THE WONDERS OF OPTICS.
------------------------------------------------------------------------
[Illustration:
Spectrum showing the absorptive power of Sodium vapour (Fig. 6).
]
[Illustration:
Solar Spectrum (Fig. 5).
]
[Illustration:
Action of a prism on a ray of light (Fig. 7).
Eng.^d by A. Robin N.Y.
]
------------------------------------------------------------------------
THE
WONDERS OF OPTICS.
BY
F. MARION.
TRANSLATED FROM THE FRENCH, AND EDITED BY
CHARLES W. QUIN, F.C.S.
ILLUSTRATED WITH SEVENTY ENGRAVINGS ON WOOD, AND A COLOURED
FRONTISPIECE.
--------------
NEW YORK:
CHARLES SCRIBNER’S SONS,
SUCCESSORS TO
SCRIBNER, ARMSTRONG, & CO.
------------------------------------------------------------------------
PREFACE.
--------------
THE present work needs but little introduction to the English public.
The author, M. F. Marion, who holds a high official scientific position
in Paris, is well known, especially in Europe, as a popular writer on
the “Wonders of Optics,” and kindred subjects. As a rule, the original
text has been strictly adhered to by the Translator, but in a few
instances certain anecdotes of a local character have been altered so as
to be more generally applicable, or condensed to make room for the
chapter on the Spectroscope, which is entirely original.
------------------------------------------------------------------------
CONTENTS.
--------------
PART I.
THE PHENOMENA OF VISION.
CHAPTER I.
PAGE
THE EYE 15
CHAPTER II.
THE STRUCTURE OF THE EYE 22
CHAPTER III.
THE ERRORS OF THE EYE 30
CHAPTER IV.
OPTICAL ILLUSIONS 36
CHAPTER V.
THE APPRECIATION OF COLOUR 44
CHAPTER VI.
ILLUSIONS CAUSED BY LIGHT ITSELF 53
CHAPTER VII.
THE INFLUENCE OF THE IMAGINATION 60
--------------
PART II.
THE LAWS OF LIGHT.
CHAPTER I.
WHAT IS LIGHT? 73
CHAPTER II.
THE SOLAR SPECTRUM 84
CHAPTER III.
OTHER CAUSES OF COLOUR 94
CHAPTER IV.
LUMINOUS, CALORIFIC, CHEMICAL, AND 100
MAGNETIC PROPERTIES OF THE SPECTRUM
CHAPTER V.
THE LAWS OF REFLECTION.—MIRRORS 106
CHAPTER VI.
METALLIC BURNING MIRRORS 117
CHAPTER VII.
LENSES 127
CHAPTER VIII.
OPTICAL INSTRUMENTS.—THE SIMPLE AND 141
COMPOUND MICROSCOPE. THE SOLAR AND
PHOTO-ELECTRIC MICROSCOPE
CHAPTER IX.
THE TELESCOPES OF GALILEO, GREGORY, 150
NEWTON, HERSCHEL, LORD ROSSE, AND
FOUCAULT
--------------
PART III.
NATURAL MAGIC.
CHAPTER I.
THE MAGIC LANTERN 173
CHAPTER II.
THE PHANTASMAGORIA 183
CHAPTER III.
OTHER OPTICAL ILLUSIONS 196
CHAPTER IV.
THE PROPERTIES OF MIRRORS 216
CHAPTER V.
CHINESE SHADOWS 223
CHAPTER VI.
POLYORAMA—DISSOLVING VIEWS—DIORAMA 231
CHAPTER VII.
THE STEREOSCOPE 236
CHAPTER VIII.
THE CAMERA OBSCURA AND CAMERA LUCIDA 242
CHAPTER IX.
THE SPECTROSCOPE 249
CHAPTER X.
SPECTRES—THE GHOST ILLUSION 264
------------------------------------------------------------------------
LIST OF ILLUSTRATIONS.
--------------
FIG. PAGE
1. Section of the Eye 24
2. A Camera Obscura 27
3. The Phenakistiscope 54
4. Disc of the Phenakistiscope 55
5. Solar Spectrum Frontispiece
6. Absorption of Light by Sodium Vapour _ib._
7. Action of a Prism on the Solar Rays _ib._
8. The Recomposition of Light 86
9. Recomposition of Light by means of a 87
Concave Mirror
10. Recomposition of Light by means of a 88
number of Mirrors
11. Newton’s Disc 89
12. Newton’s Rings 95
13. Reflection from Plane Surfaces 107
14. Refraction 108
15. Experimental Proof of Refraction _ib._
16. The Effects of Plane Mirrors 109
17. Reflection from the Surface of Water 110
18. Concave Mirror 111
19. Conjugate Foci 113
20. Virtual Focus 114
21. Concave Mirror _ib._
22. Magnifying Effect of Concave Mirrors 115
23. The Reversal of real Images _ib._
24. Diminishing Power of Convex Mirrors 116
25. Burning Mirror 124
26. Double Convex Lens 127
27. Forms of Lenses 128
28. Path of a Ray through a Convex Lens 129
29. Path of Divergent Rays through a Convex _ib._
Lens
30. Conjugate Foci 130
31. Images formed by Convex Lenses 131
32. Magnifying Property of Convex Lenses 132
33. Diminishing Effect of Concave Lenses _ib._
34. Cannon of the Palais Royal 134
35. Fresnel’s Lighthouse Apparatus 136
36. Lantern of a First-Class Lighthouse 140
37. The Compound Microscope 143
38. The Theory of the Compound Microscope 144
39. Photo-Electric Microscope 147
40. Solar Microscope 148
41. The Galilean Telescope 155
42. The Astronomical Telescope 156
43. Section of an Astronomical Telescope 157
44. Section of the Gregorian Telescope 160
45. Gregorian Telescope 161
46. Section of a Newtonian Telescope 162
47. Herschellian Telescope 164
48. Foucault’s Large Telescope 169
49. Foucault’s Small Telescope 171
50. Section of the Magic Lantern 179
51. Magic Lantern 182
52. The Phantasmagoria 184
53. The Phantascope 185
54. Phantasmagoria (ROBERTSON) 194
55. Wizard Dance 198
56. Nostradamus and Marie de Médicis 201
57. The Arrangement of the Reversing Prism 203
58. The Goat Trick 205
59. How to see through a Brick 207
60. The Polemoscope 210
61. Protection against ill-natured People 213
62. 218
63. Anamorphosis 220
64. Effect of Cut Paper-work 225
65. Seditious Toys 229
66. Diorama 234
67. 237
68. Stereoscope 238
69. The Principle of the Refracting 239
Stereoscope
70. The Camera Obscura 243
71. Section of Camera Lucida 247
72. The Spectre—an Optical Illusion 269
73. How to produce Spectres 271
------------------------------------------------------------------------
THE WONDERS OF OPTICS.
--------------
PART I.
THE PHENOMENA OF VISION.
--------------
CHAPTER I.
THE EYE.
THE Eye is at once the most wonderful and the most useful of all our
organs of sense. It is especially by means of the eye that we gain a
knowledge of the exterior world. Our other senses are far more limited
in their action: thus the sense of touch only extends to objects within
our reach; the sense of taste is only a delicate and exquisite
modification of the sense of touch; the sense of smell can only be
exercised on substances that are close to us; and the use of our ears is
limited by the distance at which the loudest sound ceases to impress
them. But the eye has the privilege of extending its dominion, whether
for mere enjoyment or for serious instruction, far beyond the limits of
this little world. Not only is it the origin of all our ideas upon every
object that comes within its ken; not only does it reveal to us our own
position and that of our surroundings; but, thanks to the discoveries of
modern science, it is able to admire, on the one hand, a world of
infinite minuteness that remained unknown to us for centuries, and, on
the other, the immeasurable immensity of the starry universe.
Admirable as the eye undoubtedly is through the possession of the power
of vision, it is also capable of enchanting us by its own particular
beauties. Not to speak of its internal mechanism, which we shall
consider very fully by and by, let us for a moment examine its outward
appearance. Have you never, dear reader, been enchanted with a pair of
soft and gentle eyes, or with a couple of black orbs veiled with long
dark lashes, or with those wondrous eyes that rival the heavens in
colour and depth, shedding on you rays of light whose mute eloquence was
irresistible? If it be true that man’s face is the canvas upon which the
affections and desires of his mind are depicted as soon as they are
formed, the eyes are unquestionably the central point of the picture,
and it is in them, as in a looking-glass, that every sentiment that
passes across our brain is reflected.
When the mind is undisturbed, says Buffon, all the parts of the face are
in a state of repose; their proportion, unity, and general appearance
indicate the pleasing harmony of our thoughts and the perfect calmness
of our mind; but when we are agitated, the human face becomes a living
picture, in which the passions that disturb us are depicted with equal
force and delicacy, a picture in which every emotion is expressed by a
stroke, every action by a letter, so to speak; in which the quickness of
the impression outstrips the will, and reveals by the most sympathetic
signs the image of our secret trouble.
It is more especially in the eyes, adds this great naturalist, that
these signs are manifested and recognised. The eye is connected with the
mind more than any other organ: it seems almost to be in contact with it
and to participate in all its movements; it expresses in obedience to it
the strongest passions and the most tumultuous emotions, as well as the
gentlest thoughts and most delicate sentiments, and reproduces them in
all their force and purity just as they have sprung into existence; it
transmits them with exquisite rapidity even to the minds of others,
where they once more become impressed with all their original fire,
movement, and reality. The eye both receives and reflects the light of
thought and the warmth of sentiment, and is at once the sense of the
mind and the tongue of the intellect. Persons who are short-sighted, or
who squint, have much less of this external intelligence that dwells in
the eye. It is only the stronger passions that can bring the other
features of the face into play, that are depicted on their physiognomy;
and the effects of fine thought and delicate feeling are rendered
apparent with much greater difficulty.
The elegant author of _L’Histoire Naturelle_ rightly thinks that we are
so accustomed only to see things from the outside, that we are hardly
aware how much this exterior view of everything influences the judgment
of even the gravest and most thoughtful of us. Thus we are apt to set
down a man as unintellectual whose physiognomy does not particularly
strike us; and we allow his clothes, and even the manner in which he
wears his hair, to influence our judgment of him. Hence, our author goes
on to say, not wholly without some show of reason, that a man of sense
ought to look upon his clothes as part of himself, because they really
are so in the eyes of others, and play an important part in the general
idea that is formed of him who wears them.
The vivacity or languor of the movement of the eyes forms one of the
chief characteristics of facial expression, and their colour helps to
render this characteristic more striking. The different colours seen in
the eye are dark hazel, or black, as it is generally called, light
hazel, blue, greenish grey, dark grey, and light grey. The velvety
substance which gives the colour to the iris is arranged in little
ramifications and specks, the former being directed towards the centre
of the eye, the latter filling up the gaps between the threads.
Sometimes they are both arranged in so regular a manner that instances
have been known in which the irises of different eyes have appeared to
be so much alike that they seemed to have been copied from the same
design. These little threads and specks are held together by a very fine
network.
The commonest colours seen in the eye are hazel and blue, and it mostly
happens that both these colours are found in the same individual, giving
rise to that peculiar greenish-grey hue that is far from being uncommon.
Buffon thinks that blue and black eyes are the most beautiful, but this
of course is a matter of taste. It is true that the vivacity and fire
which play so important a part in giving character to the eye, are more
perceptible in dark eyes than in those whose tints are lighter; black
eyes, therefore, have greater force of expression, while in blue eyes
there is more softness and delicacy. In the former we see a brilliant
fire, which sparkles uniformly on account of the iris, which is of the
same colour throughout, giving in all parts the same reflection; but a
great difference may be perceived in the intensity of the light
reflected from blue eyes, from the fact of the various tints of colour
producing different reflections. There are some eyes that are remarkable
for being almost destitute of colour, and appear to be constituted in an
abnormal manner. The iris is tinted with shades of blue and grey of so
light a hue that it appears quite white in some places. The shades of
hazel in such eyes are so light that they are hardly distinguishable
from grey and white, in spite even of the contrast of colour.
For our part, we think that the beauty of the eye consists not so much
in its colour, or even in its harmony with the rest of the face, but in
its expression.
There are also numerous instances of green eyes. This colour is, of
course, much less frequent than blue, grey, or hazel. It often happens,
too, that the two eyes vary in colour in the same individual. This
defect is not confined to the human species, being shared by the horse
and the cat. In most other animals the colour of the two eyes is always
similar. The colour of the eye in most animals is either hazel or grey.
Aristotle imagined that grey eyes were stronger than blue, that those
persons whose eyes are prominent cannot see so far as others, and that
brown eyes are less valuable in the dark than those of another tint; but
modern investigations have failed to bear out the ancient philosopher’s
ideas with regard to the human eye.
Although the eye appears to move about in every direction, it has in
reality only one movement, that of rotation round its centre, by means
of which the eyeball rises or falls, or passes from side to side at
will. In man the eyes are parallel with each other in relation to their
axes; he can consequently direct them at pleasure upon the same object:
but in most animals this parallelism is wanting. In some cases the eyes
of animals are set almost back to back, rendering it impossible for them
to see the same object with both eyes at once.
Buffon makes the remark, that after the eyes, the eyebrows contribute
more strongly than any other part of the face towards giving character
to the physiognomy, being, inasmuch as they differ in their nature from
the other features, more apparent by contrast, and hence strike us more
than any other portion of the countenance. They are, in fact, a shadow
in the picture, bringing its colour and drawing into strong relief. The
eyelashes also contribute their effect; when they are long and thick,
they overshadow the eye, making its glance appear softer and more
beautiful. The ape is the only other animal besides man that possesses
two eyelashes, the rest having them only on the upper eyelid. Even in
man they are more abundant in the upper eyelid than in the lower. The
eyebrows have but two movements, upward and downward, governed by the
muscles of the forehead. In the action of frowning we not only lower
them, but move them slightly towards each other. The eyelids serve to
protect the eyeball, and keep the cornea from becoming dry. The upper
eyelid has the power of raising and lowering itself, the lower one being
almost destitute of movement. Although the motion of the eyelids is an
effort of will, there are times when it is impossible to keep them open,
as for instance when we are overpowered by sleep, or when the eyes are
suddenly subjected to the effects of strong light. The eyelid is a most
admirable arrangement for the protection of the eye, and it is almost
impossible to admire this provision of nature too much, even when we
confine ourselves to an outward examination of it. It is not merely the
outward mechanism and motion of the eyelids, nor the colour of the eyes,
that constitutes their beauty; we have already said that the leading
characteristic of the eye was _expression_. It is this expression which
causes the eye to appear to speak, to fire up suddenly, to sparkle with
flashes of light, to languish or conceal itself underneath its lashes,
to raise itself with inspiration, or to pierce the abyss of thought,
just according to the particular sentiment governing the mind at the
moment. Hence it is expression that constitutes the true beauty of the
eye: every one knows instances of eyes which, while at rest, would never
be noticed by anybody, but which, when once animated by intense
eloquence, lend to the voice of their possessor an unexpected power,
which moves and transports the listener to an extent infinitely beyond
that resulting from the simple spoken words.
Enough, however, has been said upon the external aspect of the human
eye; we will, therefore, at once endeavour to penetrate the circle in
which are contained the wonders that this little book is intended to
describe. The object of these lines is not so much to describe the
beauty of man’s glances, nor the value of his senses, but rather to make
known those illusions to which the most sagacious of all his senses is
apt to fall a prey. But before entering the temple it was but right to
have bestowed a little admiration upon the façade. By the way, as we are
about to describe many illusory wonders, do not let us commence by
deceiving ourselves with regard to our first marvel—the eye itself. A
great philosopher calls the eyes the windows of the soul, and, although
meant as a poetical image, the saying is not far from the truth; for the
optic nerve by which we see external objects, is an extension of the
nerves of the brain, whose functions and actions are an unfathomable
mystery.
------------------------------------------------------------------------
CHAPTER II.
THE STRUCTURE OF THE EYE.
OF all the senses, says an ardent admirer of nature, the sight is
certainly that which furnishes the mind with the quickest and most
widely-extended perceptions. It is the source of the richest treasures
of the imagination, and of our ideas of the beauty, order, and unity of
the world around us. How unhappy are those whom a hard fate has deprived
of the sense of sight from their birth! Alas! the finest day and the
darkest night differ in nothing as far as they are concerned; the light
of heaven never brings joy into their hearts. The enamelled beauties of
a bed of flowers, the varied plumage of the peacock, the glories of the
rainbow are alike unknown to them. They cannot contemplate from the
mountain height the beauties of the valley beneath; the fields golden
with the harvest, the meadows smiling with verdure, and watered by
winding rivers, and the habitations of man dotted about here and there
over the surface of this magnificent picture. To them is unknown the
sight of the mighty ocean; and the innumerable legions of the cloud army
of Heaven are to them as if they did not exist. The impenetrable
obscurity which surrounds them allows them neither the contemplation of
what is grandest in man’s outward aspect, nor even the admiration of
those qualities which they themselves would hold most dear.
A strong sentiment of pity should, therefore, animate the breast of
every right-thinking man, when he considers the unhappy condition of
those who are born blind.
The eye infinitely surpasses in its complexity and beauty of structure
all the other organs of sense, and is most unquestionably the most
marvellous object that the human mind is capable of examining and
understanding. Let us first examine the external parts of this wonderful
organ. With what a singular system of entrenchments and defences do we
find the eye provided! It is itself placed in the head at a certain
depth, and surrounded on all sides by solid bone, so that it is only
with the greatest difficulty that it is hurt by accident from without.
The eyebrows also play their part as protection to the eye, and prevent
the perspiration from entering and irritating the organ. The eyelids too
are always ready to rush to the rescue, whether to protect the eye from
outward attacks, or to shade it from too strong a light during sleep.
The eyelashes not only add to the beauty of the eye, but they shade it
from the too brilliant light of the sun, and act as advanced guards to
prevent the entrance of dust or any other foreign body with which the
eyes might be injured.
But its internal structure is still more admirable. The globe of the eye
is almost spherical and measures nearly one inch in diameter. Fig. 1 is
a view of the eyeball, showing the details of its structure; the various
membranes surrounding it have been cut away in order that it may be
better examined. If we commence our examination by the exterior portion
of the front, we shall first find immediately beneath the eyelashes a
perfectly transparent membrane (C), called the _cornea_. It is a
prolongation of the hard opaque external coating of the eye, called the
_sclerotic membrane_, and marked S in the figure. The cornea is
sufficiently hard in its nature to present a strong resistance to any
violence from without.
Immediately beneath the cornea and in contact with it is the _aqueous
humour_, a thin transparent liquid occupying a small portion of the
front of the eye.
Next comes the _iris_, a circular disc perforated with a round hole in
the middle, and coloured with various shades of blue, brown, and grey.
[Illustration:
Fig. 1.—Section of the Eye.
]
The opening in the centre, which appears like a black spot when the eye
is examined, is not really an object, but simply an aperture, capable of
changing its size according to the quantity of light striking the eye.
This change of size in the opening, or pupil, as it is popularly called,
is effected by the contraction or expansion of the iris, which thus
possesses the peculiar property of exactly proportioning the amount of
light that enters the eye, so that there is never too much or too
little. It is through the pupil that the rays of light proceeding from
the various objects around us pass into the interior of the eye, and
form an image upon the retina, as will be afterwards explained.
Immediately behind the pupil is O, a bi-convex lens to transmit the rays
of light to the retina. It is generally called the _crystalline lens_.
From the crystalline lens to the back of the eyeball, is a space more or
less globular in form, containing a gelatinous diaphanous mass somewhat
resembling white of egg in appearance, and called the _vitreous humour_.
Behind the vitreous humour, and immediately opposite the pupil and lens,
is the most delicate and important of all the membranes of the eye, the
_retina_, which serves as a screen whereon are received the images of
the objects around us. This membrane is an expansion of the optic nerve
N leading from the brain, and lines the whole of the interior of the
eye. The eye is also enveloped in a second membrane (C), called the
_choroid_, which is impregnated with a black pigment. Round this is
wrapped a third membrane, the _sclerotic_ (S), which unites with the
cornea in front of the eyeball.
The crystalline lens through which all the rays pass before they reach
the retina, possesses the marvellous power of being able to modify its
curvature in such a manner as to adapt itself to the distance of the
object seen, and thus throw a distinct image on the retina. When we come
to talk of the properties of lenses, we shall see that the focus of a
lens differs for objects at different distances; if, therefore, the eye
were not provided with some such means for altering the focus of the
crystalline lens, we should only see objects distinctly at one
particular point. The crystalline lens consists of infinite numbers of
extremely thin transparent little plates, each of which is in itself
composed of fine fibres so united together as to be capable of a small
degree of compression or extension. Hence the power of the lens to alter
its form according to circumstances. It is calculated that the human eye
contains over five millions of the laminæ above referred to. With such
wonders is the world of nature replete,—wonders that we daily and hourly
pass by without examination.
