are variegated with all
the prismatic colours--the red being the highest colour in the primary,
or brightest bow, and the violet the highest in the exterior. 3. The
primary rainbow can never be a greater arc than a semicircle; and
when the sun is set, no bow, in ordinary circumstances, can be seen.
4. The breadth of the inner or primary bow--supposing the sun but a
point--is 1° 45´; and the breadth of the exterior bow 3° 12´, which is
nearly twice as great as that of the other; and the distance between
the bows is 8° 55´. But since the body of the sun subtends an angle of
about half a degree, by so much will each bow be increased, and their
distance diminished; and therefore the breadth of the interior bow
will be 2° 15´, and that of the exterior, 3° 42´, and their distance
8° 25´. The greatest semidiameter of the interior bow, on the same
grounds, will be 42° 17´, and the least of the exterior bow 50° 43´.
5. When the sun is in the horizon, either in the morning or evening,
the bows will appear complete semicircles. On the other hand, when the
sun’s altitude is equal to 42° 2´ or to 54° 10´, the summits of the
bows will be depressed below the horizon. Hence, during the days of
summer, within a certain interval each day, no visible rainbows can be
formed, on account of the sun’s high altitude above the horizon. 6.
The altitude of the bows above the horizon, or surface of the earth,
varies, according to the elevation of the sun. The altitude, at any
time, may be taken by a common quadrant, or other angular instrument;
but, if the sun’s altitude at any particular time be known, the height
of the summit of any of the bows may be found, by subtracting the sun’s
altitude from 42° 2´ for the inner bow, and from 54° 10´, for the
outer. Thus, if the sun’s altitude were 26°, the height of the primary
bow would 16° 2´, and of the secondary, 28° 10´. It follows, that the
height and the size of the bows diminish as the altitude of the sun
increases. 7. If the sun’s altitude is more than 42 degrees, and less
than 54°, the exterior bow may be seen though the interior bow is
invisible. 8. Sometimes only a portion of an arch will be visible while
all the other parts of the bow are invisible. This happens when the
rain does not occupy a space of sufficient extent to complete the bow;
and the appearance of this position, and even of the bow itself, will
be various, according to the nature of the situation, and the space
occupied by the rain.
The appearance of the rainbow may be produced by artificial means, at
any time when the sun is shining and not too highly elevated above the
horizon. This is effected by means of artificial fountains or _Jet
d’eaus_, which are intended to throw up streams of water to a great
height. These streams, when they spread very wide, and blend together
in their upper parts, form, when falling, a shower of artificial rain.
If, then, when the fountain is playing, we move between it and the sun,
at a proper distance from the fountain, till our shadow point directly
towards it, and look at the shower,--we shall observe the colours of
the rainbow, strong and vivid; and, what is particularly worthy of
notice, the bow appears, notwithstanding the nearness of the shower, to
be as large, and as far off, as the rainbow which we see in a natural
shower of rain. The same experiment may be made by candle-light, and
with any instrument that will form an artificial shower.
_Lunar Rainbows._--A lunar bow is sometimes formed at night by the rays
of the moon striking on a rain-cloud, especially when she is about
the full. But such a phenomenon is very rare. Aristotle is said to
have considered himself the first who had seen a lunar rainbow. For
more than a hundred years prior to the middle of the last century, we
find only two or three instances recorded in which such phenomena are
described with accuracy. In the philosophical transactions for 1783,
however, we have an account of three having been seen in one year,
and all in the same place, but they are by no means common phenomena.
I have had an opportunity within the last twenty years of witnessing
two phenomena of this description--one of which was seen at Perth, on
a sabbath evening, in the autumn of 1825, and the other at Edinburgh,
on Wednesday, the 9th of September 1840, about eight o’clock in the
evening--of both which I gave a detailed description in some of the
public journals. The Moon, in both cases, was within a day or two of
the full; the arches were seen in the northern quarter of the heavens,
and extended nearly from east to west, the moon being not far from the
southern meridian. The bows appeared distinct and well defined, but no
distinct traces of the prismatic colours could be perceived on any of
them. That which appeared in 1825 was the most distinctly formed, and
continued visible for more than an hour. The other was much fainter,
and lasted little more than half an hour, dark clouds having obscured
the face of the moon. These bows bore a certain resemblance to some
of the luminous arches which sometimes accompany the Aurora Borealis,
and this latter phenomenon has not unfrequently been mistaken for a
Lunar rainbow; but they may be always distinguished by attending to the
phases and position of the moon. If the moon be not visible above the
horizon, if she be in her first or last quarter, or if any observed
phenomenon be not in a direction opposite to the moon, we may conclude
with certainty that, whatever appearance is presented, there is no
lunar rainbow.