It is by means of this ingenious and inimitable structure of the eye
that external objects pass from the domain of the material world into
that of the mind, and become accessible to every faculty of our brain.
Of its own accord, and without apparently any effort of our own will,
does this marvellous mechanism adapt itself to all the variations of
distance and intensity of light, a power possessed by no instrument as
yet constructed by the hand of man—being capable, as it is, of
distinguishing instantaneously between the distance of the remotest
nebulæ and that of the letters forming this page. This wonderful organ,
writes Brewster, may be considered as being the sentinel that guards the
passage between the world of matter and that of mind, and as the medium
through which they interchange all their communications. The optic nerve
perceives the objects written on the retina by the hand of nature, and
conveys them to the brain in all their integrity of form and colour.
The path of the rays of light and the formation of images upon the
retina are shown in the preceding figure. At first sight it will be
perceived that the objects thereon depicted are in a reversed position,
that is to say, when we look at a view similar to that shown in fig. 2,
we should find, if we had any means of observing the positions of
objects reflected on our retina, that the flock of sheep coming up the
road were at the top of the eye, while the trees, the roof of the house,
and the chimney were in the contrary position. Similar reversed images
may be seen in dark rooms, by holding a screen before any little crack
or pinhole in the door or shutter of the room. In fig. 2 the keyhole of
the door is represented as playing the part of a lens. The author, in
common with almost every other boy, observed this fact at a very early
age, and the idea immediately struck him that it would be only necessary
to fix these images to procure exact representations of natural scenery;
but in making inquiries into the subject, he found that his juvenile
observations had been made a little too late, photography having already
gained the end he intended striving for.
[Illustration:
FIG. 2.—A Camera Obscura.
]
Seeing that the images of all objects appear on our retina upside down,
the student is naturally disposed to ask how it happens that we do not
see them in that position. Physiologists and natural philosophers have
advanced numerous theories on the subject. Some, with Buffon, admit at
once that it is by habit and education of the eye that we see objects
unreversed. Others, like the great physiologist Müller, imagine that as
we see everything upside down, and not a single object only, we have no
points of comparison, and practically ignore the reversal. The truth,
however, appears to be that it is the brain, and not the eye, that
possesses the power of determining the real position of what we see.
That the eye alone has no power of determining the positions of objects
by itself, may be easily proved by showing a person an astronomical
object, such as the moon through a telescope. Unless the observer has
been already familiarized with the appearance of our satellite, he will
not know whether the image he sees is reversed or not. It is the brain,
therefore, and the brain only, that has the power of determining the
position of objects around us, without taking into consideration the
reversed picture of them that is depicted on our retina. The student who
takes an interest in the structure of this important organ, would do
well to procure a sheep’s or bullock’s eye from the butchers, and
dissect it carefully with a sharp penknife and pair of scissors. The
image formed on the retina may be easily seen by cutting away the
sclerotic and choroid coatings at the back of the eye.
The ordinary distance of distinct vision for small objects, such as the
letters of a book, is from ten to twelve inches. But possibly there do
not exist two pairs of eyes in the world whose foci are the same. Even
in the same individual it frequently happens that the focal length of
the eyes differs considerably. In some persons the focus of the eye is
so reduced that they are obliged to bring the object they are examining
within six, and even four inches of their eyes, before they can see it.
This defect is known ordinarily as _short sight_, and results from the
too great convexity of the cornea and crystalline lens. It is corrected
by wearing spectacles with concave glasses. Others again, on the
contrary, place the book or object they are looking at, at a greater
distance from the eye than that named. Such people are called
long-sighted, and the defect results from the too great flatness of the
cornea and the crystalline lens. The fault is of course corrected by the
use of spectacles containing convex lenses.
Long-sightedness is generally the result of old age, and it may be taken
as a fact that the older we grow the flatter becomes the crystalline
lens. Hence short-sighted people have been known to recover their sight
perfectly as they advance in years through the natural process of the
flattening of the crystalline lens. These matters, however, will be more
fully treated of when we begin to speak of the properties of lenses of
different forms and curvatures.
------------------------------------------------------------------------
CHAPTER III.
THE ERRORS OF THE EYE.
IT is with our own organization that we shall commence our task of
exposing the illusions that we shall meet with during our optical
experiments,—in fact with that wonderful and important organ of our body
that we are apt to look upon as sure and infallible, but which we shall
find is deceiving us constantly, and hourly proving the fallacy of the
popular saying, that “every one must believe his own eyes.” In ancient
times there existed a school of sceptics who doubted everything
beginning with Pyrrho, the great theorist, and ending with the follower
of his school who doubted the existence of muscular force even after he
had received a sound box on the ear from an opponent of his system of
philosophy. If any of our readers were to become followers of Pyrrho,
they might easily do so when considering the numberless illusions we
shall describe to them, if they did not remember that if our senses are
subject to error, we have a brain to set them right: our mind, if
logical and well regulated, soon discovers errors of observation, and
speedily places our judgment on the most solid basis. We shall find
endless instances of this throughout our little book. If we are dazzled
with illusions from time to time we shall as often recover ourselves;
and no matter how beautiful or interesting these deceptions may appear,
we shall speedily be able to convince ourselves that they are unreal. In
this chapter we shall only speak of those errors of the eye of which we
have actually lost all cognizance, so effectually has our judgment
succeeded in counteracting their influence.
We all know that the first thing a child does with its eyes, even when
it is only five or six weeks old, is to turn them towards the most
brilliant object within its reach. Instinctively and without being aware
of it, the child’s eye seems to seek the light. The whole of nature,
from the lowest plant to the baby in the cradle, appears more or less
endowed with this instinct of turning towards the light.
From the time that children begin to distinguish objects, their eyes are
liable to be affected by two causes of error. Before being able to judge
of the position of things surrounding them, they see everything upside
down; they consequently acquire a false impression of the position of
objects. The next cause of error that is likely to mislead them is the
fact of their seeing everything double, a separate image of everything
being formed on each eye; and it can only be by the experience gained
through the sense of touch that they can acquire the knowledge necessary
to rectify these errors, and see those objects single which appear to
them double. This error of sight, as well as the first one, is set right
so easily in the end, that although in reality we see everything double
and upside down, we imagine that we see them single, and in their proper
positions, a state of things brought about entirely through another
sense exercising its power over our judgment; and it is hardly too much
to say that, if that sense were deprived of the power of feeling, our
eyes would deceive us, not only as to the number, but the position of
the objects within our view.
It is very easy to convince ourselves that we really see objects double,
although we imagine them to be only single. We have only to look at the
same object first with the right eye, and we shall see it directly
against some portion of the wall of the room in which we are sitting;
then looking at it with the left eye, we shall see that it covers a
different part of the wall. This experiment is easily tried, and is very
convincing. Thus we see that an image is formed on both eyes, and we
consequently see the object, whatever it may be, repeated twice. By
degrees, however, the eyes gain the power of converging their axes on
objects at different distances, so that they fall on similar portions of
each retina, and so convey a single impression to the brain. Thus, for
instance, if we look at a pencil held up at arm’s length, and then,
without changing the position of the eyeball, look at some distant
object, we shall see it double. Let us, however, converge the eyes upon
it, and the two images unite. Reverse the experiment by now looking at
the pencil without converging the eyes upon it, and we shall see that
object double in its turn. The same thing happens if we push aside one
of the eyes with the finger while looking at any object. During severe
illness it often happens that the patient from extreme weakness loses
the power of convergence, and consequently sees every thing double, and
we continually see children’s faces wearing a most distressing
appearance through having temporarily lost the power of moving the
muscles of the eye. It is a common expression to use in speaking of
drunken people, that they see double, but the saying, unlike many
others, is no metaphor; when a man gets drunk he loses his power over
the muscles of his eye, just as he does over those that sustain his
body, and the instinctive closing of one eyelid, in order that he may
see objects single, is an effort of his weakened judgment to set things
right once more.
While on this subject we may mention the experiment made by the famous
English surgeon Cheselden upon a boy who was born blind, and upon whom
he operated successfully.
This boy, who was thirteen years old at the time that Cheselden restored
to him the sense of sight, was not born absolutely blind, his affliction
having been caused by a cataract or film spread over the eyeball, which
allowed him to distinguish night from day, or black from white or
scarlet when placed in a very good light, although he was unable to
perceive the form of things around him. At first Cheselden operated on a
single eye, perfectly restoring its power; but so little idea of
distance did the new sense convey to the boy’s mind that for a long time
he imagined that everything touched his eyeball, just as those he felt
touched his skin, and it was only by the sense of touch that he could
persuade himself of the fallacy of his supposition. At first he had no
perception of form whatever, and could only recognize objects he had
already been familiar with after he had felt them all over. He was a
long time, for instance, before he could distinguish between the dog and
the cat without touching them, and was greatly surprised to find that
the persons and things he had liked best when blind were not always the
pleasantest to his newly acquired sense. His ideas of size, too, were
all at fault, and he could not, for a long time, be made to understand
how his father’s picture could be got into the back of his mother’s
watch; even after he had possessed his sight for a comparatively long
time, he could still only recognise people he had known during his
blindness by touching their faces. Whenever he saw a new object he
looked at it attentively for some time, in order, as it were, to learn
its form by heart; but his memory was at first so overtaxed that he
continually forgot his visual impressions, and mistook one thing for
another. He was more than two months before he could appreciate form as
depicted in a painting or drawing, having hitherto learned to consider
pictures as flat objects. When, however, he began to understand the
power of light and shade in producing the representations of solid
objects, he was often extremely surprised to find the surface on which
they were depicted quite flat when he touched it. The same thing
frequently happens to ourselves, when looking at the photographs of
bas-reliefs for instance. If these objects be well photographed, with
the proper arrangement of light and shade, the illusion is so complete
that the finger involuntarily touches the paper to feel if the surface
is not really raised. In the Bourse at Paris there are some figures
painted to represent bas-reliefs in so wonderful a manner, that
numberless bets have been made, lost and won, over them. When feeling
such representations of solid objects, the boy would often ask those
around him which of his senses was deceiving him, his sight or his
touch.
At first he saw everything of an enormous size, but as he saw things
larger than those around him, he found the latter diminish. He also
imagined that there was nothing beyond the room he was in, and could not
be brought to comprehend how the house could be larger. When the sight
of the second eye was restored to him a year afterwards, he at first saw
every object of an enormous size, just as in the case of the first eye;
but as he had now the perfectly educated organ to help him as well as
his sense of touch, he soon began to see things under their natural
appearances.
While he was in ignorance of what sight really meant, he was not
particularly anxious to undergo the operation, saying that he did not
think it possible to derive more pleasure from things that he liked than
he did while he was blind. But now that his sight was restored he found
every fresh object a new pleasure. When first he was shown the landscape
from the top of a high hill, he was so delighted that he exclaimed that
he had found another sense. When his second eye was operated upon, he
saw things apparently twice as large with both eyes as with the one
already restored to him. Even at first he seemed to have no difficulty
in converging the eyes on any object.
These extracts from the history of Cheselden’s patient show us how
utterly incapable the eye must be of rightly understanding the number,
position, size, and form of objects without frequently correcting our
impressions by the aid of the sense of touch.
------------------------------------------------------------------------
CHAPTER IV.
OPTICAL ILLUSIONS.
BESIDES the errors of sight already spoken of, there are other
illusions, which are either common to all persons or confined to certain
individuals, the knowledge of which will serve as a fitting prelude to
the descriptions of those which are artificial.
The following defect, for instance, is one which is little known, but
notwithstanding our ignorance of its existence it is nevertheless true
that we all suffer from it. There is in every one’s eye a blind spot,
totally incapable of experiencing the effects of the rays of light when
they impinge upon it. For objects situated opposite to this particular
spot we are as completely blind as if we had no eyes at all. To convince
yourself of the truth of this assertion it is only necessary to try the
following simple experiment.
Place upon a piece of white paper two small wafers, or two blots of ink
about an inch and a half apart. Take the sheet in your right hand, and
hold it up parallel to the lines of the eyes; shut the _left_ eye, and
fix the _right_ eye on the centre of the _left_ wafer or ink-spot. Move
the sheet of paper steadily towards the eye, until it is about two
inches and a half or three inches’ distance from it, and you will find
that in a certain position the _other_ wafer or ink-spot will disappear,
although it is evidently still in the field of view. Having discovered
this point which differs for different eyes, you will find that if you
diminish or increase the distance of the paper you will once more see
the missing object. The same thing happens if you move the eye from the
centre of the wafer. The same experiment may be repeated with the left
eye with a precisely similar result.
It has been found by experiment that this particular blind space exists
exactly over the base of the optic nerve, at the spot where it joins the
eye. (Fig. 1). Thus we see that the nerve which actually conveys the
impression of sight to the brain is in itself incapable of being excited
by light. In such cases as these Nature seems to laugh at us, and
escapes from our grasp just as we are most confident in our power of
wresting her secrets from her; indeed we may compare her to a wise and
good-natured mother, who, though always amiable and willing to instruct
those about her, sometimes smiles when her children fancy they are as
learned as she is.
If we do not perceive the constant recurrence of the phenomenon just
mentioned, it is because when both eyes are open the object whose image
falls on the blind spot in one eye is seen by the other, the insensible
portions of each eye being on opposite sides. Not only this: the spot
being always situated on the outer and indistinct portion of the image
reflected on the retina, we do not take notice of it; for as every one
has no doubt observed, it is only the small portion of the object we are
looking at exactly opposite the centre of the eye that is perfectly
distinct and clear, all the rest being confused in its details, although
quite visible.
Again, we may account for our not noticing it by the fact of our seeing
clearly only those things which specially attract our attention—a fact
first noticed by Mariotte. We see only what we wish to see with our
physical eyes, as well as those of our mind. If our attention is
attracted by a particular portion of a landscape, we see only that, and
nothing else. If it is fixed on some subject that we are contemplating
inwardly, we see nothing at all, although our eyes may not only be wide
open, but absolutely fixed on some particular object. For instance,
suppose a sportsman is out in the fields preceded by his dogs, Bran and
Ponto. If he follows the movements of Bran with attention, he becomes
the only object animate or inanimate, that depicts itself on his retina.
Ponto may jump and caper in vain: he is lost to his master’s eye as much
as if he were not there at all; his mind is entirely fixed on the beauty
of Bran’s coat, on the fit of his collar, or fifty other things, and he
sees nothing else. But let the sportsman begin to think of the number of
birds he shot yesterday, or how he will find time to get up to the
grouse in Scotland, or of that fine stag he missed when he was last
amongst the heather, and dogs, cover, and landscape will fade from his
sight as effectually as if he had been struck with blindness. Let him,
however, strike his foot against a stump, or let the dogs suddenly begin
to point, and he instantly receives back his sight, which but a few
moments before he had lost to all intents and purposes.
The phenomena of _ocular spectra_ and _complementary colours_
experienced by every one forms a curious chapter in the history of those
illusions which take their origin in the eye itself. Every one has
noticed that after looking fixedly at a bright light or a striking
colour for a few moments, the eye preserves an impression of the object
for a certain time. A very light window looked at intently for several
seconds will leave the impression of its cross-bars on the retina for
several minutes, the colour of the image changing at every movement of
the eye. The same effect may be observed when looking at the setting
sun, or a flaring gas light. If the light at which we look is coloured,
we shall see the complementary colour in the impression left on the
retina. Sir David Brewster was one of the first to notice and experiment
upon these very interesting facts.
If we cut out any simple figure, a small cross for instance, in scarlet
paper, place it upon a white background and look at it fixedly for a
minute or two, we shall find that its tint will gradually become duller.
If we now suddenly look at a piece of white paper, we shall see the
cross depicted upon it in green, which is the complementary colour to
red. It should be explained, that the complementary of any colour is
that which is necessary to make white light. Thus, blue, yellow, and red
(as we shall find out when we come to speak of the prismatic spectrum),
mixed in certain proportions, form white light; consequently the
complementary of orange, which is composed of red and yellow, will be
blue; of green, which is yellow and blue, red; of purple, which is blue
and red, yellow, and _vice versâ_. The complementary of black is white,
and of white, black as a rule; but if the white object be very
brilliant, the black spectrum will speedily become coloured. The
impression left by the setting sun is of this character. At first, while
the eye is open, the image is black, then brownish red, with a light
blue border; but if the eye be shut suddenly, it becomes green, with a
red border, the brilliancy of colour being apparently in proportion to
the strength of the impression. These spectra may be perceived for a
long time, if the eye is gently rubbed with the finger now and then.
Some eyes are more impressionable in this respect than others, and Beyle
gives an instance of an individual who saw the spectrum of the sun for
years, whenever he looked at a bright object. A modern instance of this
occurred lately to an amateur astronomer who was looking at an eclipse
of the sun. He unfortunately used a glass that was not sufficiently
smoked, and the image of the sun’s disc, with the black space caused by
the intervening moon, remained on his retina for months after. This
gentleman’s case afforded an instance of the necessity of attention in
order to see any object, for after the first few days he only became
sensible of his unfortunate mishap when his attention was called to it
by some accidental circumstance. These facts were so inexplicable to
Locke, that he consulted Newton on the subject, and was surprised to
learn that the great philosopher himself had suffered for several months
from a sun-spectrum in the eye.
Without affirming that optical illusions are the cause of all the
supposed supernatural appearances of which we have heard so much, there
is no doubt that in many instances the eye plays an important part in
deluding the brain. The following example, also cited by Beyle, will
show this clearly. A horseman dressed in black, and riding a white
horse, was trotting along a portion of the road, which through a sudden
break in the clouds was brilliantly illuminated by the rays of the sun.
The black figure of the man was projected against a white cloud, and the
horse appeared doubly brilliant from being seen against the
dark-coloured road. A person who was greatly interested in the arrival
of the horseman was watching them with great attention, when suddenly
the horse and his rider disappeared behind a wood. An instant after the
observer was terrified at seeing a _white_ cavalier on a _black_ horse
projected on a white cloud at which he was accidentally looking. It may
be readily imagined that such an occurrence, followed up by a succession
of unusual events,—such as illness, death, or any other series of
misfortunes,—might even in the present day add a chapter to the history
of the marvellous.
To the illusions to which, like the preceding, we are all subject, may
be added those resulting from some abnormal conformation, or some
disease of the eye, in those who labour under them. An example of this
occurs in the case of double or triple vision, many remarkable instances
of which are mentioned by Müller, the celebrated physiologist.
Although, as before explained, the image of an object is depicted at the
same time on both our eyes, still we only see one impression, in
consequence of the two images being carried to the brain from
corresponding portions of the retina. If this relation be disturbed by
any cause, or if the eyes are not converged exactly upon the same point,
a double image is the result. The first of these facts may be proved by
looking at the moon, for instance, with the left eye shut; on suddenly
opening it, two images will be seen for an instant. The second is
instantly proved by pushing either of the eyes aside with the finger,
when looking at any object.
It is necessary, however, to distinguish between these effects and true
double vision, as well as a certain defect which exists in the eyes of
many people, consisting in the apparent multiplication of distant
objects by the same eye. In these cases, there is a superposition of
images upon the retina, each having its proper bounds. With the majority
of individuals afflicted in this way, it only happens when they look at
a very distant object, the moon or stars for instance. There are many,
however, who suffer from it in the case of everything they look at,
whether far or near. Stephenson, who was affected with it, made it the
subject of many interesting experiments. When he looked at a clear mark
on a white ground, and gradually walked away from it, not only did the
image become indistinct, but it seemed to unfold itself into several,
independently of many others much more indistinct, more especially two
situated on each side, whose distance increased the farther he walked
away. As these latter images became more and more separated, they also
became more confused. The image seen by the right eye was a little
higher than that seen by the left. Griffin states, that after having
used the telescope for any length of time, the eye that he kept shut
always saw objects triple and double for some hours afterwards. These
phenomena are possibly connected in some way with the disposition of the
plates and fibres of which the crystalline lens of the eye is composed.
Semi-vision, or _hemiopia_ as it is called, is much more rare and more
difficult to explain than the phenomena of double vision; and consists
in the power of being able to see only the right or left half of the
object looked at, the separation being vertical when the eyes of the
observer are in the same horizontal line. Thus, in looking at the word
NEWTON, the person so afflicted would only see either the letters NEW or
TON according to which half of the eyes were defective.