The rainbow is an object which has engaged universal attention, and
its beautiful colours and form have excited universal admiration. The
poets have embellished their writings with many beautiful allusions
to this splendid meteor; and the playful school-boy, while viewing
the ‘bright enchantment,’ has frequently run ‘to catch the falling
glory.’ When its arch rests on the opposite sides of a narrow valley,
or on the summits of two adjacent mountains, its appearance is both
beautiful and grand. In all probability, its figure first suggested the
idea of _arches_, which are now found of so much utility in forming
aqueducts and bridges, and for adorning the architecture of palaces and
temples. It is scarcely possible seriously to contemplate this splendid
phenomenon, without feeling admiration and gratitude towards that wise
and beneficent Being, whose hands have bent it into so graceful and
majestic a form, and decked it with all the pride of colours. “Look
upon the rainbow,” says the son of Sirach,[15] and praise Him that
made it: very beautiful it is in the brightness thereof. It compasseth
the heaven about with a glorious circle, and the hands of the Most
High have bended it." To this grand etherial bow, the inspired writers
frequently allude as one of the emblems of the majesty and splendour
of the Almighty. In the prophecies of Ezekiel, the throne of Deity is
represented as adorned with a brightness “like the appearance of the
bow that is in the cloud in the day of rain--the appearance of the
likeness of the glory of Jehovah.” And, in the visions recorded in the
Book of the Revelations, where the Most High is represented as sitting
upon a throne; “there was a rainbow round about the throne, in sight
like unto an emerald,” as an emblem of his propitious character and of
his faithfulness and mercy. After the deluge, this bow was appointed as
a sign and memorial of the covenant which God made with Noah and his
sons, that a flood of waters should never again be permitted to deluge
the earth and its inhabitants;--and as a pledge of inviolable fidelity
and Divine benignity. When, therefore, we at any time behold “the bow
in the cloud,” we have not only a beautiful and sublime phenomenon
presented to the eye of sense, but also a memorial exhibited to the
mental eye, assuring us, that, “While the earth remaineth, seed-time
and harvest, and cold and heat, and summer and winter, and day and
night, _shall not cease_.”[16]
----On the broad sky is seen
“A dewy cloud, and in the cloud a bow
Conspicuous, with seven listed colours gay
Betokening peace with God and covenant new.--
He gives a promise never to destroy
The earth again by flood, nor let the sea
Surpass his bounds, nor rain to drown the world.”
_Milton. Par. Lost, Book XI._
SECT. 4.--REFLECTIONS ON THE BEAUTY AND UTILITY OF COLOURS.
Colour is one of the properties of light which constitutes, chiefly,
the beauty and sublimity of the universe. It is colour, in all its
diversified shades, which presents to our view that almost infinite
variety of aspect which appears on the scene of nature, which gives
delight to the eye and the imagination, and which adds a fresh pleasure
to every new landscape we behold. Every flower which decks our fields
and gardens is compounded of different hues; every plain is covered
with shrubs and trees of different degrees of verdure; and almost every
mountain is clothed with herbs and grass of different shade from those
which appear on the hills and landscape with which it is surrounded.
In the country, during summer, nature is every day, and almost every
hour, varying her appearance, by the multitude and variety of her hues
and decorations, so that the eye wanders with pleasure over objects
continually diversified, and extending as far as the sight can reach.
In the flowers with which every landscape is adorned, what a lovely
assemblage of colours, and what a wonderful art in the disposition of
their shades! Here, a light pencil seems to have laid on the delicate
tints; there, they are blended according to the nicest rules of art.
Although green is the general colour which prevails over the scene
of sublunary nature, yet it is diversified by a thousand different
shades, so that every species of tree, shrub and herb, is clothed
with its own peculiar verdure. The dark green of the forests is thus
easily distinguished from the lighter shades of cornfields and the
verdure of the lawns. The system of animated nature likewise, displays
a diversified assemblage of beautiful colours. The plumage of birds,
the brilliant feathers of the peacock, the ruby and emerald hues which
adorn the little humming-bird, and the various embellishments of many
species of the insect tribe, present to the eye, in every region of
the globe, a scene of diversified beauty and embellishment. Nor is
the mineral kingdom destitute of such embellishments. For some of the
darkest and most unshapely stones and pebbles, when polished by the
hand of art, display a mixture of the most delicate and variegated
colours. All which beauties and varieties in the scene around us are
entirely owing to that property, in every ray of light, by which it is
capable of being separated into the primitive colours.
To the same cause, likewise, are to be ascribed those beautiful and
diversified appearances, which frequently adorn the face of the
sky,--the yellow, orange and ruby hues which embellish the firmament
at the rising of the sun, and when he is about to descend below the
western horizon; and those aerial landscapes, so frequently beheld in
tropical climes, where rivers, castles and mountains, are depicted
rolling over each other along the circle of the horizon. The clouds,
especially in some countries, reflect almost every colour in nature.
Sometimes they wear the modest blush of the rose; sometimes they appear
like stripes of deep vermillion, and sometimes as large brilliant
masses tinged with various hues; now they are white as ivory, and now
as yellow as native gold. In some tropical countries, according to St.
Pierre, the clouds roll themselves up into enormous masses as white as
snow, and are piled upon each other, like the Cordeliers of Peru, and
are moulded into the shape of mountains, of caverns and of rocks. When
the sun sets behind this magnificent aërial net-work, a multitude of
luminous rays are transmitted through each particular interstice, which
produce such an effect, that the two sides of the lozenge illuminated
by them, have the appearance of being begirt with a fillet of gold;
and the other two which are in the shade, seem tinged with a superb
ruddy orange. Four or five divergent streams of light, emanating from
the setting sun up to the zenith, clothe with fringes of gold the
undeterminate summits of this celestial barrier, and proceed to strike
with the reflexes of their fires the pyramids of the collateral aerial
mountains, which then appear to consist of silver and vermilion.--In
short, colour diversifies every sublunary scene, whether on the earth
or in the atmosphere, it imparts a beauty to the phenomena of falling
stars, of luminous arches, and the coruscations of the Aurora Borealis,
and gives a splendour and sublimity to the spacious vault of heaven.
Let us now consider for a moment, what would be the aspect of nature,
if, instead of the beautiful variety of embellishments which now
appear on every landscape, and on the concave of the sky,--_one_
uniform colour had been thrown over the scenery of the universe. Let
us conceive the whole of terrestrial nature to be covered with snow,
so that not an object on earth should appear with any other hue, and
that the vast expanse of the firmament presented precisely the same
uniform aspect. What would be the consequence? The light of the sun
would be strongly reflected from all the objects within the bounds of
our horizon, and would produce a lustre which would dazzle every eye.
The day would acquire a greater _brightness_ than it now exhibits,
and our eyes might, after some time, be enabled freely to expatiate
over the surrounding landscape; but every thing, though enlightened,
would appear _confused_, and particular objects would scarcely be
distinguishable. A tree, a house or a church, near at hand, might
possibly be distinguished, on account of its elevation above the
general surface of the ground, and the bed of a river by reason of its
being depressed below it. But we should be obliged rather to guess,
and to form a conjecture as to the particular object we wished to
distinguish, than to arrive at any certain conclusion respecting it;
and if it lay at a considerable distance, it would be impossible, with
any degree of probability, to discriminate any one object from another.
Notwithstanding the universal brightness of the scene, the uniformity
of colour thrown on every object, would most certainly prevent us from
distinguishing a church from a palace, a cottage from a knoll or a
heap of rubbish, a splendid mansion from rugged rocks, the trees from
the hills on which they grow, or a barren desert from rich and fertile
plains. In such a case, human beings would be confounded, and even
friends and neighbours be at a loss to recognize one another.
The vault of heaven, too, would wear a uniform aspect. Neither planets
nor comets would be visible to any eye, nor those millions of stars
which now shine forth with so much brilliancy, and diversify the
nocturnal sky. For, it is by the contrast produced by the deep azure
of the heavens and the white radiance of the stars, that those bodies
are rendered visible. Were they depicted on a pure white ground, they
would not be distinguished from that ground, and would consequently be
invisible, unless any of them occasionally assumed a different colour.
Of course, all that beautiful variety of aspect which now appears on
the face of sublunary nature--the rich verdure of the fields, the
stately port of the forest, the rivers meandering through the valleys,
the splendid hues that diversify and adorn our gardens and meadows, the
gay colouring of the morning and evening clouds, and all that variety
which distinguishes the different seasons, would entirely disappear. As
every landscape would exhibit nearly the same aspect, there would be no
inducement to the poet and the philosopher to visit distant countries
to investigate the scenes of nature, and journeyings from one region
to another would scarcely be productive of enjoyment. Were any other
single colour to prevail, nearly the same results would ensue. Were a
deep ruddy hue to be uniformly spread over the scene of creation, it
would not only be offensive to the eye, but would likewise prevent all
distinction of objects. Were a dark blue or a deep violet to prevail,
it would produce a similar effect, and at the same time, present the
scene of nature as covered with a dismal gloom. Even if creation were
arrayed in a robe of _green_, which is a more pleasant colour to the
eye--were it not diversified with the different shades it now exhibits,
every object would be equally undistinguishable.
Such would have been the aspect of creation, and the inconveniences
to which we should have been subjected, had the Creator afforded us
light without that intermixture of colours which now appears over all
nature, and which serves to discriminate one object from another. Even
our very apartments would have been tame and insipid, incapable of
the least degree of ornament, and the articles with which they are
furnished, almost undistinguishable, so that in discriminating one
object from another, we should have been as much indebted to the sense
of touch as to the sense of vision. Our friends and fellow men would
have presented no objects of interest in our daily associations. The
sparkling eye, the benignant smile, the modest blush, the blended
hues of white and vermillion in the human face, and the beauty of
the female countenance, would all have vanished, and we should have
appeared to one another as so many moving marble statues cast nearly
in the same mould. But, what would have been worst of all, the
numerous delays, uncertainties and perplexities to which we should
have been subjected, had we been under the necessity, every moment, of
distinguishing objects by trains of reasoning, and by circumstances
of time, place, and relative position? An artist, when commencing his
work in the morning, with a hundred tools of nearly the same size and
shape around him, would have spent a considerable portion of his time
before he could have selected those proper for his purpose, or the
objects to which they were to be applied; and in every department of
society, and in all our excursions from one place to another, similar
difficulties and perplexities would have occurred. The one half of our
time must thus have been employed in uncertain guesses, and perplexing
reasonings, respecting the real nature and individuality of objects,
rather than in a regular train of thinking and of employment; and after
all our perplexities and conjectures, we must have remained in the
utmost uncertainty, as to the thousands of scenes and objects, which
are now obvious to us, through the instrumentality of colours, as soon
as we open our eyes.