Wollaston was afflicted with hemiopia on two different occasions; the
first time after violent exercise, during two or three hours, when he
could see distinctly only the left-hand halves of the objects he was
looking at. Both eyes were similarly affected, and the phenomenon only
lasted about a quarter of an hour. Twenty years afterwards he suffered
again from the same accident, but on this occasion in the contrary
manner; that is to say, he only saw the right halves of the objects he
was looking at—to use his own words, he could only see the right half of
every friend he met. At certain distances from the eye, one of two
persons would become invisible, and by simply changing his own position
or that of the persons he was near, he could make one or other of them,
or indeed both, disappear at will. It must be acknowledged that similar
tricks of Dame Nature, due to an unconscious insensibility of the eye,
are most singular, and at first sight appear to have a supernatural
origin.
Bartholin mentions the case of a hysterical woman who was afflicted with
hemiopia horizontally, and saw all natural objects cut in two, the lower
halves being invisible. In this instance it was only the left eye that
was defective.
Another interesting example of optical illusion is the luminous
sensation produced internally when the eye, or the neighbouring parts,
are struck or stimulated by friction or electricity. These appearances
are experienced even by those who have lost their sight. Müller states
that a case was submitted to a legal tribunal to decide whether the
luminous sensations which are perceptible when we rub our eyes are
really light. The matter in dispute was whether a man who was attacked
by robbers in the dark, could see and recognise them by means of the
light produced in his eyes by a violent blow on the head; but he does
not tell us how the question was decided. With regard to internal
causes, Humboldt tells us that a man whose eye had been extirpated, was
sensible of luminous appearances whenever he was galvanized. Lincke
states that a man whose eye had been removed by a surgical operation,
saw next day all kinds of luminous phenomena, which tormented him
cruelly with the idea that after all his eye had been saved. When he
shut the perfect eye, he fancied he saw with the missing eye circles of
fire, persons dancing, and similar appearances for several days. These
facts are analogous to those told of persons who have had their legs and
arms amputated, but who, notwithstanding, apparently feel pain in their
lost limbs.
------------------------------------------------------------------------
CHAPTER V.
THE APPRECIATION OF COLOUR.
MOST people understand each other sufficiently to agree in their ideas
about various colours. Thus every one agrees in saying that poppies are
red, that the sky is blue, and the leaves green; but if any one were to
assert that the sky was red, that the leaves were blue, and poppies
green, who could possibly contradict him?
This statement may appear a paradox, and an absurdity to many of our
readers, but it is really a problem that has engaged the attention of
many of our greatest philosophers. Who can prove that what I see as
yellow may not appear blue to you, or that what you see red is not green
to me? You would possibly explain the doubt by saying that because we
both agree in calling a buttercup yellow, that we see the same colour. I
call a buttercup yellow, because I have learnt since my childhood to
give this name to the particular sensation I experience when I look at
one of these flowers; but that is no proof that the sensation I feel is
similar to that felt by everybody else, and it is not merely possible,
but probable, that our personal sensations of colour are essentially
different, although the arbitrary words we use to designate them are the
same.
It may be remarked in parenthesis, that colour is not an entity, but is
simply the effect of certain properties of surface or interior structure
possessed by every substance with which we are acquainted. The old
saying, that “all cats are black in the dark,” is really a profound
philosophical truth, which is not only true of cats but of the reddest
rose that ever grew in a garden, the bluest violet that ever was
plucked, the prettiest girl that ever was kissed under the mistletoe. It
is a sad thing to think of, that when we put the candle out, and step
into bed, we become blacker than the blackest negro that was ever
emancipated. But without light there can be no colour, for there is no
material, so to speak, from which to manufacture it. White light, as we
have said before, is made up of red, blue, and yellow, and it is by the
absorption of one or all of these that all tints are formed. The surface
of a poppy leaf has the power of absorbing all the blue and a little of
the yellow, reflecting the whole of the red and the remainder of the
yellow, the mixture of the two forming scarlet. The surface of a
marigold acts differently; all the blue is absorbed, as in the case of
the poppy, and a good deal of the red with it, leaving just a little to
brighten up the yellow which is reflected with it. Some substances,
white marble for instance, have no power of splitting the light into
colours, absorbing some and reflecting others, but reflect the whole of
it in its integrity. Others again, like black velvet, absorb nearly the
whole, just reflecting sufficient to enable us to see its surface.
We began this chapter by speculating on the probability of our seeing
different colours to our neighbours, and we shall now proceed to show
that our speculations in that direction are not so absurd as they appear
to be at first sight.
The phenomenon of colour-blindness, or the insensibility of the eye to
certain colours, has been for many years past a puzzle both to the
physiologist and the philosopher. Perhaps the most remarkable case of
the sort is that mentioned first by Huddart, and quoted by Sir David
Brewster, of a shoemaker named Harris, living at Maryport, in
Cumberland, who was utterly incapable of distinguishing any colour at
all, and saw everything white, grey or black. The first time that Harris
noticed this defect, was when he was about four years old; having found
the stocking of a playmate in the street, he returned it to him at his
cottage, and noticed that every one said it was a red stocking, but he
could not understand why they should call this particular stocking red,
as it seemed to him to be like every other. This circumstance remained
in his mind, and a few more similar observations confirmed his
suspicions that he had some defect of sight that prevented him from
seeing as others did. He also observed that other children pretended to
distinguish cherries from their leaves by what they called their colour,
whilst he could see no difference between them, except those of shape
and size. He also noticed that by means of the difference of colour,
others could distinguish cherries on a tree at a much greater distance
than he could; whilst he, on the contrary, could see other things at
greater distances than his companions. Harris had two brothers, whose
eyes were similarly defective; one of these, that Huddart examined,
mistook green for yellow constantly, and orange for light green.
In the _Philosophical Transactions_ Scott describes a similar defect in
his own powers of vision. He states that he was unable to distinguish
green, and that the colours known as crimson and pale blue presented no
difference of hue. He further confesses his inability to see any
difference between bright green and bright red, although he could
distinguish between red and yellow, dark blue, and almost every shade of
blue, except sky-blue. He goes on to relate how he married his daughter
to a worthy young man of his acquaintance, and that the day before the
wedding the bridegroom came to his house in a full suit of black, as he
thought. He was greatly displeased to see him appear in mourning on such
an occasion, and took an opportunity to remonstrate with him on the
subject. But what was his surprise to hear his daughter exclaim, in loud
tones of counter remonstrance, that she had rarely seen her lover in a
coat of such a pretty colour, and that her father’s eyes must deceive
him on this as on many other occasions. Scott’s father, his maternal
uncle, one of his sisters, and two of his sons had the same defect of
sight. Dr. Mitchell mentions the case of a naval officer who for his
ordinary uniform chose a blue coat and waistcoat and red trousers, fully
believing that they were all of the same colour. A tailor of Plymouth,
also mentioned by Dr. Mitchell, mended a black silk waistcoat with a
piece of crimson, and another put a red cloth collar to a blue coat.
Several celebrated men have suffered from colour-blindness. Amongst them
may be mentioned Dugald Stewart, the great philosophical writer; John
Dalton, the originator of the atomic theory; and Troughton, the
philosophical instrument maker. Dugald Stewart first discovered the
defect on hearing a member of his family admire the contrast of colour
between the leaves and fruit of a Siberian crab-tree, while he could see
no difference between them, except in point of form and size. John
Dalton could not distinguish blue from crimson, and he could only see
two colours, blue and yellow, in the prismatic spectrum. Troughton could
see no difference between dark crimson, bright orange, and yellow—in
fact, he could only distinguish blue from yellow.
In an article on this subject, published in the _Magasin Pittoresque_
for 1846, a Swiss physician gives some interesting examples, which are
worth repeating. In the solar spectrum obtained by passing a ray of
light through a triangular prism, and which is composed of the following
colours,—red, orange, yellow, green, blue, indigo, and violet,—Dalton
could only see yellow, blue, and violet. Rose-colour by day appeared to
him a pale blue, but at night it seemed to take an orange hue. By day
crimson seemed to be dirty blue, and red cloth dark blue. Dr. Whewell
having asked him one day to describe the colour of the doctor’s scarlet
gown, Dalton pointed to the trees around them, and declared he could
distinguish no difference in their colour; and one day having dropped a
stick of red sealing-wax in the grass, he had the greatest difficulty in
finding it again. Since Dalton’s time over five hundred distinctly
marked instances of this imperfection have been noticed, and Professor
Prévost, of Geneva, has named it _Daltonism_, an extremely
unphilosophical piece of pathological nomenclature, which has
unfortunately received the sanction of too many great physiologists to
be abolished. Blindness might just as well be called _Homerism_ or
_Miltonism_.
Colour-blindness is much more frequent than is generally supposed, for
those who are afflicted with it are mostly ignorant of the defect, and
frequently practise trades or professions in which perfect sensibility
to the different hues of colour is quite indispensable. An instance of
this occurred some time since in the case of an engine-driver, who
allowed his engine to run into a luggage train, through not noticing the
red danger signal. At his examination it was proved that he was
colour-blind, and could not distinguish red from green. Partial
colour-blindness is, no doubt, the cause of the frequent disputes that
we hear about the tints of certain objects; to say nothing of the
glaring instances of bad taste in the arrangement of colour that are
now-a-days so common. Out of forty boys at a school at Berlin who were
examined by Leebech, he found five who were quite confused in their
notions of colour, and could not distinguish between ordinary shades of
the same hue. This affliction is in many cases hereditary, descending
from father to son. It is singular that instances of colour-blindness
are much more common amongst men than amongst women, for out of over
five hundred cases there were only four in which females were the
sufferers. It seems also that persons with grey eyes are more frequently
colour-blind than those whose eyes are blue or brown. To the list of
great men who were colour-blind, we must not forget to add the
celebrated Italian historian, Sismondi.
Physiologists consider that there are two kinds of colour-blindness,—one
where only two colours are seen, the other where more than two are
perceptible. Daubeny Turberville, an oculist of Salisbury, mentions a
case of the former, in which a young girl, like the Maryport shoemaker
mentioned by Brewster, could only distinguish between black and white,
everything between the two being of different shades of grey. This girl,
singularly enough, could see to read in twilight a quarter of an hour
after her companions. This sharpness of sight appears to be not at all
uncommon amongst those who are colour-blind. Spurzheim mentions the ease
of a whole family who were afflicted in the same way as Turberville’s
patient. All the male members of Troughton’s family were equally
incapable of distinguishing any colours but blue and yellow.
The cases of colour-blindness where more than two colours are
distinguishable, are much more common. Goethe, the great German poet,
who dabbled a great deal in optics, knew two young men who, although
they possessed powerful sight, and could distinguish between white,
black, grey, yellow, and orange, were at a loss when the shades between
dark red and rose-colour were in question. A piece of dried carmine
appeared bright red to them, and a faint carmine hue on a white shell,
and a rose-leaf, light blue; the leaves of trees and grass appeared
yellow, and they confounded rose-colour, blue, and violet together.
Goethe supposed them to be incapable of perceiving blue and its several
hues, and called their defect by a high sounding Greek name,
_akyanoblepsy_, or blue-blindness. Péclet mentions two other persons,
also brothers, who likewise were incapable of distinguishing between
blue, violet, and rose-colour. Like Professor Whewell, they confounded
the dull scarlet of the trousers of the French infantry with the leaves
of the trees. Yellow appeared to them more brilliant than any other
colour. Doctor Sommer and his brother could not distinguish between red
and its derivatives and other colours; they could only distinguish
between yellow, blue, white, and black. Doctor Nicholl mentions a child
that could only see red, yellow, and blue, in the spectrum. It could
distinguish green, but called it brown when it was dark, and pink when
it was pale. The same physiologist knew a man who called red green, and
brown dark green. A young lady who was an amateur artist, could not
perceive a piece of scarlet cloth hanging on a hedge that was close to
her, although others could see it plainly half a mile off. One day she
gathered, as a great curiosity, a lichen which she supposed to be of a
bright scarlet hue, but which was in reality of a beautiful green.
Another time she could see no difference between carmine and prussian
blue. A gardener living at Clydesdale, who began life as a weaver, was
compelled to give up his first trade because in daylight he confounded
all light colours; yellow and its varieties he could distinguish
perfectly, but he was incapable of seeing any difference between red,
blue, pink, brown, and white. Another man, who was a silk-weaver, had to
change his trade, because he could not distinguish between red, pink,
and sky blue. A Genevese artist whom circumstances compelled to paint a
portrait by candle-light, used yellow for pink in laying on his flesh
tints, with a pleasing result that may be readily imagined. In fact, the
instances of colour-blindness mentioned by physiological writers are
almost innumerable, and I should only weary my readers if I related all
the authentic cases of this singular affliction. One instance, however,
which was very carefully observed by Wartmann, a distinguished German
oculist, merits our attention. The afflicted person, whom Wartmann
speaks of as D., was thirty-three years old. Those of his brothers and
sisters whose hair was fair suffered from the same infirmity, but those
whose hair was dark were exempt from it. Like so many others who are
colour-blind, he could not distinguish between cherries and their
leaves, and confounded a sea-green piece of paper with a scarlet ribbon
placed near it. A rose of the ordinary hue appeared greenish-blue. Being
anxious to see if reflected, refracted, and polarized light exercised a
different action on his retina, Wartmann tried him first with the
prismatic spectrum, but he could only distinguish four colours,—blue,
green, yellow, and red. He could distinguish perfectly the peculiar
black lines seen crossing the spectrum in certain places, and known by
the name of Fraunhofer’s lines. He then placed in his hands thirty-seven
pieces of differently coloured glass, but he could only distinguish four
varieties. The colours produced by polarized light seemed to give the
patient quite as much trouble as those produced in the ordinary way.
Chocolate brown appeared reddish brown; purple, dark blue; and violet, a
dirty blue. When colours were illuminated by sunlight, they seemed to
him to be redder than usual, even green and blue appearing red.
In considering cases of colour-blindness, it is very difficult not to be
misled into using wrong terms, as applied to colour, for we have no
possible means of knowing what colour it is that is really seen by the
patient. Thus, for instance, Dr. Whewell could not distinguish between
red and green. But what colour did he really see? Did he see the leaves
and cherries both red or both green, or was it some colour between the
two that was impressed upon his retina? Again, great care must be
exercised in placing implicit reliance on the statement of persons who
are colour-blind, for we must recollect that their only means of
conveying the results of what they experience is by the use of an organ
that is confessedly defective, and which is quite likely to deceive
them, and us too, without their being parties to the deception.
The cause of colour-blindness is completely unknown; philosophers and
physiologists are still in the realms of hypothesis concerning this
peculiar optical defect. As yet, the most careful observation has failed
to detect any difference between the eyes of those who are colour-blind,
and the eyes of ordinary persons, that could in any way account for this
singular affection of the sense of sight.
------------------------------------------------------------------------
CHAPTER VI.
ILLUSIONS CAUSED BY LIGHT ITSELF.
WHEN playing about the Christmas fire, children frequently amuse
themselves by whirling round and round a piece of wood, one end of which
they have previously lighted and blown out. In proportion as the
movement becomes more rapid, the path of the red-hot end becomes more
and more connected, until at last a burning ring is formed, in every
part of which the shining charcoal appears to be at the same time. The
only way of accounting for this illusion is by supposing that the image
formed by the burning stick upon the retina remains there for an
appreciable period, the impression made by it at one part of its journey
remaining until it returns to its former position. The power possessed
by the retina of retaining impressions explains a large number of
illusions of the same kind. The chord of a musical instrument, for
instance, when struck, appears to occupy a longer space during the time
it vibrates, than when it is at rest. A rapidly revolving wheel appears
almost solid on account of the combined images of the spokes seeming to
unite into one homogeneous mass.
The persistence of luminous impressions upon the retina has given rise
to the invention of a number of well-known optical toys, amongst which
may be mentioned the _phenakistiscope_, the _thaumatrope_, the
_phantascope_, and many others.
[Illustration:
FIG. 3.—The Phenakistiscope.
]
The phenakistiscope may be described (figs. 3 and 4) as consisting of an
iron pin _a b_ turning easily on its axis, and passing through two holes
in a brass rod _t g_, bent twice at right angles. Attached to one end of
the pin is a disc of cardboard, divided into several equal sectors, and
pierced near its circumference with as many similar sized rectangular
holes (fig. 4). In each sector the same scene is represented, with this
difference, that the movements of the objects are so arranged as to be
progressive from one extreme to the other. The disc being fastened to
the pin _a b_ (fig. 3) by the screw _v_, with the figures facing
outwards towards _a_, the whole apparatus is held before a looking-glass
by the handle _m_. If the disc be now rotated by the button _b_, and the
eye placed opposite one of the square holes in the card, the figures on
the disc will appear to move more or less quickly according to the rate
at which it is rotated. The three bricklayers in fig. 4 will be seen to
pass their bricks from one to the other with perfect regularity if the
drawing has been made carefully. Numberless other designs may be made
for this little instrument, such as a windmill in full sail, a man
working a pump, a conjurer swallowing knives—in fact, any scene with
objects in motion may be drawn, and will cause infinite amusement for
the long winter evenings.
The time during which the impression of any object remains upon the
retina appears to be in direct proportion to its brilliancy. For a
burning coal it is stated to be about the tenth of a second;
consequently, if the stick mentioned at the beginning of the chapter is
rotated ten times in a second, a continuous luminous ring will appear to
be formed. That the time necessary for producing a distinct impression
varies with the brilliancy of the object, may be readily guessed from
the fact that an electric spark is perfectly visible, although its
duration can hardly be measured, while a cannon-ball in flight is only
perceptible to the practised eye of the artilleryman, owing to its
reflecting only a small quantity of very diffused light.
[Illustration:
FIG. 4.—Disc of the Phenakistiscope.
]
The second instrument, the thaumatrope, is constructed on the same
principle. It consists of a certain number of circular discs of card
three or four inches across, which are capable of being turned on their
axes with great rapidity by means of the finger and thumb and a couple
of silk threads fixed at opposite sides of their circumference. On each
of these discs a design is painted, one-half appearing on one side, and
the other half on the other, in such a manner that the two parts form a
single picture. You may have, for instance, Harlequin on one side and
Columbine on the other, but on turning the card you will see them
together. The body of a Turk may be drawn on one side and his head on
the other, and, by rotating the card, the head suddenly finds a pair of
shoulders to fit it. A sentence may be divided in the same way, or the
words, or even the letters, may be divided between the opposite sides of
the card: in fact, like the phenakistiscope, the designs applicable to
this little instrument are endless.
The third of these instruments, the phantascope, is constructed in
accordance with the peculiar power possessed by the eyes of adapting
themselves to the distance of the objects they are looking at. Everybody
must have noticed that in order to see objects plainly that are placed
at different distances we insensibly alter the position and focus of the
eyes, and that, consequently, objects even in the same plane as those we
are looking at are not perceived by us until something calls our
attention to them, and causes us to alter the position and focus of our
eyes and fix our gaze on them. For instance, in looking at a canary in a
cage, we have but a confused idea of the wires, which we will suppose to
be midway between the bird and the observer. But if anything attracts
our attention to the wires we lose sight of the bird, or at any rate see
it only as a confused mass. If this experiment is made with care, it
will be perceived that the object seen confusedly is always double,—a
fact that may be verified by interposing the finger between the eyes and
any object. When we look at the finger, the distant object will seem to
be doubled; if we look at the object, it is the finger that undergoes
duplication.
We know by experience that when we look at an object and press one of
the eyeballs slightly with the finger, the image of it becomes doubled.
The explanation of this phenomenon is not very easy, but it is generally
supposed that in the case of ordinary vision the two eyes produce the
sensation of a single image in consequence of the two impressions being
formed at corresponding parts of each retina, and that habit causes us
to see only a single object in such a case. But when the eyes are so
disposed as to be capable of seeing distant objects distinctly, the two
images formed by a near object are no longer found in the corresponding
portions of each retina, and so produce the sensation of double vision.
The same thing happens when either of the eyes is momentarily displaced.
These phenomena have given rise to the construction of a very simple
instrument, the phantascope, with which many interesting experiments may
be performed, and which was invented some years since by Dr. Lake, an
eminent physician of New York.
In the middle of one of the edges of a thin piece of wood, say six
inches or a foot in length, which serves as a base for the instrument,
is fixed a rod fourteen or sixteen inches long, upon which slide a
couple of ferules capable of being fixed at any height by means of
thumb-screws. Each of these ferules holds a piece of cardboard five or
six inches long, and of any convenient breadth, in a horizontal
position. The upper card is pierced in a longitudinal direction with a
slit rather less than a quarter of an inch broad, and about three inches
long; that is to say, a little wider than the distance between the
centres of two eyes. The second card has a similar slit of the same
length, and corresponding vertically with the one above it; the width,
however, in this instance being only about the eighth of an inch. In
addition, the lower card should be marked with a fine line drawn across
the centre, which we shall call the index.