In short, without colour, we could have had no books nor writings:
we could neither have corresponded with our friends by letters, nor
have known any thing with certainty, of the events which happened in
former ages. No written revelation of the will of God, and of his
character, such as we now enjoy, could have been handed down to us
from remote periods and generations. The discoveries of science, and
the improvements of art, would have remained unrecorded. Universal
ignorance would have prevailed throughout the world, and the human mind
have remained in a state of demoralization and debasement. All these,
and many other inconveniences and evils would have inevitably followed,
had not God painted the rays of light with a diversity of colours,
And hence we may learn, that the most important scenes and events in
the universe, may depend upon the existence of a single principle in
nature, and even upon the most minute circumstances, which we may be
apt to overlook, in the arrangements of the material world.
In the existing state of things in the visible creation, we cannot but
admire the Wisdom and Beneficence of the Deity, in thus enabling us
to distinguish objects by so easy and expeditious a mode as _that of
colour_, which in a moment, discriminates every object and its several
relations. We rise in the morning to our respective employments,
and our food, our drink, our tools, our books, and whatever is
requisite for our comfort, are at once discriminated. Without the
least hesitation or uncertainty, and without any perplexing process
of reasoning, we can lay our hands on whatever articles we require.
Colour clothes every object with its peculiar livery, and infallibly
directs the hand in its movements, and the eye in its surveys and
contemplations. But, this is not the only end which the Divine
Being had in view, in impressing on the rays of light a diversity
of colours. It is evident, that he likewise intended to minister to
our _pleasures_, as well as to our wants. To every man of taste, and
almost to every human being, the combination of colours in flowers, the
delicate tints with which they are painted, the diversified shades of
green with which the hills and dales, the mountains and the vales are
arrayed; and that beautiful variety which appears in a bright summer
day, on all the objects of this lower creation--are sources of the
purest enjoyment and delight. It is colour, too, as well as magnitude,
that adds to the _sublimity_ of objects. Were the canopy of heaven of
one uniform hue, it would fail in producing those lofty conceptions,
and those delightful and transporting emotions, which a contemplation
of its august scenery is calculated to inspire. Colours are likewise of
considerable utility in the intercourse of general society. They serve
both for ornaments, and for distinguishing the different ranks and
conditions of the community: they add to the beauty and gracefulness
of our furniture and clothing. At a glance, they enable us at once to
distinguish the noble from the ignoble, the prince from his subjects,
the master from his servant, and the widow clothed with sable weeds
from the bride adorned with her nuptial ornaments.
Since colours, then, are of so much value and importance, they may
be reckoned as holding a rank among the noblest natural gifts of the
Creator. As they are of such essential service to the inhabitants of
our globe, there can be no doubt that they serve similar or analogous
purposes throughout all the worlds in the universe. The colours
displayed in the solar beams are common to all the globes which
compose the planetary system, and must necessarily be reflected, in
all their diversified hues, from objects on their surfaces. The light
which radiates from the fixed stars displays a similar diversity of
colours. Some of the double stars are found to emit light of different
hues;--the larger star exhibiting light of a ruddy or orange hue,
and the smaller one a radiance which approaches to blue or green.
There is therefore reason to conclude, that the objects connected
with the planets which revolve round such stars--being occasionally
enlightened by suns of different hues--will display a more variegated
and splendid scenery of colouring than is ever beheld in the world on
which we dwell; and that one of the distinguishing characteristics of
different worlds, in regard to their embellishments, may consist in
the splendour and variety of colours with which the objects on their
surfaces are adorned. In the metaphorical description of the glories
of the New Jerusalem, recorded in the Book of Revelation, one of the
chief characteristics of that city is said to consist in the splendour
and diversity of hues with which it is adorned. It is represented
as “coming down from heaven, _prepared as a bride adorned for her
husband_,” and as reflecting all the beautiful and variegated colours
which the finest gems on earth can exhibit; evidently indicating, that
splendour and variety of colouring are some of the grandest features of
celestial scenery.
On the whole, the subject of colours, when seriously considered,
is calculated to excite us to the adoration of the goodness and
intelligence of that Almighty Being whose wisdom planned all the
arrangements of the universe, and to inspire us with gratitude for the
numerous conveniences and pleasures we derive from those properties and
laws he has impressed on the material system. He might have afforded
us light, and even splendid illumination, without the pleasures
and advantages which diversified colours now produce, and man and
other animated beings might have existed in such a state. But, what
a very different scene would the world have presented from what it
now exhibits! Of how many thousands of pleasures should we have been
deprived! and to what numerous inconveniences and perplexities should
we have been subjected! The sublimity and glories of the firmament, and
the endless beauties and varieties which now embellish our terrestrial
system, would have been for ever unknown, and man could have had little
or no incitement to study and investigate the works of his Creator.
In this, as well as in many other arrangements in nature, we have a
sensible proof of the presence and agency of that Almighty Intelligence
“in whom we live, and move, and have our being.” None but an infinitely
Wise and Beneficent Being, intimately present in all places, could
thus so regularly create in us by means of colour, those exquisite
sensations which afford so much delight, and which unite us, as it
were, with every thing around us. In the diversity of hues spread
over the face of creation, we have as real a display of the Divine
presence as Moses enjoyed at the burning bush. The only difference
is, that the one was out of the common order of Divine procedure, and
the other in accordance with those permanent laws which regulate the
economy of the universe. In every colour, then, which we contemplate,
we have a sensible memorial of the presence of that Being “whose
Spirit garnished the heavens and laid the foundations of the earth,”
and whose “merciful visitation” sustains us every moment in existence.
But the revelation of God to our senses, through the various objects
of the material world, has become so familiar, that we are apt to
forget the Author of all our enjoyments, even at the moment when we
are investigating his works and participating of his benefits. “O that
men would praise Jehovah for his goodness, and for his wonderful works
towards the children of men.”
PART II.
ON TELESCOPES.
CHAPTER I.
HISTORY OF THE INVENTION OF TELESCOPES.
The telescope is an optical instrument for viewing objects at a
distance. Its name is compounded of two Greek words,--τηλε, which
signifies, _at a distance_, or _far off_, and σχοπειν, _to view_, or to
_contemplate_. By means of telescopes, remote objects are represented
as if they were near, small apparent magnitudes are enlarged, confused
objects are rendered distinct, and the invisible and obscure parts of
very distant scenes are rendered perceptible and clear to the organ
of vision. The telescope is justly considered as a grand and noble
instrument. It is not a little surprising that it should be in the
power of man to invent and construct an instrument by which objects,
too remote for the unassisted eye to distinguish, should be brought
within the range of distinct vision, as if they were only a few yards
from our eye, and that thousands of august objects in the heavens,
which had been concealed from mortals for numerous ages, should be
brought within the limits of our contemplation, and be as distinctly
perceived, as if we had been transported many millions of miles from
the space we occupy, through the celestial regions. The celebrated
Huygens remarks, in reference to this instrument, that, in his opinion,
‘the wit and industry of man has not produced any thing so noble and
so worthy of his faculties as this sort of knowledge; (namely of the
telescope) insomuch that if any particular person had been so diligent
and sagacious as to invent this instrument from the principles of
nature and geometry,--for my part, I should have thought his abilities
were more than human; but the case is so far from this, that the most
learned men have not yet been able sufficiently to explain the reason
of the effects of this casual invention.’
The persons who constructed the first telescopes, and the exact
period when they were first invented, are involved in some degree of
obscurity. It does not certainly appear that such instruments were
known to the ancients, although we ought not to be perfectly decisive
on this point. The cabinets of the curious contain some very ancient
gems, of admirable workmanship, the figures on which are so small,
that they appear beautiful through a magnifying glass, but altogether
confused and indistinct to the naked eye: and, therefore, it may
be asked, if they cannot be _viewed_, how could they _be wrought_,
without the assistance of glasses? And as some of the ancients have
declared that the moon has a form like that of the earth, and has
plains, hills, and valleys in it,--how could they know this--unless by
mere conjecture, without the use of a telescope? And how could they
have known that the _Milky Way_ is formed by the combined rays of an
infinite number of stars? For Ovid states, in reference to this zone,
‘its ground-work is of stars.’ But whatever knowledge the ancients may
have possessed of the telescope or other optical glasses, it is quite
evident that they never had telescopes of such size and power as those
which we now possess; and that no discoveries in the heavens, such as
are now brought to light, were made by any of the ancient astronomers;
otherwise some allusions to them must have been found in their writings.
Among the moderns, the illustrious Friar Bacon appears to have acquired
some rude ideas respecting the construction of telescopes. ‘Lenses and
specula’ says he, ‘may be so figured that one object may be multiplied
into many, that those which are situated at a great distance may be
made to appear very near, that those which are small may be made to
appear very large, and those which are obscure very plain; and we can
make stars to appear wherever we will.’ From these expressions, it
appears highly probable, that this philosopher was acquainted with
the general principle both of telescopes and microscopes, and that he
may have constructed telescopes of small magnifying power, for his
own observation and amusement, although they never came into general
use. He was a man of extensive learning, and made so rapid a progress
in the sciences, when attending the university of Paris, that he
was esteemed the glory of that seat of learning. He prosecuted his
favourite study of experimental philosophy with unremitting ardour; and
in this pursuit, in the course of twenty years, he expended no less
than £2000 in experiments, instruments, and in procuring scarce books.
In consequence of such extraordinary talents, and such astonishing
progress in the sciences, in that ignorant age, he was represented,
by the envy of his illiterate fraternity, as having dealings with
the devil; and, under this pretence, he was restrained from reading
lectures, and at length, in 1278, when sixty-four years of age, he was
imprisoned in his cell, where he remained in confinement for ten years.
He shone like a single bright star in a dark hemisphere--the glory of
our country--and died at Oxford, in the year 1294, in the eightieth
year of his age. ‘Friar Bacon,’ says the Rev. Mr. Jones, ‘may be
considered as the first of English philosophers; his profound skill in
mechanics, optics, astronomy, and chemistry, would make an honourable
figure in the present age. But he is entitled to further praise, as
he made all his studies subservient to theology, and directed all his
writings, as much as could be, to the glory of God. He had the highest
regard for the sacred scriptures, and was persuaded they contain the
principles of all true science.’