Things being thus arranged, if we place two similar objects—two A’s, for
instance—upon the wooden stage of the instrument, about three inches
apart, and look at them through the two slits, we shall see them as
under ordinary circumstances; but on fixing our eyes intently on the
index of the lower card, and gradually raising it, we shall see the two
A’s become double, the two images of each letter separating themselves
more and more the nearer the lower card approaches the upper one, until
the last two of the images will coalesce, and appear to be placed on the
lower cardboard, the other two remaining in their proper place. The eyes
must be kept firmly fixed upon the index, otherwise the illusion
disappears immediately, and two A’s only are seen in their true position
on the base of the instrument. This is an instance of the production of
an image in a place where it certainly does not exist. This illusion is
seen best when the upper screen is about ten inches from the object, the
lower screen being just half-way between; but, as in most of these
cases, the distances will differ according to the focus of the
observer’s eyes. The proper distances once being found, the experiment
may be varied in a hundred different ways. For example, instead of two
letters and a line we may have two flowers on the stage, and the figure
of a flower-pot on the intermediate screen. If the two flowers are
painted different colours, they will unite and form a mixed tint. Thus a
red and yellow flower will give an orange image, a blue and yellow a
green image, and so on. A perpendicular stroke and a horizontal one will
give a cross. A few experiments with this little instrument will throw a
light upon many of the obscurer points that exist amongst the phenomena
of vision, and will show conclusively that the two eyes rarely see in
the same manner, and that it is sometimes one, and sometimes the other,
that sees most distinctly. A couple of pieces of cardboard, pierced with
suitable slits and held in the hand may be substituted for the apparatus
above described, but of course they will be much more difficult to use,
and will give less satisfactory results.
------------------------------------------------------------------------
CHAPTER VII.
THE INFLUENCE OF THE IMAGINATION.
THE above facts show plainly that optical illusions find their source in
the very mechanism of the organs of sight, and that without going
farther than the eye itself we may discover numberless examples of these
phenomena. We shall presently bring before our readers the innumerable
means devised by art for deceiving the sense of sight and impressing us
with sensations that are purely imaginary. But before describing these
numerous pieces of apparatus we must still remain for a short time
within the domain of man’s faculties, and describe some of the illusions
that we are subjected to by those powers of the imagination that are
supposed to hold in check the five senses of the body. Our imagination,
however, plays us as many tricks as our eyes, and, like them, is
alternately false and true. Touch, taste, smell, hearing, and sight, are
all supposed to be under its powerful influence for good or evil; but
they are all deceived by it in turn, more especially the sense of sight,
which we generally boast of as being the most trustworthy of them all.
Were we to describe all the labyrinths into which our imagination is
continually leading us, we might easily extend this little volume to one
of treble the size. But our purpose is not so much to write a history of
all the hallucinations to which the imagination is subject, but to cull
from those already existing the most interesting instances in which this
great faculty is alternately the victim and the tyrant of the sense of
sight.
Amongst many works on this subject we may cite that of Brière de
Boismont on “Hallucinations, Apparitions, Visions, &c.,” from which we
shall draw largely in the following pages. The examples we shall give
will be those only in which the victims of the hallucination were in the
full enjoyment of their mental faculties, and could healthily analyze
the sensations and impressions to which they were subjected.
One of the first of these bears upon those diseases of the eye to which
allusion was made in Chapter IV. Towards the end of 1833, a poor
washerwoman who was tormented grievously with rheumatic pains gave up
her business, and took to sewing for her livelihood. Being but little
accustomed to this kind of work, she was compelled to sit over her
needle late at night in order to save herself from starving. The
unwonted strain upon the eyes soon brought on ophthalmia, which speedily
became chronic. Nevertheless, she continued her work, and fell a prey to
_diplopia_, or double sight in each eye. Instead of a single needle and
thread, she saw four continually at work, everything else about her
being similarly multiplied. At first she took no notice of the singular
illusion, but at last both imagination and sight joined arms against the
judgment, and the poor creature imagined that Providence had taken pity
on her forlorn condition, and had worked a miracle in her favour by
bestowing on her four pair of hands in order that she might do four
times her usual amount of work.
The following is another instance of the passage of illusion into
hallucination. A man fifty-two years old, of a plethoric constitution,
after having suffered from a defect in his visual functions that caused
him to see objects sometimes double, and at others upside down, suddenly
showed signs of cerebral congestion, and threatened apoplexy. By proper
treatment, however, he was saved for a time from the latter catastrophe,
but he became permanently afflicted with strabismus, or squinting, and
he suffered from a singular hallucination. His eyelids would contract,
and his eyeballs would roll from side to side at more or less distant
intervals. On these occasions he imagined he saw the figures of
different persons that he knew moving about, and would even follow them
outside his door into the other rooms of the house. He was perfectly
aware that these appearances were merely the effect of the imagination,
but this did not in any way detract from their appearance of reality.
The man afterwards died from an attack of apoplexy.
The following examples are also cases of singular optical deception,
some of them being so extraordinary as to trench upon the supernatural,
and in the days of ignorance would have given those who were their
victims the character of unearthly personages.
A certain English painter, who in some sort inherited the palette of Sir
Joshua Reynolds, and believed himself superior in many respects to the
great master, used to boast that in one year he painted over three
hundred portraits, large and small. This fact seemed to Wigan a physical
impossibility, and he questioned him closely as to the secret of his
astonishing rapidity of execution, for he never required more than one
sitting from his patrons. Wigan states that he saw him paint a miniature
of a well-known personage in eight hours, which was incomparable in its
fidelity to nature and finished execution. Wigan asked him to give him
some details of the method he adopted, and he gave him the following
answer: “When a sitter presents himself, I look at him attentively for
half an hour, sketching the outlines of his features on my canvass
during the time. I have no occasion for a longer sitting, and I pass on
to some one else. When I wish to continue the first portrait, I take the
sitter in my imagination, and I seat him in the chair, where I see him
as distinctly as if he were really there, and I can even heighten a
tint, or soften down a clumsy form at will, without altering the
likeness. I look from time to time at the imaginary figure, and I go on
painting. I stop now and then to examine his position, absolutely as if
the original were before me; for every time I look towards the chair I
see the sitter. This method of proceeding has rendered me very popular;
and as I have always succeeded in catching the likeness of my patrons,
they have been simply enchanted at my sparing them the tedious sittings
exacted by other painters. Little by little I have begun to lose the
distinction between the real and imaginary sitter, and I have often
maintained stoutly that my patrons had already sat to me on the previous
day. At last I became convinced that it was the real sitters that I saw,
and thenceforth all became confusion. I suppose my friends took alarm at
my hallucinations, for I remember nothing of what happened during the
thirty years that I remained in the madhouse. This long period has left
no trace on my memory, except indeed the last six months of my
confinement. It seems to me, however, that when my friends talk of
having visited me I have some vague recollection of the fact; but it is
a subject that I do not care to pursue.”
The most remarkable feature of the case is, that this artist after a
lapse of thirty years resumed his pencil, and painted almost as well as
when he was forced by madness to abandon his art.
This faculty of being able to evoke shadows, with which to people one’s
solitude, may be carried so far as to transform real persons into
phantoms. Hyacinth Langlois, a distinguished artist, living at Rouen,
tells us that Talma, with whom he was extremely intimate, confided to
him that, whenever he went upon the stage, he had the power, by mere
force of will, to cause the clothes and flesh of his numerous auditory
to disappear, and become transformed from living beings into so many
skeletons. When his imagination had peopled the house with these
singular phantoms, the emotion he felt was so great that it gave his
dramatic powers still greater force, and enabled him to produce the
wonderful effects that have made his name so famous.
Wigan says, that he once knew a most intelligent and amiable man, who
could at will evoke his own image. He often laughed at seeing his second
self standing before him, the phantom appearing to laugh as heartily as
himself. This illusion was for a long time a matter of amusement to him,
but at last he became persuaded that he was haunted by his own double.
His second self appeared to hold arguments with him continually, and
beat him frequently on various points of dispute, a matter which
mortified him excessively, as he was rather proud of his powers of
reasoning. This gentleman, although always considered as being somewhat
eccentric, was never put under the slightest restraint, and at last the
creature of his imagination so tormented him, that he resolved not to
live through another year. He consequently paid all his debts, arranged
his affairs, and waited pistol in hand until the clock struck twelve on
the 31st of December, and then deliberately blew out his brains.
In _Abercromby on the Mind_ we read an account of the observations made
by a gentleman who was the victim of illusions during the whole of a
pretty long life. If he met a friend in the street, he was unable to
tell at first whether he saw a real human being or only a phantom. By
close examination he could detect a difference between the real person
and the creature of his imagination, the features of the former being
sharper and more defined than those of the phantom; but in general he
was obliged to test the reality of the figure he saw by the senses of
touch and hearing. He was able, by concentrating his thoughts upon the
appearance of any friend, to call up his image; a power which extended
even to scenes that he had witnessed. Although he could produce these
hallucinations at will, he was powerless in making them disappear; and
when once he succeeded in calling forth these creatures of his
imagination, he never could tell how long the delusion would last. This
gentleman was in the prime of life, a good man of business, and
otherwise in a perfect state of mental and bodily health. A member of
his family possessed the same faculty, but in a minor degree.
In 1806, General Rapp, when returning from the siege of Dantzic, having
occasion to speak to the Emperor Napoleon, walked into his private room
without being announced, and found him in such a profound state of
abstraction, that he remained for some time unperceived by his imperial
master. The General, seeing him thus perfectly motionless, fancied he
must be ill, and purposely made a slight noise. Napoleon instantly
turned his head, seized the General by the arm, and pointing upwards,
exclaimed, “Do you see it up there?” The General, hardly knowing what to
say, remained silent; but the Emperor repeated his question, and he was
obliged to reply, that he saw nothing. “What,” said the Emperor, “you
don’t see anything? You don’t see my star shining before your eyes?” And
becoming more and more animated, he went on to say, that the mysterious
visitor had never abandoned him, that he saw it throughout all his great
battles, that it always led him onward, and that he was never happy but
when he was gazing at it.
That such hallucinations have no real existence as far as the eye goes,
is proved by the fact of many people who have lost their sight, being
subject to them. It is hardly to be wondered at that those who by
accident have been deprived of their sight, should wish so ardently to
see once more the persons and sights they have taken pleasure in, that
they should at last create for themselves illusions of this character.
The same thing has frequently occurred with those whose sight is more or
less weak. An old man of eighty, who was purblind, never sat down to a
table during the last years of his life, without seeing around him a
number of his friends who had long been dead, dressed in the costume of
fifty years before. This old man had but one eye, which was extremely
weak, and wore a pair of green preservers, in the glass of which he
continually saw his own face reflected.
Doctor Dewar, of Stirling, mentioned to Abercromby a very remarkable
instance of this species of hallucination. The patient, who was quite
blind, never walked in the street without seeing a little old woman
hobbling on before him and leaning on a stick. This apparition always
disappeared when he entered his house.
Similar illusions frequently happen to every one, even the most healthy
amongst us, but a little consideration soon puts them to flight. It
would be useless to mention the numberless cases in which a square tower
has appeared round, or where the landscape has suddenly seemed to recede
from the sight. Such illusions as these have been long well known, and
appreciated at their proper value; but there are others whose true cause
has remained a mystery, until explained by the progress of science, such
as the Spectre of the Brocken, the Fata Morgana, and the mirage.
Analogous appearances have been seen in Westmoreland and other
mountainous districts, the inhabitants imagining that the air was full
of troops of cavalry, and whole armies even; such illusions resulting
simply from the shadows of men and horses passing over an opposite
mountain being thrown on the fog.
A vast number of different circumstances give rise to these illusions,
such as a strong impression, or the recollection of some striking event,
which may easily cause them, by the association of ideas. Wigan relates,
that being at a _soirée_ held at the house of M. Bellart, a few days
after the execution of Marshal Ney, the groom of the chamber, instead of
calling out the name of _M. Maréchal aîné_ (M. Maréchal, senior),
announced the arrival of M. le Maréchal Ney. A shudder passed through
the company, and many of them declared, that for an instant they saw the
face and figure of the dead man in place of those of his involuntary
representative.
When the mind is thus prepared, the most familiar objects are
transformed into phantoms. Ellis relates an anecdote of this kind, which
he heard from an eye-witness, who was a ship’s captain of
Newcastle-on-Tyne. During a voyage that he made, the ship’s cook died.
Some days after the funeral, the chief mate came running to him in a
great fright, with the news that the ship’s cook was walking on the
water, astern of the vessel, and that all the crew were on deck looking
at him. The captain, who was angry at being disturbed with so
nonsensical a tale, answered sharply, that they had better put the ship
about and race the ghost to Newcastle. His curiosity, however, was
presently aroused, and he went upon deck and looked at the spectre. He
frankly avowed that for some moments he saw what really appeared to be
his old shipmate, just as he knew him in life, with his walk, clothes,
cap and figure perfectly resembling those of the dead man. The panic
became general, and every one was struck motionless for a time. He had
the presence of mind, however, to seize the helm and put the ship about,
when as they neared the ghost, they found the absurd cause of their
fright was a broken mast from some wreck, which was floating after them
in an upright position. If the captain had not boldly sailed up to the
supposed ghost, the story of the dead cook walking upon the water would
have continued to this day to terrify half the good inhabitants of
Newcastle.
Such facts as these are innumerable, and we shall mention a few more
which will explain a host of stories found in various ancient and modern
authors.
Ajax was so angry at the arms of Achilles being awarded to Ulysses, that
he became furious, and, seeing a herd of pigs, drew his sword and fell
upon them, taking them for Greeks. He next seized a couple of them and
beat them cruelly, loading them at the same time with insults, imagining
one of them to be Agamemnon, his judge, and the other Ulysses, his
enemy. When he came to himself, he was so ashamed at what he had done,
that he stabbed himself with his sword.
Theodoric, blinded by jealousy and yielding to the base solicitations of
his courtiers, ordered that Symmachus, one of the most upright men of
his time, should be put to death. The cruel order had hardly been
executed, when the king was seized with remorse, and bitterly reproached
himself with his crime. One day a new kind of fish was put upon the
table, when the king suddenly cried out that he saw in the head of the
fish the absolute resemblance of that of his victim. This vision had the
effect of plunging the king into a state of melancholy that lasted his
whole life.
Bessus once, when surrounded by his guests and giving himself up to the
enjoyment of the feast, ceased suddenly to listen to the flattering
speeches of his courtiers. He apparently listened with great attention
to some sound that was heard by no one else, and suddenly leaping from
his couch, mad with rage, he seized his sword and rushing at a swallow’s
nest that was near, beat it down, killing the poor birds inside it,
crying out that these insolent birds dared to reproach him with the
murder of his father. Surprised at such a sight, his courtiers gradually
disappeared, and it became known some time afterwards that Bessus was
really guilty, and that the senseless action he had performed simply
resulted from the voice of conscience.
The illusions of sight and hearing are often found to take an epidemic
form, and historians relate an immense number of anecdotes bearing on
this particular phase of self-delusion. One of the commonest of them is
that which transforms the clouds into armies and figures of all kinds.
Religious prejudices, optical phenomena, physical laws that are still
unknown, dangerous fevers, derangements of the brain, afford a natural
explanation of these hallucinations.
We have borrowed most of these examples from Brière de Boismont’s works,
for the special purpose of showing how easy it is to deceive the
imagination, and to demonstrate the facility with which the sense of
sight is led astray without the intervention of complicated apparatus.
In addition, we may quote instances from Brewster, showing the ease with
which the imagination enables us to see distinct forms in a confused
mass of flames, or in a number of shadows superposed upon each other.
This great philosopher gives us an anecdote of Peter Heamann, a Swedish
pirate and murderer. One day that his crew were repairing some
unimportant portion of the ship, after having pitched the place well he
took the brush in order to tar the other parts of the vessel, which were
much in want of such treatment; but as soon as he spread the pitch over
the timbers of the ship, he was thunderstruck at seeing apparently
reflected in its shining surface the image of a gallows with a headless
man beneath. The head belonging to the body was lying before it, and the
body itself was depicted with every limb—legs, thighs, and arms—perfect.
He frequently told his crew of these illusions, adding that it was
evidently a prediction of the fate in store for them. He was often in
such a state of terror, that on calm days he would drop down into the
hold and wrap himself up in a spare sail in order not to catch sight of
the horrible image that he constantly saw in the shining surface of the
tar.
The imagination really seems to create for itself a sort of mental
visual organ which is in intimate relation with that of the body, and
which often takes its place so efficiently—as in the case of dreams—that
the mind is utterly unable to perceive the substitution. It is on
account of this that practical opticians are so unsparing in their
endeavours to predispose their spectators to being deceived.
When both the body and mind are healthy, the relative intensity of the
two kinds of impressions is very unequally divided, mental images being
more evanescent and comparatively weak, and with persons of ordinary
temperament incapable of effacing or disturbing the reflections of
visible objects. The affairs of life could not go on if the memory
introduced amongst them brilliant representations of the past in the
midst of ordinary domestic scenes or the objects familiar to us. We may
account for this by supposing that the set of nerves which carries the
efforts of the memory to the brain cannot execute their functions at the
same time as those which take cognizance of the images reflected on the
retina. In other words, the mind cannot accomplish two separate
functions at one and the same time, and the mere act of directing the
attention to one class of subjects causes all others to become instantly
imperceptible. The exercise of the mind in these instances is, however,
so rapid that the alternate appearance and disappearance of the two
different impressions is completely unnoticed. Thus, for instance, while
looking at the dome of St. Paul’s, if our memory suddenly evokes the
image of some other object, Mont Blanc for instance, the picture of the
cathedral, although still depicted on our retina, is momentarily effaced
by the effort of the will, although we may not change the position of
our eyes during the time. While the memory continues to dwell on the
picture it has called up, it is seen with sufficient distinctness,
although its details may be somewhat misty and its colours confused; but
as soon as the wish to see it passes away the whole disappears, and the
cathedral is seen in all its former distinctness.
In darkness and solitude, when surrounding objects produce no images
that can interfere with those of the mind, these latter are more lively
and distinct: and when in addition we are half asleep and half awake,
the intensity of mental impressions approaches that of visible objects.
In the case of persons of studious habits who are continually employed
in mental effort, these images are more distinct than with those who
follow the ordinary avocations of life, and during their working hours
rarely see the objects round them. The earnest thinker, absorbed by
meditation, is in a manner deprived for the time of the use of his
senses. His children and servants pass in and out of his study without
his seeing them, they speak to him without his hearing them and they may
even try to rouse him from his reverie without success; and yet his
eyes, ears, and nerves received the impression of light, sound, and
touch. In such instances, the mind of the philosopher is voluntarily
occupied in following out an idea which interests him profoundly; but
even the most unlearned and thoughtless of us sees the images of dead or
absent friends with his mind’s eye, or even fantastic figures which have
nothing to do with the train of thought he may be pursuing. It is with
these involuntary apparitions as with spectres of the imagination:
although they are intimately connected with some thought that has passed
through our mind unperceived, it is impossible to trace a single link of
the chain connecting them together.
------------------------------------------------------------------------
PART II.
THE LAWS OF LIGHT.
--------------
CHAPTER I.
WHAT IS LIGHT?
EVERYBODY knows the effects of the action of light, without, however,
understanding precisely what constitutes light itself. Any formal
definition would rather puzzle than help the student; we must therefore
content ourselves with saying that light is that effect of force which
causes us to perceive external objects.
A man who was blind from his birth, and upon whom the operation for
cataract had been successfully performed, had accustomed himself for a
long time to imagine the nature of those unknown phenomena that his
affliction had prevented him from observing. He had arranged in his mind
the various definitions that had been given to him as to the nature of
light, and having combined them, he fancied he had acquired some notion
of what the sense of vision really meant. But what was the astonishment
of the surgeon who had restored to him his fifth sense, when he asked
him to give his opinion upon the effects of light, to see him take up a
lump of sugar and reply that it was under that form that he had imagined
it to himself.
As for us who have the happiness of possessing the sense of sight, we
know this mysterious agent more by the enjoyment that we have derived
from it, than from any analysis we have made of its nature. It is an
endless chain that connects us with the entire universe; a bond that
laughs at distance and spans the abysses of space. By means of light we
can appreciate the beauties of hue and form, and by its power we touch
as it were the inaccessible. It constitutes the most intimate connexion
between ourselves and external objects—a connexion that seems even to
alter our temper, disposition, and character, according to the
variations of its intensity. The dull and foggy days of winter, those
days when sleet and rain struggle in the atmosphere, spread like a veil
over us, and throw a shadow upon our life. The return of the bright
spring sun, the reappearance of light and blue sky, on the contrary,
open up our hearts and minds, gay nature enchants us once more, and a
feeling of fresh happiness prepares us for the coming glories of the
newly risen year.