The next person who is supposed to have acquired a knowledge of
telescopes, was Joannes Baptista Porta, of Naples, who flourished
in the sixteenth century. He discovered the _Camera Obscura_--the
knowledge of which might naturally have led to the invention of the
telescope; but it does not appear that he ever constructed such an
instrument. Des Cartes considers James Metius, a Dutchman, as the
first constructor of a telescope, and says, that ‘as he was amusing
himself with making mirrors and burning-glasses, he casually thought
of looking through two of his lenses at a time, and found that distant
objects appeared very large and distinct.’ Others say that this great
discovery was first made by John Lippersheim, a maker of spectacles at
Middleburg, or rather by his children, who were diverting themselves
with looking through two glasses at a time, and placing them at
different distances from each other. But Borellus, who wrote a book ‘on
the invention of the telescope,’ gives this honour to Zacharias Jansen,
another spectacle-maker in the same town, who, he says, made the
first telescope in 1590. Jansen was a diligent inquirer into nature,
and, being engaged in such pursuits, he was trying what use could be
made of lenses for those purposes, when he fortunately hit upon the
construction. Having found the arrangement of glasses which produced
the effect desired, he enclosed them in a tube, and ran with his
instrument to prince Maurice, who, immediately conceiving that it might
be of use to him in his wars, desired the author to keep it a secret.
Such are the rude conceptions and selfish views of princely _warriors_,
who would apply every invention in their power for the destruction
of mankind. But the telescope was soon destined to more noble and
honourable achievements. Jansen, it is said, directed his instrument
towards celestial objects, and distinctly saw the spots on the surface
of the moon, and discovered many new stars, particularly seven pretty
considerable ones in the Great Bear. His son Joannes is said to have
noted the lucid circle near the lower limb of the moon, now named
_Tycho_, from whence several bright rays seem to dart in different
directions. In viewing Jupiter, he perceived two, sometimes three, and
at the most four small stars, a little above or below him, and thought
that they performed revolutions around him. This was, probably, the
first observation of the satellites of Jupiter, though the person who
made it was not aware of the importance of his discovery.[17]
It is not improbable that different persons about Middleburgh hit upon
the invention, in different modes, about the same time. Lippersheim
seems to have made his first rude telescope by adjusting two glasses
on a board, and supporting them on brass circles.[18] Other workmen,
particularly Metius and Jansen, in emulation of each other, seem to
have made use of that discovery, and by the new form they gave it, made
all the honour of it their own. One of them, considering the effects
of light as injurious to distinctness, placed the glasses in a tube
blackened within. The other, still more cautious, placed the same
glasses within tubes capable of sliding one in another, both to vary
the prospects, by lengthening the instrument, according to the pleasure
of the observer, and to render it portable and commodious. Thus, it
is probable that different persons had a share in the invention, and
jointly contributed to its improvement. At any rate, it is undoubtedly
to the Dutch that we owe the original invention. The first telescope
made by Jansen, did not exceed fifteen or sixteen inches in length, and
therefore its magnifying power could not have been very great.
The famous Galileo has frequently been supposed to have been the
inventor of the telescope, but he acknowledges that he had not the
honour of being the original inventor, having first learned from
a German, that such an instrument had already been made; although,
from his own account, it appears that he had actually re-invented
this instrument. The following is the account, in his own words, of
the circumstances which led him to construct a telescope. ‘Nearly
ten months ago (namely in April or May 1609) it was reported that a
certain Dutchman had made a perspective through which many distant
objects appeared distinct as if they were near: several effects of
this wonderful instrument were reported, which some believed and
others denied: but, having it confirmed to me a few days after by
a letter from the noble John Badoverie, at Paris, I applied myself
to consider the reason of it, and by what means I might contrive a
similar instrument, which I afterwards attained to by the doctrine of
refractions. And, first, I prepared a leaden tube, to whose extremities
I fitted two spectacle-glasses, both of them plain on one side, and
on the other side, one of them was spherically convex, and the other
concave. Then applying my eye to the concave, I saw objects appear
pretty large and pretty near me. They appeared three times nearer and
nine times larger in surface than to the naked eye: and soon after I
made another, which represented objects about sixty times larger, and
eight times nearer; and, at last, having spared no labour nor expense,
I made an instrument so excellent, as to show things almost a thousand
times larger, and above thirty times nearer, than to the naked eye.’
In another part of his writings, Galileo informs us that ‘he was at
Venice when he heard of Prince Maurice’s instrument, but nothing of
its construction; that the first night, after he returned to Padua,
he solved the problem, and made his instrument the next day; and soon
after, presented it to the Doge at Venice, who, to do him honour for
his grand invention, gave him the ducal letters which settled him for
life in his lectureship at Padua; and the Republic, on the twenty-fifth
of August in the same year (1610) more than tripled his salary as
professor.’
The following is the account which this philosopher gives of the
process of reasoning, which led him to the construction of a
telescope:--‘I argued in the following manner. The contrivance consists
either of one glass or more--one is not sufficient, since it must
be either convex, concave, or plane; the last does not produce any
sensible alteration in objects, the concave diminishes them; it is true
that the convex magnifies, but it renders them confused and indistinct;
consequently one glass is insufficient to produce the desired effect.
Proceeding to consider two glasses, and bearing in mind that the plane
glass causes no change, I determined that the instrument could not
consist of the combination of a plane glass with either of the other
two. I therefore applied myself to make experiments on combinations of
the two other kinds; and thus obtained that of which I was in search.’
If the true inventor is the person who makes the discovery by reasoning
and reflection, by tracing facts and principles to their consequences,
and by applying his invention to important purposes, then, Galileo may
be considered as the real inventor of the telescope. No sooner had he
constructed this instrument--before he had seen any similar one--than
he directed his tube to the celestial regions, and his unwearied
diligence and ardour were soon rewarded by a series of new and splendid
discoveries. He descried the four satellites of Jupiter, and marked the
periods of their revolutions; he discovered the phases of Venus, and
thus was enabled to adduce a new proof of the Copernican system, and
to remove an objection that had been brought against it. He traced on
the lunar orb, a resemblance to the structure of the earth, and plainly
perceived the outlines of mountains and vales, casting their shadows
over different parts of its surface. He observed, that when Mars was
in quadrature, his figure varied slightly from a perfect circle;
and that Saturn consisted of a triple body, having a small globe on
each side--which deception was owing to the imperfect power of his
telescope, which was insufficient to show him that the phenomenon was
in reality a ring. In viewing the sun, he discovered large dark spots
on the surface of that luminary, by which he ascertained that that
mighty orb performed a revolution round its axis. He brought to view
multitudes of stars imperceptible to the naked eye, and ascertained
that those nebulous appearances in the heavens which constitute the
Milky Way, consist of a vast collection of minute stars, too closely
compacted together to produce an impression on our unassisted vision.
The results of Galileo’s observations were given to the world in
a small work, entitled ‘_Nuncius Sidereus_,’ or, ‘News from the
starry regions,’ which produced an extraordinary sensation among
the learned. These discoveries soon spread throughout Europe, and
were incessantly talked of, and were the cause of much speculation
and debate among the circles of philosophers. Many doubted; many
positively refused to believe so novel and unlooked-for announcements,
because they ran counter to the philosophy of Aristotle, and all the
preconceived notions which then prevailed in the learned world. It
is curious, and may be instructive, to consider to what a length of
absurdity, ignorance and prejudice carried many of those who made
pretensions to learning and science. Some tried to reason against
the facts alleged to be discovered, others contented themselves, and
endeavoured to satisfy others, with the simple _assertion_ that such
things were not, and could not possibly be; and the manner in which
they supported themselves in their incredulity was truly ridiculous.
‘O my dear Kepler,’ says Galileo in a letter to that astronomer,
‘how I wish we could have one hearty laugh together. Here at Padua
is the principal professor of philosophy, whom I have repeatedly and
urgently requested to look at the moon and planets through my glass,
which _he pertinaciously refuses to do_, lest his opinions should be
overturned. Why are you not here? what shouts of laughter we should
have at this glorious folly! and to hear the professor of philosophy at
Pisa labouring with the Grand Duke with logical arguments, as if with
magical incantations, to charm the new planets out of the sky.’ Another
opponent of Galileo, one Christmann, says in a book he published, ‘We
are not to think that Jupiter has four satellites given him by nature,
in order, by revolving round him, to immortalize the Medici who first
had notice of the observation. These are the _dreams of idle men_,
who love ludicrous ideas better than our laborious and industrious
correction of the heavens. Nature abhors so horrible a chaos; and
to the truly wise, such vanity is detestable.’ One Martin Horky, a
would-be philosopher, declared to Kepler, ‘I will never concede his
four new planets to that Italian from Padua, _though I should die for
it_;’ and he followed up this declaration, by publishing a book against
Galileo, in which he examines four principal questions respecting the
alleged planets; 1. Whether they exist? 2. What they are? 3. What they
are like? 4. Why they are? The first question is soon disposed of by
declaring positively that he has examined the heavens with Galileo’s
own glass, and that no such thing as a satellite about Jupiter exists.
To the second, he declares solemnly that he does not more surely
know, that he has a soul in his body than that reflected rays are
the sole cause of Galileo’s erroneous observations. In regard to the
third question, he says, that these planets are like the smallest fly
compared to an elephant; and finally, concludes on the fourth, that
the only use of them is to gratify Galileo’s ‘thirst of gold,’ and to
afford himself a subject of discussion. Kepler, in a letter to Galileo,
when alluding to Horky, says, ‘He begged so hard to be forgiven, that I
have taken him again into favour upon this preliminary condition--that
I am to show him Jupiter’s satellites, AND HE IS TO SEE THEM,
and own that they are there.’
The following is a specimen of the reasoning of certain pretended
philosophers of that age against the discoveries of Galileo. Sizzi, a
Florentine astronomer, reasons in this strain: ‘There are seven windows
given to animals in the domicile of the head, through which the air
is admitted to the rest of the tabernacle of the body to enlighten,
to warm and to nourish it; two nostrils, two eyes, two ears, and a
mouth; so in the heavens, or the great world, there are two favourable
stars, two unpropitious, two luminaries, and Mercury alone undecided
and indifferent. From which and many other similar phenomena in nature,
such as the seven metals, &c., we gather that the number of planets
is _necessarily seven_. Moreover, the satellites are invisible to
the naked eye, and therefore can exert no influence on the earth, and
therefore would be useless, _and therefore do not exist_. Besides, as
well the Jews as other ancient nations have adopted the division of
the week into seven days, and have named them from the seven planets.