This intimate connexion between the light of heaven and the human mind,
hallowed as it is by our desire to rise towards the Source of all light,
might be made the subject of many eloquent pages; and it would be an
interesting and useful task to show the gradual progress of mankind from
those ancient people who trembled at the approach of darkness, and who
fervently saluted the dawn with prayers and praises, down to the
philosophers of the present age, who investigate its effects with so
much reverential joy. But we must cease paying any more attention to the
superficial action of this marvellous force which in these latter days
has become, in the hands of man, the source of so many illusions and the
origin of a complete world of rich and brilliant pictures, but which
after all only exist in the imagination.
It was believed for a long time that light was a compact mass of tiny
particles emitted by luminous bodies, which struck our eyes and so
produced the phenomenon of vision. These particles or molecules were
naturally thought to be extremely minute, and the objects illuminated by
them were supposed to throw them off as if they were endowed with
elasticity. Under this hypothesis, light was a material body. The
illustrious Newton was the first propagator of this theory; the last was
M. Biot, a French philosopher, lately dead.
The undulatory theory has now-a-days completely superseded the
corpuscular hypothesis. It was first started about the year 1660 by the
Dutch philosopher Huyghens, who has left behind him numerous treatises
on optics, and the properties of light, as well as a curious account of
the inhabitants of the other members of the solar system, including a
minute description of the various planetary manners and customs. At the
beginning of the present century, Fresnel showed, by the most brilliant
discoveries the superiority of this theory, and shortly after Arago
confirmed him in his demonstrations. According to the undulatory
hypothesis, light is not a mass of molecules emitted by a luminous body,
but simply the vibration of an elastic fluid which is conceived to fill
the whole of space. A comparative example may assist you in
understanding this theory more clearly. If you throw a stone into a
smooth piece of water, there will form around the point where the stone
fell, a series of circular undulations, starting from the centre and
gradually enlarging themselves. If a loud noise is suddenly heard, the
same effect is produced round the point from whence the sound proceeds.
A series of waves are formed which spread not only horizontally, as on
the surface of the water disturbed by the stone, but in every direction.
In fact, in the case of sounds, the waves are so many gradually
increasing spheres. In the case of light, when a luminous body is placed
in space, the ether which surrounds it is thrown into a state of
vibration, and the motion is immediately propagated in all directions,
with extreme velocity. It is these undulations that produce upon our
eyes the sensation of light. We may therefore say that light, like
sound, is movement, while darkness, like silence, is absolute rest.
Many people still believe that light is propagated instantaneously, and
cannot bring themselves to imagine that we do not see a flame the moment
we light it, but only an instant after. I have myself spoken to
well-educated people possessed of good judgment and a certain amount of
elementary knowledge, who could never bring themselves to believe that
we see the stars, not as they now exist, but as they appeared at the
particular moment when the luminous wave by which we are enabled to
perceive them left their surface, and which only reaches us after
travelling through space a certain number of years, days, or hours,
according to their distance. It is extremely useful and interesting to
form a correct idea upon the way in which light is propagated.
The determination of the prodigious quickness with which the waves of
light move through space, says Arago, is undoubtedly one of the happiest
results of modern astronomy. The ancients believed that it moved with
infinite velocity, and their view of the subject was not, like so many
of the questions relating to physics, a mere opinion without proof; for
Aristotle, in mentioning it, brings forward the apparently instantaneous
transmission of daylight. This notion was disputed by Alhazen, in his
_Treatise on Optics_, but only by meta-physical weapons, which were
again opposed by several very worthless arguments, by his commentator,
Porta, although he admitted the immateriality of light. Galileo seems to
have been the first amongst modern philosophers who endeavoured to
determine the velocity of light by experiment. In the first of his
dialogues, _Delle Scienze Nuove_, he announces by the mouth of Salviati,
one of the speakers present, the ingenious means he had employed, and
which he thought quite sufficient to solve the question. Two observers
with lights were placed at the distance of one mile from each other; one
of them extinguished his light, and the other as soon as he perceived it
extinguished his. But as the first observer saw the second light
disappear the instant he had extinguished his own, Galileo concluded
that light was propagated instantaneously through a distance double that
which separated the two observers. Certain analogous experiments that
were made by the members of the Academy _Del Cimento_, but at three
times the distance, led to precisely the same conclusions.
These attempted proofs seem at first sight to be absurd, when we think
of the vastness of the problem to be solved; but we must judge these
experiments with less severity, when we consider that almost at the same
epoch, men of such well-deserved repute as Lord Bacon believed that the
velocity of light, like that of sound, was sensibly altered by the force
and direction of the wind.
Descartes, whose theories upon light had so much analogy with those
known under the name of the undulatory hypothesis, believed that light
was transmitted instantaneously throughout any distance, and endeavours
to prove his position by proofs that he thought he had obtained whilst
observing an eclipse of the moon. It must be acknowledged, however, that
his very ingenious train of reasoning proves that whether the
transmission of light is instantaneous or not, it is at least too
considerable to be determined by experiments made on the earth, like
those of Galileo, and which he vainly hoped would have solved the
question.
The frequent occultations of the first satellite of Jupiter, the
discovery of which was almost consequent upon that of lenses, furnished
Römer with the first means of demonstrating that light was propagated by
perceptible degrees.
In tracing out the history of human knowledge, says Dr. Lardner, we have
frequently to point out with some little surprise, joined to a feeling
of profound humility, the important part played by chance in the
advancement of science. In searching zealously after mere trifles which,
when found, are of no consequence, we frequently lay our hands on
inestimable treasures. The frequency of this fact impresses the mind
with the notion that some secret and unceasing power exists, in
accordance with which human knowledge and science are continually
progressing. It is in physical, as in moral philosophy. In our
ignorance—like the dog mentioned by Æsop, which, seeing in the water the
reflection of the prey it held in its mouth, dropped the substance and
tried to seize the shadow—we are continually searching after trifles;
but, more fortunate than the animal of whom we have been speaking, the
shadow that we try to seize is often transformed into a rich treasure.
We can say with every confidence that “the Providence which shapes our
ends,” knows our wants better than we do ourselves, and bestows on us
the things we _ought to have_ asked for instead of those we _have_ asked
for. We shall find a very simple proof of this in the history of the
discovery of the velocity of light.
A short time after the invention of the telescope and the consequent
discovery of Jupiter’s satellites, Römer, a celebrated Danish
astronomer, was engaged in a series of observations, the object of which
was to determine the time which one of these bodies took to revolve
round its planet. The method employed by Römer was to observe the
successive occultations of the satellite, and to notice the interval
that elapsed between each of them. But it at last happened that the
interval between the two occultations, which was about forty-five hours,
became prolonged by periods of 8, 13, and 16 minutes, during that half
of the year when the earth was receding from the planet, while it became
proportionally cut short during the rest of the year. Römer was struck
by a happy idea; he suspected instantly that the moment when he remarked
the disappearance of the satellite was not always coincident with the
instant when it really took place, but that it sometimes appeared to
happen later—that is to say, after an interval of time sufficiently long
to allow the light that had left the satellite immediately after its
disappearance, to reach the eye of the observer. Hence it became evident
that the farther off the earth was from the satellite, the longer was
the interval of time between its disappearance and that of the arrival
of the last portions of its light upon the earth; but that the moment of
the disappearance of the satellite is that of the commencement of the
occultation, and that the moment of the arrival of the last portions of
light is that when the commencement of the occultation is observed.
It was thus that Römer explained the difference between the calculated
and observed time of the occultation, and he saw that he was on the
threshold of a great discovery. In a word, he saw that light propagated
itself through space with a certain velocity, and that the fact we have
just mentioned furnished the precise means of measuring it.
Thus the occultation of the satellite was retarded one second for every
185,000 miles that the earth is distant from Jupiter; the reason being,
that a ray of light takes a second to travel this distance, or, in other
words, because the velocity of light is at the rate of 185,000 miles per
second.
It must be remembered when considering this subject, that in any system
of undulations or vibrations, no matter through what medium they are
propagated, their movement is simply a change of form, and not a
transmission of matter. The waves which spread round a central point
when a stone is thrown into the water, give one the idea that the water
which forms the wave really moves towards the observer. But it is not
so, as may be readily proved by placing on the surface a floating body,
which we shall find is but little, if at all, influenced by the
undulations of the water. The appearance of rolling waves given on the
stage by means of a painted cloth, to which an undulatory motion is
given, is an instance of this apparent movement. In the case of the
floating body, which would follow the movements of the water, we shall
find that wave after wave rolls to the shore, in the same way as the
painted marks on the imitation sea keep their place, although the cloth
itself undulates. The waves of the sea even appear to the eye to be
endowed with a progressive motion, but an instant’s observation will
convince us of our error; for if such were the case, every object
floating on the ocean would be gradually carried on shore. A vessel
floating on the waves is not carried along by them, at least not until
it reaches within a few yards of the shore, where the water is really in
motion; but out in the open sea a floating body will alternately rise on
their crests, and fall into the valleys that separate them. The same
effect may be observed with any object floating on the water. If,
however, in addition to being in a state of undulation the sea is really
in motion from the effects of a current, or from any other cause, the
floating object will of course be carried along by it—in fact, the two
movements are quite independent of each other, and may take place in
similar or contrary directions. It is very important that we should be
able to distinguish at an early period the exact difference between true
movement and mere undulation; and we must remember that although the
waves of light are propagated at the rate of 185,000 miles a second,
still there is no transmission of any material substance at this
marvellous rate. The same observation applies to sonorous vibrations
transmitted through the air.
Thus we are constrained to admit peaceably the truth of the undulatory
hypothesis as compared with the corpuscular theory. I say _peaceably_,
because I am forcibly reminded by the contrast I have made between the
two theories of an anecdote related of one of the greatest monsters who
ever walked this earth, but who was afterwards struck down in the midst
of his power by the hand of a weak girl. I allude to the infamous Marat,
who one day presented himself at the house of Dr. Charles, a celebrated
natural philosopher, of the time of the first French Republic, in order
to advance certain notions of his own against the optical principles
that Newton has left behind in his _Principia_, and other works—also, to
oppose certain theories connected with electrical science. Dr. Charles,
who did not approve of Marat’s wild notions, undertook to convince him
of his errors. But instead of discussing the matter peaceably, Marat
allowed himself to be carried away by his temper, which was naturally
very violent. Every argument advanced by his antagonist seemed to
increase his rage, until at last he lost all control over himself, drew
his small sword, and rushed upon his opponent. The doctor, who was
unarmed, had to exercise all his powers to prevent himself from being
wounded, and being much more stoutly built than Marat, he at last
succeeded in throwing him down, and wresting his sword from him, which
he immediately took care to break. Whether it was the violence of the
fall, the shame he felt at being doubly beaten, or the effects of his
fit of passion, does not appear, but Marat fainted. Assistance was
called, and he was carried home to his house, his offence against all
the laws of propriety being forgiven by his more talented and
better-tempered adversary.
There are many persons, no doubt, whom we should astonish, and possibly
enrage, by asserting positively that we could cause darkness by means of
light, that silence could be produced by sound, or cold by heat. These
are daring paradoxes, and at first sight appear almost as reasonable as
that of Anaxagoras, a Greek philosopher, who asserted that snow was
black. But as I hope that most of my readers do not possess the
passionate temper of the French tribune, I will confide to them a little
secret that will make these paradoxes plain. It is called by natural
philosophers the theory of interference.
The experiments connected with this subject are exceedingly difficult to
perform, and require the aid of apparatus far beyond the reach of the
ordinary student. It is a case where theory and description are much
easier than practice.
If a ray of electric light is thrown upon a screen, it is possible to
direct another ray upon the same spot in such a manner that they will
extinguish each other mutually. The reason of this phenomenon may be
understood, if we remember that light is caused by undulatory movement,
and that by opposing two series of waves to each other in such a manner
that their vibrations coming in contact produce rest, we can easily see
how the waves of light of one ray may be stopped by those of a second.
Going back to our illustration of the eddies on a pool of water, it is
easy to prove that by throwing a second stone into the water we form
another series of undulations; which are mutually destroyed when they
encounter each other. It is the same with the peculiar fluid which,
existing throughout space, is thrown in a state of undulation by
incandescent bodies; by opposing one set of waves to another we obtain
rest as a result.
This fact was first observed by Grimaldi in 1665, and Dr. Thomas Young
was the first to offer an explanation. Fresnel used it with great
success at the beginning of the century to demonstrate the truth of the
undulatory theory, by showing that it could not be explained by any
other.
------------------------------------------------------------------------
CHAPTER II.
THE SOLAR SPECTRUM.
THE white light that the glorious orb of day spreads over the face of
nature is the original source of all those brilliant and sombre colours
with which the works of the Creator are beautified. To the rays of the
sun we owe not only the whiteness of the lily, but the scarlet of the
field poppy, the modest blue of the timid violet, the splendour of the
peacock’s plumage, the cool green of the meadows, and the purple and
gold of the distant mountains. For, as we have hinted before, this white
light, which seems of itself so destitute of colour, is productive of
every hue that the eye of man is capable of appreciating.
It may seem that I am bestowing too much praise upon our own sun; but if
you are surprised that I should seek to exalt this brilliant globe of
ever-burning fire, I must ask you to recollect, that though the starry
heavens are full of suns as vast and important as ours, and possibly
affording brilliant colourless light to worlds full of inhabitants,
there are others that give forth rays that are far from being white.
Some are as green as emeralds, others are as blue as sapphires, while
others give out a warm light like a ruby or topaz. The worlds which
surround these can only receive light of a certain colour, or at any
rate they are restricted to a few shades and hues. Imagine living in a
world where everything was always _couleur de rose_, or in which the
inhabitants were continually looking blue! A residence in either of them
for a short time would undoubtedly cause us to appreciate the relative
value of our own little sun, small as it is in comparison with some of
the mighty orbs floating about in space.
The fact that the light of the sun is the source of all the changing
hues to be found on the surface of the earth season after season was
first discovered by Newton, and his experiments are easily repeated with
a very few and inexpensive appliances.
A small round hole is made in the window-shutter of a room, facing the
sun, and the pencil of light proceeding from it is allowed to fall upon
the surface of a three-sided prism, held in a horizontal position, and
placed at a distance of a few inches from the aperture (fig. 5,
Frontispiece). The pencil of light does not pass through the prism as if
it were a plate of glass with parallel sides, but in virtue of the laws
of refraction, of which we have already spoken, it is turned out of its
natural course, and is thrown upon the wall in the direction indicated
in the figure. The pencil of light is not only turned aside, but it is
also widened out into a band which is truly painted with all the colours
of the rainbow, every tone and hue being of the most marvellous
brilliancy. This long coloured stripe, which constitutes one of the most
beautiful sights that the science of optics can afford us, is known to
scientific men by the name of the solar spectrum.
Before going into the causes that produce these colours, let us first
examine their number and position. Beginning at the top, we shall find
that they run in the following order:—Violet, indigo, blue, green,
yellow, orange, red. The red being lowest is called the least
refrangible of them all; or, in other words, in passing through the
prism it was bent less out of its course than its companions. Violet,
being at the top, is of course the most refrangible. The cause of the
separation of the colours of white light is consequently only the effect
of their individual character. They were, so to speak, so many streams
flowing together until an unexpected deviation in their course caused
them to separate. This change in the direction of their flow brought out
their personal individuality, and they at once became completely
disunited.
Every single tint in the prismatic spectrum is simple, and cannot be
decomposed. This may be shown by passing any of them through another
prism, when it will be found that no change will take place in the
colour or size of the pencil. Hence those worlds already spoken of,
whose light of day is red, blue, or green, never see any colours but
these. (Fig. 6, Frontispiece).
It is just as easy to reunite the colours into which white light is
decomposed, by applying a second prism in a reversed position to the
pencil of coloured light, as it is to separate them in the first
instance. The method of accomplishing this is shown in fig. 7,
Frontispiece.
[Illustration:
FIG. 8.—The Recomposition of Light.
]
Another experiment in the same direction consists in reuniting the
colours by causing them to pass through a double convex lens, behind
which is placed a screen of ground glass, or a card (fig. 8). By
advancing and withdrawing this screen we can easily find the exact spot
where the rays reunite, and form a dazzling spot of white light. This
point is called the focus, from a Latin word, signifying “fire-place,” a
term which will put the student in mind of the frequently repeated
experiment of burning a piece of paper with an ordinary
magnifying-glass.
Instead of using a lens, you can, if you please, employ a concave
mirror, using the ground glass or cardboard screen, as before. The
colours reflected by the mirror unite at its focus, and produce a
brilliant white spot in just as conclusive a manner as in the other
experiment.
[Illustration:
Fig. 9.—Recomposition of Light by means of a Concave Mirror.
]
A fourth experiment, which is somewhat more difficult for the student to
accomplish, consists in causing every one of the seven different colours
to be reflected from a separate mirror.
The mirrors in this case are concave, and are so mounted as to be
capable of being moved in any direction. By directing each of the seven
rays, one by one, upon the same point, you may observe the gradual
decomposition of the coloured light. The effect obtained by adding the
last colour to the mixture is quite magical, the white circle being
produced from two brilliantly-coloured spots.
[Illustration:
Fig. 10.—Recomposition of Light by means of a number of Mirrors.
]
A fifth experiment, first devised by Newton, is also within the reach of
the student. On a disc of cardboard the centre and border of which have
been previously painted black, are pasted seven strips of paper, painted
as nearly as possible of the same colour as the components of the
spectrum—or if the student is anything of an artist he may paint the
disc in imitation of the spectrum, carefully shading off the tints into
each other. If the disc be now rapidly rotated the colours will
disappear, and a greyish hue will be seen, which will approach more
closely to white, the nearer the colours on the disc are to those of the
spectrum. This experiment is not precisely the same in principle as the
preceding ones, for it is evident that the colours on the disc do not
mix, but only the impressions they form upon the retina. We have already
said that such impressions remain on the eye for one-tenth of a second
or there-abouts; the disc must therefore revolve at least ten times a
second, or the effect will not be perceived.
[Illustration:
FIG. 11.—Newton’s Disc.
]
From these experiments it follows that the colours with which all
natural substances are clothed, ought not to be looked upon as belonging
to them absolutely, but only as a property dependent on the reflection
and absorption of light from their surfaces. The leaves of plants, for
instance, must not be regarded as being really green in themselves, but
as being capable of absorbing certain portions of light, and reflecting
others. Grown in the dark, the green substance contained in the plant
and its leaves becomes white, and no longer possesses the property of
absorbing red light, and reflecting green. A green leaf placed in red
light becomes almost black, from its power of absorbing light of that
colour; in the blue it reflects a much greater proportion of the
coloured ray. A very striking experiment may be performed with a
substance known to chemists as the iodide of mercury. If a little of
this salt, which is of a brilliant red, be placed in a watch-glass, and
heated over a spirit-lamp, it will gradually sublime, and a card held
over it will be covered with a number of light yellow crystals. In this
case no change of composition has taken place, but simply a change in
the power the salt possesses of reflecting some rays and absorbing
others. By simply scratching the surface of the card with a pointed
piece of wood, the yellow crystals become transformed once more into the
red variety; not only this, the transformation gradually spreads, like a
red cloud, over the whole of the deposit. There are some other salts
known to chemists which possess the property of dichroism, or double
colour. The double cyanide of platinum and barium, for instance, appears
violet when viewed in one direction, and yellow in another. Change of
temperature is often sufficient to change the colour of bodies—white
oxide of zinc, for example, becomes bright yellow when heated. Such
instances might be supplied _ad infinitum_, but enough has been said to
prove that colour, after all, is only an appearance, and not an
essential property of bodies.
We have already spoken of complementary colours, or those which it is
necessary to add together in order to produce white light. Blue, for
instance, is complementary to orange, red to green, violet to yellow,
and _vice versa_. But it is not by the aid of the palette that this can
be proved, for in the case of coloured pigments the arrangement of their
atoms interferes in some way with the success of the experiment, and it
is only by means of the colours of the spectrum that such recompositions
can be effected.
Although most philosophers consider that there are seven colours in the
spectrum, there are others who do not admit it, but assert that there
are really only three, red, yellow and blue—which by the superposition
of their edges produce the intermediate hues of green and orange.
Perhaps it would be nearer to the truth to say that the spectrum is
composed of an infinite number of colours of different hues.
We have already stated that every one of these colours is
indecomposable, and that there are certain worlds illuminated by a
single colour only, instead of possessing the infinite number of tints
enjoyed by the inhabitants of the solar system. An idea of this effect
can easily be gained in a very simple but surprising manner by inserting
panes of glass of different colours in the hole of the shutter of a dark
room. If the light is yellow, you will find that all those objects that
are capable of reflecting yellow light are coloured by it, while those
which are bright red or blue become almost black by absorbing the only
light present. If we could procure an object which was perfectly
complementary in colour to the yellow glass, it would appear perfectly
black. The same experiment may be repeated with the other colours. After
remaining in this coloured light for some time, if you suddenly pass out
into daylight the complementary colour will tinge everything around you.
Instead of using a room into which coloured light only is admitted,
lamps burning with a coloured flame may be employed. Brewster mentions
the following experiment, which is a very striking one:—Fill a
spirit-lamp with alcohol in which has been dissolved as much common salt
as the spirit will take up; on being lit it will be found to burn with a
livid yellow flame. A room lighted entirely with one or two lamps of
this kind will form a laboratory for some very singular experiments. It
should, if possible, be hung with pictures in water and oil colours, and
the persons present ought to wear nothing but the brightest colours, and
the table be ornamented with the gayest of flowers. The room being first
lighted with ordinary daylight, the lamps above mentioned should be
brought in, and the daylight carefully excluded, when an astonishing
metamorphosis will take place. The spectators will be hardly able to
recognise each other; the furniture of the room, and every other object
contained in it, will reflect but a single colour. The flowers will lose
their brilliant tints, the paintings will appear as if they were drawn
in Indian ink. The brightest purple, the purest lilac, the richest blue,
the liveliest green, will be converted into a monotonous yellow. The
same change will take place in the countenances of those present; a
livid paleness will spread over their faces, whether young or old, and
those who are naturally of an olive complexion will hardly appear
changed at all. Every one will laugh at the appearance of his
neighbour’s face, without thinking that he is just as great a subject of
laughter to them. If, in the midst of the amusement caused by this
experiment, the light of day is admitted at one end of the room, the
other end being still lighted with the salt-lamp, every one will appear
to be half-illuminated with the livid colour which has caused so much
surprise, the other portion of their figure and clothes being of the
natural hue. One cheek, for instance, will appear animated with its
usual brilliancy, while the other will be that of a corpse; one side of
a lady’s dress will be brilliant blue or green, as the case may be, the
other a colour that it would puzzle an artist to give a name to. The
experiment may be varied by admitting the white light through several
small holes in the shutter of the room, every luminous spot painting the
place where it falls in its natural colours, and the yellow spectators
will become spotted with the most singular tints and hues. If a magic
lantern is used to throw on the walls of the room and the clothes of the
company any luminous figures, such as those of flowers or animals, they
will be coloured with these figures in the tint of the wall or fabric
upon which they fall, yellowish colours of course escaping the
transformation. If nitrate of strontia be substituted for the salt, a
crimson tint will be spread over everything. In fact, a lamp prepared in
this way will form a source of endless amusement. It is not necessary to
use alcohol for the purpose; wood-spirit or methylated alcohol will
serve the purpose equally well. If a lamp is not to be had, a few pieces
of cotton-wool, tied on wires and dipped in the salted spirit, will do
almost as well.
------------------------------------------------------------------------
CHAPTER III.
OTHER CAUSES OF COLOUR.
THE colours of the spectrum are to the sense of sight what the tones of
the gamut are to the sense of hearing. On the one hand, the differences
in the lengths of the sonorous waves constitute the variety of note
perceptible by the ear; on the other, the differences in the lengths of
the luminous waves constitute the variety of colour perceptible by the
eye. By and by, we shall learn both the length and rapidity of these
vibrations, but it will be as well first to describe the experiments
made in this direction by the immortal Newton himself.
Every one has, doubtless, at one period of his life, amused himself with
blowing soap-bubbles by means of a tobacco-pipe and a little lather—a
sufficiently childish amusement, you will possibly say, but one narrowly
connected with the most intricate secrets of the science of optics.
These little globes, so fragile that they disappear in a breath, hardly
seem worthy of the attention of a thinker, and still less the
examination of a philosopher; but it is nevertheless true that Newton
made experiments on the colours shown on the surface of these apparently
insignificant objects which ended in the most brilliant discoveries,
just as on seeing an apple fall he began a train of thought which only
terminated in the enunciation of the hypothesis of the earth’s power of
gravity.
All transparent substances, whether liquid, solid, or gaseous, become
coloured with the most brilliant hues as soon as they are reduced to
plates of extreme thinness. In the soap-bubble it is the oleaginous
particles floating on the surface which thus become coloured, but Newton
showed that thin plates of air were similarly capable of showing colour,
and that the thinner the plates were the more brilliant were the tints.
We may see this in the soap-bubble, which becomes more beautiful as it
gets larger and thinner. By placing a convex lens of large size on a
flat plate of glass, Newton observed that rings of different colours
were formed round the spot where the two pieces of glass touched.
[Illustration:
Fig. 12.—Newton’s Rings.
]
By measuring the convexity of the lens and the diameter of the various
rings, Newton was enabled to tell to a minute fraction the exact
thickness of the plate of air corresponding to the different colours.
The glasses being placed in position, a ray of a particular colour—red,
for instance—was thrown upon the surface. The result was a black spot at
the point where the two surfaces touched, and surrounding it at various
distances were several rings alternately red and black. Calculating the
thickness of the plates of air at the part where the dark rings made
their appearance, Newton found that their dimensions were in the
proportion of the even numbers two, four, six, eight, &c.; while the red
rings showed figures corresponding to the odd numbers. Although
trammelled by the corpuscular theory, Newton’s deductions from these
experiments show that they can only be accounted for by the undulatory
hypothesis. Thus the thickness of the plate of air at the first red ring
is that of the red wave, the thickness at the second that of two red
waves, and so on; so that in order to arrive at the thickness of the red
wave we need only measure the distance between the portions of the
glasses where the first red ring occurs.
This experiment, was applied to the measurement of all the waves.
Whenever they were reflected on the glasses a parallel series of rings
was formed, but it was found that the first ring was more or less
distant from the central spot, according to the colour used. The red
ring was the largest; the orange, yellow, green, blue, indigo, and
violet, following in the same sequence as in the spectrum. The word
“thickness” seems hardly fit to apply to dimensions arrived at by Newton
in his experiments, so infinitely small do they appear to be, yet their
correctness has never been impugned, although the experiments have been
repeated by the philosophers of all countries. The waves of red light
are so small that 40,000 of them go to an inch, and those of violet
light situated at the other end of the spectrum are still smaller,
measuring only the 60,000th part of an inch.
The waves of the other colours are between these two, while the wave of
white light, which is a mixture of them all, is just half-way between
the two.
Thus was the physical cause of the various hues of colour discovered by
this great man, revealing as it does the singular and mysterious analogy
between sound and light. The rays of light, like the waves of sound,
produce a different effect, according to their length, by causing
quicker or slower pulsations in the nerves of sight, just as musical
sounds vibrate upon the drum of the ear with different velocities.
This is not all, for the relationship between sound and light does not
cease here: we have as yet only spoken of the size of the undulations,
and have only shown how their dimensions are connected with the
sensation of colour; but there are other things to be considered, for on
investigation we find that not only do the different coloured waves vary
in the length of their undulations, but also in the number that take
place in a given time.
The perception of sound is produced by the action of the drum of the
ear, which vibrates sympathetically with the pulsations of the air that
have been originated by the vibrations of the sounding body; and the
perception of light is produced in a similar manner by the vibrations
originating in a luminous body, and propagating themselves through the
luminous ether until they reach the nerves of sight. The number of these
pulsations taking place in the eye has been accurately determined in the
following manner. Let us suppose that we are looking at a coloured
object—let us say, a red railway signal-lamp; from the lamp to our eye
there flows a continuous line of luminous undulations; these undulations
enter the eye and become depicted on the retina. For every wave that
passes through the pupil, there is a separate and corresponding
vibration of the optic nerve, and the number of these vibrations that
take place in the course of a second can be easily calculated if we know
the velocity of light and the breadth of the waves. We have before found
that light travels at the rate of 185,000 miles per second; it therefore
follows, that a series of undulations 185,000 miles long pass through
the pupil every second; consequently the number of vibrations per second
is arrived at by calculating how many waves measuring the 40,000th of an
inch—that being the length of a wave of red light—are contained in
185,000 miles. The following table, showing the number of waves passing
into the eye per second for the different colours, will interest the
student:—
Extreme red 458,000,000,000,000 waves per
second.
Red 477,000,000,000,000 "
Orange 506,000,000,000,000 "
Yellow 535,000,000,000,000 "
Green 577,000,000,000,000 "
Blue 622,000,000,000,000 "
Indigo 658,000,000,000,000 "
Violet 699,000,000,000,000 "
Extreme violet 727,000,000,000,000 "
Whatever theory we may adopt to explain the phenomena of light, we
arrive at conclusions that strike the mind with astonishment and
admiration. According to the corpuscular hypothesis, it was supposed
that the molecules of light were endowed with the power of attraction
and repulsion, that they possessed poles and centres of gravity like the
earth, and that they had other physical properties that could only be
given to ponderable matter. Starting with these notions, it is difficult
to divest oneself of the idea of sensible size, or to induce the mind to
conceive particles so extremely small as those of light would
necessarily be if the theory of emission were accepted. If a particle of
light weighed a grain, it would produce by means of its enormous
velocity the effects of a cannon-ball weighing 120 lbs., travelling at
the rate of 300 yards per second. How infinitely small would be these
particles, seeing that the most delicate optical instruments are
submitted to their action for years without being injured!
If we are astonished at the extreme smallness and prodigious rapidity of
the luminous molecules whose existence is necessitated by the
corpuscular theory, the numerical results of the undulatory hypothesis
are not less surprising. The extreme smallness of the distance between
the waves, and the inconceivable quickness of their undulations,
although both are easily calculated, must raise in the mind of the
student feelings of the utmost wonder and admiration.
Colour, then, simply results from the difference in the rate of
vibration of the rays, as Professor Tyndall observes in his lectures on
the “Analogy between Sight and Sound,” the impression of red being
produced by waves that undulate a third less rapidly than those which
produce the sensation of violet.
------------------------------------------------------------------------
CHAPTER IV.
LUMINOUS, CALORIFIC, CHEMICAL, AND MAGNETIC PROPERTIES OF THE SPECTRUM.
THE solar spectrum may be compared to a battle-field with an army drawn
up upon it ready for action. In the centre we find the luminous rays, on
one side the light troops which produce chemical effect, and on the
other the heating rays, which may be compared to squadrons of heavy
cavalry. Close by the light brigade are the magnetic rays, which are a
corps of skirmishers, sometimes appearing, and at others hiding
themselves from view in a very mysterious manner.
But to drop metaphor, we shall find on examination of the spectrum that
the three great forces—heat, light, and chemical effect—are regularly
distributed over three different portions of this wonderful band of
colour.
Before Fraunhofer the intensity of the light of different parts of the
spectrum remained undetermined with any degree of accuracy; but this
philosopher, by the use of a very delicate photometer, obtained the
results given below.
The maximum of luminous effect is situated just at the junction of the
yellow and orange. Taking this spot as its starting-point, it gradually
decreases on each side until it ceases altogether at the extreme red and
violet.
With respect to the calorific portion of the spectrum it was for a long
time supposed that the heat-giving properties of any part were in direct
proportion to the amount of its luminous effect; but Sir John Herschel
proved by a long series of experiments that the heat of the spectrum
gradually increased from the extreme violet to the extreme red, and that
passing this point it still further increased until it attained its
maximum at a point where not a single ray of light existed. From these
grand experiments he adduced the important conclusion, that in solar
light there existed invisible rays, which produced heat, and which
possessed even a less degree of refrangibility than the extreme red
rays. Sir John Herschel then tried, but unsuccessfully, to determine the
exact refrangibility of the invisible heat rays.
Sir Henry Englefield compared these results, and obtained the following
figures:——
Blue 56 deg. Fahr.
Green 58 "
Yellow 62 "
Red 72 "
Beyond the 79 "
red
Bérard obtained similar results, but he at first found that the maximum
of heat was just at the end of the extreme red, and that beyond it the
air was only about one-fifth warmer than the ordinary temperature. Sir
John Herschel attributed these discordant results to Bérard having used
a thermometer with too large a bulb; he accordingly repeated his
experiments with other instruments with long narrow bulbs, and arrived
at similar results to those obtained by the English philosopher.
We will now pass on to the physical properties of the other end of the
spectrum. Towards the end of the last century, Scheele, a Swedish
philosopher, remarked that chloride of silver was blackened more quickly
by the violet portion of the spectrum than by any other. In 1801, Ritter
of Genoa, in repeating certain experiments made by Herschel, found that
a much stronger blackening effect was produced at a point beyond the
violet, and that the discoloration was produced with less intensity by
the violet and still less so by the blue, the change gradually
decreasing till the red ray was reached. He also found that when
slightly blackened chloride of silver was exposed to the effects of the
red rays, or even in the space beyond, its colour was restored to it.
From these facts he drew the conclusion that in the solar spectrum there
existed two kinds of rays, one at the red extremity, which favoured
oxygenation; the other, at the blue end, which possessed the contrary
properties. He also found that when phosphorus was placed in the
invisible rays beyond the red, it gave off fumes of oxide, which were
immediately extinguished when it was transferred to the other end.
On repeating the experiment with chloride of silver, Lubeck found that
the tint varied according to the colour in which it was placed. Beyond
or in the violet ray it became brownish red, in the blue it became
bluish or bluish grey, in the yellow it remained white, or became
slightly yellow and reddish in or beyond the red ray. When he used
prisms of flint glass, the chloride of silver was discoloured beyond the
visible limits of the spectrum.
Without being aware of Ritter’s experiments, Dr. Wollaston obtained the
same results by acting on chloride of silver with violet light. In
continuing his researches he discovered that gum guaiacum was also
influenced by the chemical rays of light.
The magnetic influence supposed to be exerted by the solar rays still
remains without positive proof, although numbers of philosophers have
experimented in this direction. More than fifty years ago Dr. Morichini
announced that the violet rays of the solar spectrum possessed the
property of magnetizing steel needles that were previously free from
magnetism. He produced this effect by concentrating the violet rays upon
one-half of each needle with a convex lens, taking care to keep the
other half concealed beneath a screen. After having continued this
experiment for more than an hour, the needles were found to be quite
magnetic.
Dr. Somerville tested Morichini’s experiments by covering one-half of an
unmagnetized needle an inch long with a piece of paper, and exposing the
uncovered half to the violet rays of the spectrum, and found that the
needle became magnetic in the course of a couple of hours, the exposed
end being the north pole. The indigo rays produced almost the same
effect, but the blue and green rays were much less powerful. When the
needle was exposed to the yellow, orange, red, and invisible rays beyond
the red, no magnetic effect was produced, although the experiment was
continued for three days. Pieces of chronometer and watch springs were
submitted to the same influences with a similar result; but when the
violet rays were concentrated upon the needles and pieces of spring with
a lens, the time necessary for magnetizing them was greatly reduced.
Baumgartner of Vienna and Christie of Woolwich also repeated these
experiments. The latter philosopher found that when a needle of
magnetized steel, copper, or even glass, vibrated by force of torsion in
the rays of the sun, the arc of vibration diminished much more quickly
than when the experiment was conducted in the shade. The sun’s rays
appeared to have the greatest effect upon the magnetized needle. From
these results Christie concluded that the solar rays were capable of
exerting a certain amount of magnetic influence.
These experiments were afterwards fully confirmed by those of Barlocci
and Zantedeschi. The former found that a natural magnet which was
capable of supporting a pound weight, had its power almost doubled by
exposure to strong sunlight for four-and-twenty hours. Zantedeschi
exposed a magnet which would carry fifteen ounces to the sun for three
days, and increased its power two and a half times. These experiments
seem almost to decide the fact of the power of white and violet light to
induce magnetic force; but a series of researches by a philosopher who
without doubt is greater than any of those already mentioned, seems to
throw some doubt on the facts we have related above.
Before concluding, we must add a few more facts relating to the
existence of invisible rays at both ends of the spectrum. “The visible
portion of the spectrum,” says Dr. Tyndall, in one of his Royal
Institution lectures, “simply marks an interval of radiant action, the
rays existing in which bear such a relation to our visual organs, as to
be capable of exciting in them the sensation of light. Beyond this
interval, in both directions, right and left, the radiant action
continues to exercise itself, but the rays emitted are dark, in
consequence of their exerting no influence on our eye. Those that exist
beyond the red ray are capable of producing heat, while those that are
beyond the violet excite chemical action. These invisible violet rays
can be actually made perceptible to the eye, or, in other words, the
undulations or waves proceeding from this end of the spectrum can be
made to strike against certain substances and induce luminous
vibrations, so as to connect the dark space beyond the violet with a
brilliantly illuminated band. I have here a substance capable of
effecting this change. The lower half of this sheet of paper has been
moistened with a solution of sulphate of quinine, the other half being
left in its ordinary condition. I will now hold the paper in such a
manner that the line that separates the prepared half from the other
shall cut the spectrum in two halves horizontally. The upper half will
remain unaltered and may be readily compared with the lower half, upon
which you will see the spectrum prolonged beyond its ordinary limits.
The effect produced is the addition of a splendid band of fluorescent
light, which extends over a space of several inches, which but an
instant before was a dark mass. I withdraw the prepared paper, and the
light disappears; I replace it, and the light shines forth once more;
showing us in the most brilliant way that the visible limits of the
ordinary spectrum are not the limits of radiant action.
“I plunge a pencil into the solution of sulphate of quinine, and I pass
it over the paper. You see that wherever the solution falls, the light
bursts forth. The existence of these rays has been known for a long
time. Young was familiar with them, and subjected them to experiment;
but it is to Professor Stokes that we are indebted for a complete series
of researches on this subject. It was he who first made those invisible
rays visible, as we have done.”
In the same way the Professor proceeded to show that the heat rays were
invisible by passing a beam of sunlight through a solution of iodine in
spirits of wine, which, although it completely stopped all light,
allowed the heat rays to pass uninterruptedly. By collecting these
invisible rays into a focus by means of a lens, Dr. Tyndall was enabled
to ignite various combustible bodies.
Thus we see the reason why certain rays produce certain effects on the
eye, each particular degree of refraction causing a different set of
vibrations, resulting in a different sensation for every part of the
spectrum, and reproducing the effect of various colours on the optic
nerve. In the following chapters we shall conclude our account of the
different colours in the spectrum and of the laws of light.
------------------------------------------------------------------------
CHAPTER V.
THE LAWS OF REFLECTION.—MIRRORS.
WHEN a ray of light falls obliquely on any polished surface, as that of
a mirror, a piece of water, a plate of burnished metal, or any other
reflecting substance, the ray, like an elastic ball, is immediately
projected in a contrary direction to that in which it fell. Moreover,
the direction in which it is reflected is at right angles to the
surface, and in the same plane as that of the ray in the first instance.
This experiment may be tried very easily, and will show the reason for
the two following laws.
1. The angle of incidence is equal to the angle of reflection, and _vice
versâ_.
2. Reflection can only take place in one direction—in that of the
incident rays, both of which are always in a plane perpendicular to the
reflecting surface.
The following figure will assist the student in performing experiments
on the reflection of light from flat surfaces.
The ray A B falling obliquely on the horizontal mirror, is reflected
upwards at the same angle in the direction B C. This may be proved
geometrically by placing a graduated circle in a vertical position in
the plane A B C, when we shall find that the angle A B D formed by A B
(the incident ray) with the perpendicular D B is equal to the angle
formed by this perpendicular line and the reflecting ray B C. You may
also prove in the same way that these three lines are all in the same
vertical plane.
[Illustration:
FIG. 13.—Reflection from Plane Surfaces.
]
Let us now examine the effects of light reflected from plane surfaces.
We must first, however, notice a certain optical illusion to which we
are continually falling a prey, almost without our knowledge. We always
fancy objects to be in reality in the place where we see them, and, in
spite of our having already enumerated a large number of these
deceptions, we must still add one more to the list. In reality we rarely
see objects in the place where they really are; for if by the effect of
reflection, refraction, or any other cause, the rays of light are made
to deviate from their course, we no longer see the object from which
they proceed in its real position, but in the direction taken by the
luminous pencil at the moment of entering the eye.
[Illustration:
FIG. 14.—Refraction.
]
[Illustration:
FIG. 15.—Experimental Proof of Refraction.