Now, if we increase the number of the planets, this whole system
falls to the ground.’ The opinions which then prevailed in regard to
Galileo’s observations on the moon, were such as the following:--Some
thought that the dark shades on the moon’s surface arose from the
interposition of opaque bodies floating between her and the sun, which
prevent his light from reaching those parts; others imagined that, on
account of her vicinity to the earth, she was partly tainted with the
imperfections of our terrestrial and elementary nature, and was not
of that entirely pure and refined substance of which the more remote
heavens consist: and a third party looked on her as a vast mirror, and
maintained that the dark parts of her surface were the reflected images
of our earthly forests and mountains.
Such learned nonsense is a disgrace to our species, and to the
rational faculties with which man is endowed, and exhibits, in a most
ludicrous manner, the imbecility and prejudice of those who made bold
pretensions to erudition and philosophy. The statement of such facts,
however, may be instructive, if they tend to guard us against those
prejudices and pre-conceived opinions, which prevent the mind from the
cordial reception of truth, and from the admission of improvements in
society which run counter to long-established customs. For the same
principles and prejudices, though in a different form, still operate
in society and retard the improvement of the social state, the march
of science, and the progress of Christianity. How ridiculous is it for
a man, calling himself a philosopher, to be afraid to look through a
glass to an existing object in the heavens, lest it should endanger
his previous opinions! And how foolish is it to resist any improvement
or reformation in society, because it does not exactly accord with
existing opinions, and with ‘the wisdom of our ancestors.’
It is not a little surprising, that Galileo should have first hit on
that construction of a telescope which goes by his name, and which
was formed with a _concave_ glass next the eye. This construction of
a telescope is more difficult to be understood, in theory, than one
which is composed solely of convex glasses; and its field of view is
comparatively very small, so that it is almost useless when attempted
to be made of a great length. In the present day, we cannot help
wondering that Galileo and other astronomers, should have made such
discoveries as they did with such an instrument, the use of which must
have required a great degree of patience and address. Galileo’s best
telescope, which he constructed ‘with great trouble and expense,’
magnified the diameters of objects only thirty-three times; but its
length is not stated--which would depend upon the focal distance of the
concave eye-glass. If the eye-glass was two inches focus, the length
of the instrument would be five feet four inches; if it was only one
inch, the length would be two feet eight inches, which is the least we
can allow to it--the object-glass being thirty-three inches focus, and
the eye-glass placed an inch within this focus. With this telescope,
Galileo discovered the satellites of Jupiter, the crescent of Venus,
and the other celestial objects to which we have already alluded.
The telescopes made in Holland, are supposed to have been constructed
solely of _convex_ glasses, on the principle of the astronomical
telescope; and, if so, Galileo’s telescope was in reality a new
invention.
Certain other claimants of the invention of the telescope, have
appeared, besides those already mentioned. Francis Fontana, in his
‘celestial observations,’ says, that he was assured by a Mr. Hardy,
advocate of the parliament of Paris, a person of great learning and
undoubted integrity, that on the death of his father, there was found
among his things an old tube, by which distant objects were distinctly
seen, and that it was of a date long prior to the telescope lately
invented, and had been kept by him as a secret. Mr. Leonard Digges,
a gentleman who lived near Bristol, in the seventeenth century, and
was possessed of great and various knowledge, positively asserts in
his ‘_Stratoticos_,’ and in another work, that his father, a military
gentleman, had an instrument which he used in the field, by which he
could bring distant objects near, and could know a man at the distance
of three miles. Mr. Thomas Digges, in the preface to his ‘Pantometria,’
published in 1591, declares, “My father, by his continual painful
practices, assisted by demonstrations mathematical, was able, and
sundry times hath by proportional glasses, duly situate in convenient
angles, not only discovered things far off, read letters, numbered
pieces of money, with the very coin and superscription thereof, cast by
some of his friends of purpose, upon downs in open fields, but also,
seven miles off, declared what hath been done that instant, in private
places. He hath also, sundry times, by the sun-beams, fired powder
and discharged ordnance half a mile and more distant, and many other
matters far more strange and rare, of which there are yet living divers
witnesses.”
It is by no means unlikely, that persons accustomed to reflection,
and imbued with a certain degree of curiosity, when handling
spectacle-glasses, and amusing themselves with their magnifying powers
and other properties, might sometimes hit upon the construction
of a telescope; as it only requires two lenses of different focal
distances to be held at a certain distance from each other, in order
to show distant objects magnified. Nay, even one lens, of a long
focal distance, is sufficient to constitute a telescope of a moderate
magnifying power, as I shall show in the sequel. But such instruments,
when they happened to be constructed accidentally, appear to have been
kept as secrets, and confined to the cabinets of the curious, so that
they never came into general use; and as their magnifying power would
probably be comparatively small, the appearance of the heavenly bodies
would not be much enlarged by such instruments--nor is it likely that
they would be often directed to the heavens. On the whole, therefore,
we may conclude that the period when instruments of this description
came into general use, and were applied to useful purposes, was when
Galileo constructed his first telescopes.