]
For instance, if the ray A B is bent during its passage to the eye at B,
and consequently reaches it in the direction B C, it is at A´, and not
at A, that we shall see the object from which it proceeds. Every ray of
light which passes out of a medium of a certain density into another of
a different density is bent from its primary course, or, in scientific
language, it is refracted. The experiments we made in a former chapter
on the properties of the prism are founded on this principle. The law
may be easily illustrated by allowing a ray of light to fall upon the
surface of a vessel of water, as shown in the preceding figure.
[Illustration:
FIG. 16.—The Effects of Plane Mirrors.
]
The light of the stars and planets undergoes a similar deviation when
passing in its course through the earth’s atmosphere; and at the moment
we see the rising of the sun, the moon, or a star, they are in reality
still below the horizon. Our eyes consequently are still deceiving us,
no matter what part of the domain of optics we may enter.
There are two kinds of mirrors—plane and curved. We will first examine
the properties of the former sort, being those which are ordinarily
applied to the usages of every-day life.
[Illustration:
FIG. 17.—Reflection from the Surface of Water.
]
In the figure in the preceding page we have a young lady looking at her
reflection in a tall cheval glass. Every point upon the surface of her
clothes and face is reflected back to her eye from the surface of the
tin amalgam which has been applied to the back of the mirror by the
looking-glass maker, for the purpose of rendering the image of the
object more brilliant than if the glass alone were used. The rays which
proceed from every one of these points strike upon the surface of this
metallic layer, are stopped by its opacity, and are reflected back to
the eye at an angle equal to that at which they strike the surface. The
image seen by the eye is formed, consequently, by the reflection of
every one of these rays; and as we always see objects in the direction
taken by the luminous ray at the moment it enters the eye, we fancy we
see objects before us that are really behind, or on each side of us. For
instance, the ray starting from the left foot of the young lady in the
figure is reflected from the point indicated on the surface of the
glass, but the eye does not stop here, but sees the foot at an equal
distance beyond the mirror.
The same thing takes place, not only with glass, but with all substances
having polished surfaces. Still water, which to all intents and purposes
has a polished surface, reflects the objects within its range as
perfectly as a mirror.
The preceding observations apply to all plane reflecting surfaces; but
there are other sorts of mirrors, whose effects are of a more
interesting nature, and which we must hasten to describe—we allude to
those whose surfaces are either convex or concave.
Curved mirrors are made of a great variety of shapes, but for the
present we shall only describe those which are spherical. Spherical
mirrors may of course be either concave or convex.
[Illustration:
FIG. 18.—Concave Mirror.
]
Suppose the arc M N (fig. 18) to be movable round the point O, this
revolution will describe the surface of the mirror. The central point C
of the hollow sphere of which the mirror forms part, is called the
centre of curvature, the line O L the principal axis. By remembering
these very simple definitions, we shall be able to understand the action
of these mirrors without the slightest difficulty.
To understand how the rays of light are reflected from the surface of
the mirror N M at the point F, which is called the focus, we have only
to consider the mirror as consisting of an infinite number of facets,
all inclined towards that particular point, and forming by reason of
their immense numbers a regular spherical surface. In considering the
mirror from this point of view, we can immediately see that, on account
of the inclination of the supposed facets, the rays that they receive
are all reflected back again at the same point; and it may be proved
geometrically, that when the incident rays are parallel the focus will
be situated somewhere on the line O C, its position depending on the
curvature of the mirror.
If, therefore, we receive on a spherical mirror a pencil of sunlight,
the rays which compose it may be regarded as parallel, the sun being at
so great a distance from the earth; it follows that these rays will all
be reflected together in a particular point, viz., at F, and if any
object be placed there it will be illuminated with great brilliancy. The
laws governing the reflection of heat being nearly similar to those
regulating the action of light, the rays reflected from a burning body
will ignite any inflammable substance placed at the point F. The focus
for parallel rays is called the principal focus of a mirror. Having
described the effects of parallel rays, let us now see what happens when
the source of light is close to the mirror. If it is placed at a very
small distance, the luminous rays are divergent instead of parallel, and
their meeting point becomes changed in accordance with the laws laid
down at the beginning of this chapter. That is to say, the focus will
approach more or less to the centre of curvature C, according as the
source of light is placed nearer to or further from the mirror;
consequently, in the case of the candle in fig. 19, instead of uniting
at F, the rays will meet at _f_, a point situated somewhat nearer the
mirror than the principal focus. If, instead of placing the light at A,
we place it at _f_, we shall find the rays will be concentrated at the
point A. Thus the foci are consequently related to each other, and are
hence called _conjugate foci_. It will be readily seen that a spherical
mirror may have an infinite number of conjugate foci, according to the
distance of the source of light. It is also clear, that if we cause the
light to approach the mirror, the focus will also approach it.
[Illustration:
FIG. 19.—Conjugate Foci.
]
Continuing our experiment, we shall find that when the candle passes the
principal focus so as to be between it and the mirror, the reflected
rays first become parallel and then divergent, and cannot consequently
produce any focus beyond the mirror, but are reflected in the way shown
in fig. 20.
In experimenting on the plane mirror, we imagined we saw the object at a
certain distance behind it; the same thing happens when we see ourselves
reflected in a concave mirror, and the particular point at which we
suppose we see our reflection is called the virtual focus.
[Illustration:
FIG. 20.—Virtual Focus.
]
If instead of a candle we place our head before a concave mirror, we
shall see ourselves magnified as in fig. 21.
[Illustration:
Fig. 21.—Concave Mirror.
]
We shall easily see how this happens by tracing the paths of the rays in
fig. 22.
[Illustration:
Fig. 22.—Magnifying effect of Concave Mirrors.
]
[Illustration:
Fig. 23.—The Reversal of real Images.
]
The rays, for instance, which proceed from the forehead at the point _a_
are reflected from the point _o_ to the eye in such a way as to appear
to proceed from a point beyond the mirror, A. In the same manner the
rays reflected from the chin appear to take their origin from the point
B. If, on the other hand, we place ourselves at a distance from the
principal focus, we shall produce a reversed and diminished image of our
face. This image is not illusory, like the preceding ones, but is real,
and may be received upon a screen, as shown in fig. 23.
We may easily follow the path of the rays as shown in the figure, and we
shall see that the rays forming the images of the church-tower and the
terrace below, cross at a certain point.
Convex mirrors produce precisely opposite effects, and give a diminished
image instead of a magnified one, as may be perceived on examining fig.
24.
[Illustration:
FIG. 24.—Diminishing power of Convex Mirrors.
]
------------------------------------------------------------------------
CHAPTER VI.
METALLIC BURNING MIRRORS.
THE classical student will remember that Archimedes burned the fleet of
Marcellus, by means of burning-glasses, from the heights of the
fortifications of his native city of Syracuse. Unfortunately, any
account of the system of catoptrics, or the science of reflections,
employed by the ancient Syracusan in their construction is lost to us,
and many modern writers have gone so far as to doubt the fact
altogether. The knowledge of the properties, however, of concave mirrors
which we have just been acquiring, will enable us to form a pretty good
guess as to the means adopted by Archimedes for the destruction of the
enemy’s fleet. The ancients, not having the means of either casting or
grinding such enormous mirrors, must have constructed them of a large
number of small ones, so arranged that the images of the sun reflected
by them would all fall in the same place, or nearly so. In this case,
the larger the number of mirrors, the greater would be the burning
effect. In order to explain the reflection of rays incident upon the
surface of concave mirrors, we supposed them to consist of an immense
number of plane mirrors placed in a curve, so that the reflected rays
might all meet in one point; but on examining into the history of
burning mirrors, we find that the plan has been adopted in reality in a
great number of instances. We have also said, that the reflection of the
heating rays was governed by similar laws to those influencing the rays
of light; consequently, by directing a pencil of sunlight upon the
surface of a concave mirror, we obtain the maximum of light and heat at
the focal point.
Many modern writers give the ancients too little credit for their
knowledge of optical principles, and late investigations seem to prove
that the old school of philosophers were much more learned in these
matters than has been generally supposed. The discovery of a rock
crystal double convex lens in an Egyptian tomb of great antiquity is an
instance of this. Descartes wrote a little treatise to prove that the
stories related of the burning mirrors of Archimedes were pure
fabrications, although many Latin authors have described them both as
being used by that philosopher and in more modern times; Dion, for
instance, who lived in the early part of the sixth century, states that
at the siege of Constantinople, Proclus burnt the fleet of Vitalian with
mirrors of brass; but the opinion of Descartes seemed to outweigh all
other testimony. Buffon, who wished to sift the matter thoroughly,
constructed for himself, after many previous experiments on the laws of
reflection, a series of mirrors that closely imitated those ascribed to
Archimedes. His first memoir, “On the Invention of Mirrors capable of
burning at a great Distance,” was published in the Transactions of the
French Academy of Sciences for 1747. A few years later he combated both
theoretically and practically the opinion of Descartes, in a memoir
containing an account of an immense number of experiments. Before
speaking of the extraordinary effects of burning mirrors, it will be as
well to do justice to the predecessors of the learned naturalist we have
just mentioned, by quoting a passage from the works of Father Kircher,
who, 128 years previously, experimented in this direction with great
patience and perseverance, and tried to prove that the stories related
of Archimedes were true. “The larger the surface of a mirror,” says this
philosopher (who, like Huyghens, was a practised astronomer), “the more
light it reflects from the objects opposite to it. If it is only a foot
square, it will throw a square foot of light upon any wall or screen
placed before it. Experiment shows that this light is composed of an
infinite number of rays reflected from different points on the surface
of the mirror. Direct the rays from a second mirror upon the same place
as those from the first, and the light and heat will clearly be doubled.
They will become trebled if you direct the rays from a third mirror upon
the same spot, and so on _ad infinitum_. In order to prove that the
intensity of the light and heat is in direct proportion to the number of
reflecting surfaces employed, I took five mirrors, and found that on
exposing them to the sun I obtained with only one, less heat and light
than if I used direct sunlight. With two the light and heat increased
considerably; three gave as much heat as an ordinary fire, and four gave
me a still greater effect. I therefore concluded that by multiplying
these plane mirrors, I not only obtained greater effects than those got
by using parabolic, hyperbolic or elliptical mirrors, but that I could
use them upon objects at a much greater distance. With five mirrors I
could obtain these effects at a distance of 100 feet, but what terrible
phenomena would have taken place had I used one thousand instead of
five?” He ends by begging mathematicians to experiment in this direction
with greater care than they had hitherto done.
After Kircher we may cite as an experimentalist with these terrible
instruments the French philosopher Villette, who constructed several
mirrors, in direct imitation of those of Archimedes, for Louis XIV. and
other sovereigns. The _Journal des Savants_ for 1679 gives an account of
his principal metallic burning mirror in the most eulogistic terms,
adding an instance of ignorance which is singularly quaint and curious.
It is of the fourth and most perfect of Villette’s mirrors that the
_Journal des Savants_ speaks, the first having been bought by Tavernier,
and presented to the Shah of Persia, who considered it as one of the
rarest and most precious curiosities that he possessed: the second was
sold to the King of Denmark, and the third was given by M. Villette to
Louis XIV., from whom he received the praises and rewards that were due
to his talent and perseverance. “It was thirty-four inches in diameter,
and vitrified flints and bricks almost instantaneously, no matter how
large they were. It consumed the greenest wood, burning it to ashes in
an instant, and fused the most refractory metals with equal ease and
quickness. Steel, no matter how hard, resisted its power no more than
other metals, and melted so quickly that one part burnt away in
inconceivably brilliant sparks, some of them forming stars as large as a
franc piece, leaving a flowing mass of metal behind. The last made by
Villette was still more powerful, being larger and more carefully made.
It was forty-four inches in diameter, and three inches and a line deep.
Its burning point, or focus, was situated at a distance of three feet
seven inches from the surface, and was apparently as large as a five-sou
piece; and it was at this spot, where the rays of light and heat were
concentrated into so small a space, that the wonderful effects of its
violent power became manifest, the spot of light being of such
brilliancy that the eyes could no more withstand its brightness than
that of the sun. Besides the property of burning which it possessed in
so wonderful a degree, it was capable of exhibiting other effects just
as curious as those already related. It had the power of sending the
images of objects to a distance of fifteen feet or more, so that a man
looking at himself in this mirror with a stick or sword in his hand, saw
the image of them suspended in the air, apparently ready to strike the
observer. On seeing such an effect for the first time, the observer
could hardly fail to experience the greatest surprise, and even fear;
and it is stated that the king having placed himself, sword in hand,
before one of these mirrors, in order to observe the effect, was
surprised to find himself face to face with an armed hand apparently
directed against him. When he advanced, the hand seemed to spring
forward to meet him. The king could not conceal his surprise and fright,
and afterwards felt so ashamed at being terrified with a mere shadow
that he ordered the mirror to be taken away, and could never be
prevailed upon to look into it again.” The _Journal des Savants_ then
goes on quaintly to remark on the various startling effects produced by
these mirrors, winding up by stating that its powers of reflection were
so great, that at night the light of a torch or flambeau was reflected
so perfectly that an observer placed at four hundred feet distant could
read the smallest print.
It also mentions a curious piece of superstition on the authority of a
scientific writer of the name of Robertson, who states that it happened
at Liége. In reading the accounts of these experiments we can see how
easily the minds of individuals were affected in those days by the
wonderful. It happened while one of Villette’s mirrors was at Liége,
that the latter end of the summer was somewhat rainy, and great fears
were entertained that a bad harvest and dear bread would be the result.
Certain evil-minded people, who had taken a fancy to the mirror and
wished to possess it by unfair means, spread the report that the
continual rain was entirely caused by its action on the clouds and sun,
and that the coming famine must be laid upon the shoulders of its owner
and inventor. This absurd idea took such forcible possession of the
minds of the populace of Liége, that great mobs collected together,
uttering all kinds of maledictions against the mirror and its inventor,
and at last became so violent that they attacked Villette’s house with
the intention of smashing his great work, and administering to the
unfortunate philosopher the chastisement they supposed he deserved.
Happily, however, for M. Villette and his mirror, Liége was governed in
those days by the Prince Bishop of Cologne, who was a man of great
enlightenment. He put the crowds round M. Villette’s to flight by armed
force, but he found that the conviction that all the coming mischief
would result from the unlucky mirror was so strong, that he was obliged
to issue a pastoral peremptorily declaring that the idea had originated
with a number of malicious people, who spared no pains to propagate it
for their own bad purposes, and that it was a mischievous and dangerous
error to ascribe to a mirror a power which only belonged to the
Almighty.
In 1747, Buffon performed many extraordinary experiments with burning
mirrors, which were more surprising than any that had hitherto been
described. They were mostly performed at the _Jardin des Plantes_, at
Paris, of which institution Buffon was director; and many of them are
worth describing.
[Illustration:
FIG. 25.—Burning Mirror.
]
On the 3rd of April, at about two o’clock in the afternoon, the great
mirror was mounted on its stand, and was found to be capable of setting
a plank of wood on fire at a distance of 138 feet, when 128 glasses were
used, although the light was weak at the time, and the sun was covered
with mist. In pursuing these experiments great care had to be taken to
prevent the by-standers placing themselves within range of its terrible
power, for several were nearly blinded by looking at the brilliant focal
point of the instrument. The next day, at eleven in the forenoon,
although the sun was still covered with mist and fleecy clouds they were
able to produce such a heat at 150 feet distant, with 154 glasses, that
a pitched plank began to smoulder and would have burnt into flame had
not the sun disappeared at that particular moment. On the fifth of
April, at three in the afternoon, with the light much in the same weak
condition as it was on the other days, they succeeded in igniting at 150
feet distant, a heap of shavings of deal mixed with charcoal and
sulphur, in less than a minute and a half, with 154 glasses. When,
however, the sun shone with its natural power, a few seconds were
sufficient to effect these results.
On the 10th, when the sun was shining pretty powerfully, a pitched pine
plank was easily fired with 128 glasses, at 150 feet distant. In this
case the ignition was very sudden, and extended over the whole of the
radiant spot forming the focus, which at the distance named measured 16
inches in diameter. The same day at half-past two, a pitched elm plank
covered in some places with chopped wood, was set fire to with extreme
rapidity, and burnt with such violence that it had to be dipped in water
before it could be put out. In this experiment 148 glasses were used, at
a distance of 150 feet.
On the 11th of April, the burning point was fixed at 20 feet distant
from the mirror, and combustible substances were easily burnt with only
12 glasses. With 21 glasses a half-burnt elm plank was set fire to, and
with 45 a piece of tin weighing six pounds was almost immediately
melted. Silver sheet was fused, and an iron plate was made red-hot with
117 glasses. In giving an account of these interesting experiments,
Buffon expresses his conviction that at 50 feet it would have been easy
to have melted metals if all the glasses of the mirrors had been used.
When used at that distance, the burning spot was six to seven inches in
diameter. He also noticed that when metals were melted, part of them
were dissipated in brilliant vapour, which was so thick as to cast a
shadow on the ground, although it seemed to be as bright as the sun
itself. When the sun was at its full strength, and all the glasses were
brought into requisition, wood was set on fire at a distance of over 200
feet, and metals and minerals were fused at 40 and 50 feet. Hence the
possibility of making and using these mirrors as Archimedes was said to
have done, was proved practically by the great naturalist. Fig. 25
represents a burning mirror in action.
Robertson, an English philosopher, residing in France during the days of
the first Republic, reconstructed the mirrors described by historians as
being used by Archimedes, and the results he obtained were thought
sufficiently important by the Council of the Department of Ourthe to
merit an attentive examination by two members of their body, who
reported in favour of their being used as instruments of war.
It would be possible to pursue this subject still further, and give an
account of numerous experiments made on burning mirrors by various
philosophers, but we must not forget that it is light and heat that we
have more especially to deal with in the present work. Already we have
possibly strayed from our path a little too far, but the two influences
are so closely connected with each other that it is almost impossible to
speak of them separately when reflection is in question.
------------------------------------------------------------------------
CHAPTER VII.
LENSES.
THE word lens is derived from the Latin name of the seed of the _Ervum
lens_, or ordinary lentil. When eating this wholesome vegetable, almost
every one has noticed that its shape is exactly that of a double convex
lens, as represented in the following figure:—
[Illustration:
FIG. 26.—Double Convex Lens.
]
Perhaps it would be more correct if we were to say that a double convex
lens is like a lentil, rather than turn the comparison the other way,
seeing that this little seed has given its name not only to the
particular-shaped glass depicted above, but also to some five others
more or less analogous to it.
In fig. 27 we have the different forms of lenses shown in section. The
first is the _double convex lens_, the second the _plano-convex_, the
third and sixth the _concavo-convex_, the fourth the _double concave_,
and the fifth the _plano-concave_. A _crossed lens_ is a double convex
lens whose one side is more convex than the other. The third lens is
also called _meniscus_.
[Illustration:
FIG. 27.—Forms of Lenses.
]
The properties of the first, second, and third are similar; that is to
say, they cause parallel rays of light passing through them to converge
at a certain point, called their focus; while the three others have a
divergent action on rays passing through them. By examining the path of
the rays through these lenses, we shall find that the first three
magnify objects seen through them, while the latter have the contrary
effect.
As in the case of the curved mirrors, the rays falling on the surface of
a convex lens may be either parallel, divergent, or convergent. In the
case of parallel rays, as depicted in the following figure, they are
represented as meeting at a point beyond the lens, which is called the
sidereal focus, or the focus for parallel rays. It is generally found by
causing the image of the sun or of some distant object to be thrown by
the lens upon a screen, or by knowing the curvature of the faces, and
the refractive power of the glass.
Every ray on striking the surface of the lens is refracted inwards,
until it meets with its companions at the focus F, in accordance with
the law of refraction, by which a ray of light passing from one
transparent medium, such as air, to another which in this instance is
glass, becomes refracted or bent in proportion to the relative density
of the two mediæ. The nearer the ray passes to the edge of the lens, the
more it is refracted, the angle of incidence being greater; the ray
through the exact centre being uninfluenced by the form of the glass.
Hence they all meet in a single point. Figs. 29 and 30 show the path of
the rays when they are divergent and convergent.
[Illustration:
FIG. 28.—Path of a Ray through a Convex Lens.
]
[Illustration:
FIG. 29.—Path of divergent Rays through a Convex Lens.
]
If the rays of light are not parallel, as in the case of the source of
light being near the lens, they do not converge so rapidly as when they
proceed from a distant object, consequently the focus for near objects
is longer in proportion to their distance. In fig. 29 for instance, if a
candle be placed as shown, and a screen on the other side of the lens, a
point will be found where the image of the candle is seen upon it in a
reversed position. The distance between these two points is always
relative, and they are called conjugate foci. Thus, the candle may
change places with the screen with a similar effect, as long as the
exact position of the two points is preserved. If the candle is placed
farther off, we must diminish the distance between the screen and the
lens, and _vice versâ_. In fact, the nearer the object, the longer the
focus; the farther it is off, the shorter the focus. Half an hour’s
experiment with a double convex lens, a piece of white cardboard, and a
small candle, will teach the student more about the properties of convex
lenses than a chapter of explanation. A common magnifying-glass, or even
an old spectacle lens, will serve the purpose of more expensive
instruments.
[Illustration:
FIG. 30.—Conjugate Foci.
]
We now proceed to speak of the images formed by lenses. In fig. 31 we
have a flower placed on one side of a lens. As it is not at an infinite
distance, the rays sent out by its various parts are convergent, and not
parallel, consequently they do not meet at the sidereal focus, but at a
point beyond it, according to the rule already laid down. The rays
proceeding from the exact centre of the flower striking the lens exactly
in the middle at right angles, suffer no change, the others being
refracted in proportion to their angles of incidence.
[Illustration:
FIG. 31.—Images formed by Convex Lenses.
]
The rays proceeding from the flower cross each other at a certain point:
hence the image on the screen is reversed. The dimensions of the image
will depend on the distance of the object from the lens. This is a fact
we meet with every day, when using an opera-glass or a telescope. Images
formed by convex lenses upon a screen are called by opticians _real
images_, in contradistinction to those which are the result of mere
reflection, as in the case of plane mirrors. These latter are known as
_virtual images_ and are produced by convex lenses as well as by plain
reflecting surfaces. In fig. 32, for instance, the unreversed image of
the insect seen by the eye is not a real image, but a virtual one,—a
fact that might be easily proved by placing a screen in the position of
the eye, when it would be found that no image would be formed.
When using an ordinary magnifying-glass we see the virtual image of the
object we are looking at, but in the case of a telescope or opera-glass
we see the real image of the object, formed by the large lens in front,
and reversed again by the arrangement of small lenses next to the eye.
[Illustration:
FIG. 32.—Magnifying Property of Convex Lenses.
]
Double concave lenses produce effects which are just the reverse of
those we have been considering. Instead of increasing in thickness from
the edges to the centre, they follow the contrary plan, and increase
from the centre to the edges. Consequently, instead of the rays meeting
at the focus, they diverge from each other, and gradually spread out, as
shown in fig. 33.
[Illustration:
FIG. 33.—Diminishing Effect of Concave Lenses.
]
The above figure shows the path of the rays proceeding from the vase,
and meeting the eye at such an angle that the virtual image is greatly
diminished. Concave lenses, as the student has no doubt already guessed,
do not give real images.
[Illustration:
FIG. 34.—Cannon of the Palais Royal.
]
The effects produced by the action of concave mirrors may be produced
with just as much facility by convex lenses. If a body is placed in a
focus of a lens which receives the direct rays of the sun, the heat as
well as the light will be concentrated at one point; and if the object
is combustible, it will take fire sooner or later, according to the size
of the lens. All the experiments mentioned by Buffon as being produced
by a concave mirror are equally obtainable with a concave lens. When of
sufficient diameter, the most refractory metals, such as platinum or
iridium, may be melted and dissipated into vapour. Before lucifer
matches and vesuvians were as common as they are now, it was not at all
unusual to find smokers carrying a small burning-glass and a piece of
tinder, for the purpose of lighting their pipes or cigars; and there
hardly exists a boy who has not lighted a bonfire in the fields or
playground by means of an old spectacle lens or telescope glass.
Amongst other applications of this property of lenses may be mentioned
that of causing guns to fire at a certain time, by arranging a small
burning-glass above the touch-hole. In the Gardens of the Palais Royal,
at Paris, there is such a gun, so arranged that on sunny days it fires
exactly at noon, or, in other words, at the moment the sun comes to the
meridian. Every fine day towards twelve o’clock, crowds of Parisians who
have nothing to do may be seen bending their steps towards the Palais
Royal to set their watches by the gun, which they believe to be superior
as a time-keeper to the finest chronometer in the world. There they
stand, most of them old fellows with a scar or two about their faces,
showing that they have nobly won the rest they appear to enjoy so
innocently and calmly with watch in hand, leaning against the railings,
and waiting with impatience the moment when true solar noon is indicated
by the sharp report of the little piece. Their belief in the correctness
of solar time is something astonishing; and if a bystander were to
insinuate, no matter how delicately, that solar time varied slightly
every now and then, he would either receive a smile of pitying contempt,
or else he would be called out upon the spot. Fig. 34 gives a pretty
view of the celebrated cannon of the Palais Royal.
[Illustration:
FIG. 35.—Fresnel’s Lighthouse Apparatus.
]
We now come to another application of the refracting power of lenses, in
the way of concentrating rays, which is infinitely more valuable to
humanity than either of those we have just mentioned; we mean the
construction of enormous refracting apparatuses for lighthouse purposes.
The first lighthouse of which we have any record is that which was
erected on the island of Pharos, by Ptolemy Philadelphus, in the year
470 of the foundation of Rome. This was merely a tower, upon the top of
which fires were kept burning at night; but as the world progressed, the
blazing tar-barrel or wood fire gave place to the carefully-constructed
lamp and silvered reflector apparatus, which are fast disappearing in
their turn before the electric or Drummond light and the refracting
apparatus constructed by Fresnel, who was the first to endeavour to
abolish the old-fashioned and inefficient metallic mirror from the
lanterns of lighthouses. Fig. 35 shows a section of Fresnel’s apparatus.
A is a plano-convex lens of about a foot in diameter, whose focus
corresponds with those of the concentric lenticular rings of glass which
surround it, and which are seen more plainly in fig. 36. These rings,
which are ground and polished with the greatest accuracy, are somewhat
in the shape of an ordinary quoit, and are equivalent to a plano-convex
lens with the centre portion cut out. This arrangement is so powerful
that the distance at which a light provided with it can be seen is only
limited by bad weather, the state of the atmosphere and the distance of
the horizon. It is common for such lights to be seen at a distance of
between fifty and sixty miles. The apparatus is mostly arranged in the
form of an octagon, and is generally provided with additional reflecting
mirrors at those parts above the light which are out of the range of the
lenses. The light shining fully in eight directions at one time, can
scarcely be missed by any ship within range; but in order to guard
against any possibility of accident, the optical apparatus is often made
to revolve by clockwork, so that every point of the ocean is illuminated
in turn. By using coloured glasses, or by causing the light to disappear
at distinct intervals, different lighthouses may be identified by ships
that are out of their reckoning. Fig. 36 represents the interior of the
lantern of a first-class lighthouse, showing the arrangement of the
lenticular rings round the central lens. If ever the student should pass
through Havre, he should not miss the opportunity of seeing this noble
apparatus, which is one of the finest ever manufactured.
[Illustration:
FIG. 36.—Lantern of a First-class Lighthouse.
]
------------------------------------------------------------------------
CHAPTER VIII.
OPTICAL INSTRUMENTS.—THE SIMPLE AND COMPOUND MICROSCOPE. THE SOLAR AND
PHOTO-ELECTRIC MICROSCOPE.
THE lenses and mirrors whose properties we have been considering in the
previous chapters, have been combined in different ways for the purpose
of examining objects too small or too distant to be perceived by the
human eye. To instruments used for the former purpose the name of
microscope has been given, from two Greek words signifying _small_ and
_to see_. In like manner the name of telescope is also derived from two
Greek words, meaning _distant_ and _to see_. Besides these two classes
of optical instruments, others have been devised to facilitate the
depicting of natural objects, either by means of the pencil or of
photography, or to amuse the eye by optical illusions. Thus we have the
camera obscura, the camera lucida, the magic lantern, the
phantasmagoria, and numberless other instruments of the same sort, most
of which will be described in the latter part of this book.
There are two sorts of microscopes, the simple and the compound; the one
consisting of a single convex lens, and the other of several
combinations of both convex and concave lenses.
When speaking of convex lenses, we described the properties of the
ordinary magnifying-glass, or simple microscope. The uses of this
instrument are almost too well known to need description. It is used by
old people, the lenses of whose eyes have become flattened by old age,
by watchmakers for examining the minute portions of their work, by
jewellers for the same purpose, and by most people for examining maps,
engravings, and photographs. Simple microscopes are generally mounted in
horn, ivory, or metal handles for convenience’ sake. Some simple
microscopes consist of two or more lenses mounted together in order to
increase the magnifying power. The student must distinguish between
several lenses mounted together in this way, and the true compound
microscope, which is a comparatively complicated optical arrangement, as
we shall see presently. When two single lenses are thus mounted
together, the power of the combination is equal to the powers of each
added together.
There is good reason for supposing that the simple microscope is a
comparatively ancient invention. Seneca, who lived in the first century,
declares that in his time it was well known that, when writing was
looked at through a globe full of water, it appeared larger and blacker.
In the eighth century we find the use of magnifying spectacles for old
people common in most countries, and yet it was only at the beginning of
the seventeenth century that a true optical instrument, in the form of a
telescope, was invented. It only needed the placing of two magnifying
glasses in a line to discover the principle of the telescope, but nearly
a thousand years elapsed after the first introduction of these glasses
before an accident rendered the principle evident.
In fig. 37 we see the commonest form of microscope in the hands of an
observer; and by examining the following figure and tracing out the path
of the rays, we shall easily discover the principles on which its action
depends.
[Illustration:
FIG. 37.—The Compound Microscope.
]
The object to be looked at is placed at _a_ (fig. 38), on a piece of
thin glass usually called a _slide_. A small converging lens placed at
_b_ collects the rays proceeding from the object, and transmits them as
far as _c d_, where they come under the influence of a second converging
lens B, which causes them to spread out still more before they reach the
eye. Consequently we not only see the image of the object magnified by
the lens _b_, but still more enlarged by the action of the lens B, and
appearing considerably enlarged at C D. The lens placed in front of the
object is called the _objective_ or _object-glass_; that placed nearest
the eye, the _eye-piece_. These names apply equally to the similar
lenses used in telescopes and other optical instruments. The instrument
shown in fig. 38 is the simplest possible compound microscope, and is
very rarely used. The eye-piece is generally constructed of two lenses,
and the object-glass of as many as eight; the object in multiplying the
lenses being, not only to increase the magnifying power, but to decrease
certain defects inherent in all lenses whose surfaces are parts of
spheres.
[Illustration:
FIG. 38.—The Theory of the Compound Microscope.
]
The amplification depends mainly upon the power of the objective, but
different eye-pieces are also used to increase the apparent size of the
objects to be examined. Thanks to the investigations of modern
philosophers, we are enabled to magnify objects to 2,000 times their
diameter with perfect distinctness; that is to say, the surface of the
object appears to occupy 4,000,000 times its natural extent. Under such
a power a hair would appear about six inches thick, a fine needle would
look like a street post, and a grain of sand like a mass of rock.
Although it is possible to employ compound microscopes of such a high
magnifying power in the investigation of certain classes of objects, all
ordinary preparations are best seen under a power of 500 or 600
diameters. It would be utterly impossible to give our readers the
slightest idea of the benefits conferred on the human race by this
marvellous instrument. Suffice it to say, that no naturalist or surgeon
ever attempts the most simple investigation into the structure of any
body without the aid of the microscope. It has already shown us that a
world of creatures exists which, although invisible to the eye of man,
are possessed of wonderful forms, colour, and beauty of structure, and
is daily adding to our knowledge in this direction. We can hardly submit
any substances to this marvellous instrument without discovering animal
or vegetable life of the most vivid character. A drop of scum from the
surface of a stagnant pool is instantly seen to be peopled with animal
and vegetable life, when submitted to microscopic examination. At one
moment a rolling ball glistening like glass slowly revolves past our
view; then a little fellow like a piece of spiral spring screws his way
along, backing when he meets with an obstacle; or a shuttle-shaped
vegetable, apparently made of glass, with green balls inside him, slowly
works his way from side to side, or, possibly, a mad battledore-shaped
being dashes past at an inconceivable rate.
As it is indispensable that the object should be well lighted, a concave
mirror is placed below it to reflect the rays of light from a lamp or
white cloud, through the object when it is transparent. When it is
opaque, it is illuminated by the rays of light being concentrated upon
it by means of a convex lens. The name microscope appears by common
consent to be applied more particularly to the compound instrument, the
epithet of magnifier or magnifying-glass being kept for simple
microscopes, although they are all, strictly speaking, _microscopes_.
In the ordinary compound microscope, it is only possible for one person
to see the object to be examined at once; for popular exhibitions of
microscopic objects the reflecting microscope has been devised, by means
of which the images of the objects to be looked at are thrown upon a
screen. The principle of this instrument is the same as that of the
magic lantern and phantasmagoria, of which we shall speak presently.
Fig. 39 (see next page) represents the photo-electric microscope, so
called from the objects being reflected by the electric light.
The jars seen on the ground are the cells of a voltaic battery, by which
the electricity is generated. The luminous rays starting from the
incandescent charcoal points are reflected through the tube and its
lenses by the reflector placed at the back of the instrument, and are
concentrated upon the object to be magnified. The image thus produced
passes through a second system of converging lenses, and is projected
upon the screen magnified some millions of times according to the power
of the object-glass employed.
[Illustration:
FIG. 39.—Photo-Electric Microscope.
]
“The experiments made with the photo-electric microscopes,” says M.
Ganot, “are amongst the most curious and pleasing to be found in the
whole range of physical science. With this instrument it is possible to
show the smallest objects magnified almost indefinitely to an unlimited
number of spectators. A human hair will appear as large as a broomstick,
an ordinary flea will look the size of a sheep, and the tiny cheese
mite, as well as the smallest animalcules, will be visible in all their
beauty of form and colour as clearly as if they were seen with the naked
eye. One of the most remarkable experiments to be made with this
instrument is that which shows the circulation of the blood. The tail of
a live tadpole is inserted between two plates of glass, or on an
instrument specially made for the purpose, and placed in the microscope
armed with a somewhat low power. The spectator immediately perceives
upon the screen a mass of rivers and rivulets, all flowing with the red
corpuscles forming the blood of the animal, and rushing through its
veins and arteries with inconceivable rapidity. Another interesting
experiment consists in dissolving a small quantity of sal-ammoniac in
warm water, and passing a small portion of the solution across a warm
glass slide. When placed in the microscope the water gradually
evaporates, leaving behind a mass of feathery crystals, whose growth may
be watched atom by atom, each crystalline molecule grouping itself
around the others in forms resembling a mass of fern-leaves.”
The apparatus we have been describing is sometimes illuminated with the
rays of the sun, as in the following figure.
[Illustration:
FIG. 40.—Solar Microscope.
]
It is then called the solar microscope, and exhibits objects with great
beauty and clearness. The use of the sun’s rays, however, has, in our
own country at least, been entirely superseded by the electric and lime
light. The latter method of illumination, which consists in projecting a
stream of oxygen and hydrogen upon a ball of lime, is cheaper and more
certain than the electric light, although the latter is possibly the
more brilliant of the two. The construction of the solar microscope
differs but little from the instrument already described, and may be
readily understood from the foregoing figure. The large mirror is placed
outside the window of the room in which the microscope stands, so that
the solar rays are reflected upon the surface of a series of convergent
lenses, and from thence on to another mirror, from which it is again
reflected through the microscope. As the position of the sun is
constantly changing, it is necessary to connect the outside mirror with
a train of clockwork. It may be mentioned that an instrument of this
kind, for reflecting the sun’s rays, is called a heliostat.
The student will, no doubt, at once perceive that if we concentrate the
light of the sun upon an object, we shall also concentrate the heat, and
either melt or consume it. A screen is therefore used in such cases,
which will allow the light to pass while holding back the rays of heat.
A solution of alum is found to answer the purpose admirably.
------------------------------------------------------------------------
CHAPTER IX.
THE TELESCOPES OF GALILEO, GREGORY, NEWTON, HERSCHEL, LORD ROSSE, AND
FOUCAULT.
IF history has failed to furnish us with the name of the inventor of the
microscope, we have very exact information as to the first experimenters
upon the powers of the telescope.
“In the archives of the Hague,” says Arago, “we find documents, by the
aid of which Van Swieten and Moll have come to a decisive conclusion as
to the first and true inventor of the telescope.”
We read in these documents that a spectacle-maker of Middleburg, named
John Lippershey, addressed a petition to the States-General on October
2, 1606, in which he asked leave to take out a patent, which should
constitute him the only maker of this instrument, or which should confer
upon him an annual pension, on the condition of not manufacturing them
for other nations. The petition qualifies the instrument as serving to
see distinct objects, as had already been explained to the members of
the States-General.
On the 4th of October, 1608, the States-General appointed a deputy from
each province to experiment on the new instrument, which was placed on a
tower of the palace belonging to the Stadtholder. Huggard says that the
first telescopes experimented on were a foot and a half in length.
On the 6th of October, the commission declared the instrument of
Lippershey to be useful to the nation, but demanded that it should be
made for two eyes instead of one.
On the 9th of December, Lippershey, having announced that he had solved
the problem, Van Dorth, Magnus, and Van der Au were ordered to verify
the fact, which they did by making a very favourable report on the 11th
of the same month. The binocular instrument was therefore found to
answer.
In reading the extracts from the archives of the Hague, given by Moll,
we may remark with great pleasure the promptitude with which the
commissioners of the States-General examined Lippershey’s instruments.
But their satisfaction soon gave way to displeasure, when they found a
large number of opticians making these instruments, and selling them to
foreigners, like so much spice from the East Indies. Later on one feels
indignant at finding the commissioners of the States-General to be so
wanting in proper feeling as to decide that the telescope must be
considered imperfect until it could be used with both eyes, without
either winking or seeing the reflection of the pupils in the eye-pieces.
Consequently, instead of being permitted to expend his talent on
perfecting the optical powers of the single telescope, Lippershey saw
himself condemned to waste his time upon the double instrument. The
States-General finished by giving Lippershey 900 florins; but they
refused him a patent, on the ground that it was already notorious that
other opticians had commenced the manufacture of similar instruments.
Amongst others who were rivals of Lippershey, we must mention John
Adrian Metius, the son of Adrian Metius, of Amsterdam, who discovered
that the nearest relation of the circumference of a circle to its
diameter was 355 to 113. He addressed a letter to the States-General on
the 17th of October, 1608, conceived in the following terms:—
“After two years’ labour and thought I have succeeded in making
an instrument, by the aid of which objects which are too distant
to be visible by the eye, are seen plainly. The one I show,
although constructed out of bad materials, and simply as an
experiment, is, in the judgment of the Stadtholder and of
several other persons, as good as the one lately presented to
the States-General by a citizen of Middleburg. I am sure of
improving it still further in the course of time, and I beg to
ask for a patent by which any person who is not already in
possession of this invention will be forbidden, under pain of a
heavy fine and confiscation, to make or sell similar instruments
for twenty-two years.”
The States-General refused to grant the patent in this case also, but
enjoined Metius to perfect his instrument, reserving to themselves the
power to reward him in the future if they thought fit.
In Italy, Galileo is generally supposed to have discovered independently
the method of making a telescope on the principle of the Dutch
philosophers, about the beginning of 1609, having received a very
imperfect account of these instruments somewhere about that time. It may
be remarked that in his letter to the chiefs of the Venetian Republic,
giving an account of the properties of these new instruments, Galileo
states that, if necessary, they could be made specially for the use of
the navy and army belonging to the state. But secrecy was useless, for
telescopes were already made and sold in Holland at a cheap rate.
Besides, Galileo makes no allusion to the labours of his Dutch
predecessors, either in a prior letter handed down to us by Venturi, or
in the decree of the Venetian Senate, dated August 5, 1609.
The Italian commentators are in error when they attribute the second
discovery of the telescope to the knowledge that Galileo possessed of
the laws of refraction, and that it was by deductions therefrom that he
was enabled to construct his first instruments.
Huyghens says, in his _Treatise on Dioptrics_, “I will unhesitatingly
place that man above all mortals, who, by the aid of his own reflections
and without the aid of accident, first succeeded in constructing a
telescope.”
“Let us see,” says Arago, when speaking on this subject, “if Lippershey
and John Adrian Metius were men of unparalleled powers.”
Hieronymus Saturnus tells us that an unknown man of genius called upon
Lippershey, and ordered from him a number of convex and concave lenses.
At the time agreed upon the man returned, and chose two, one convex and
the other concave, and, placing them one before his eye and the other at
some distance from it, drew them backwards and forwards, without giving
any explanation of his manœuvres, paid the optician, and left the place.
As soon as he was gone, Lippershey began immediately to imitate the
experiments of the stranger, and soon found that distant objects were
brought apparently nearer, when the lenses were placed in certain
positions. He next fastened them to the ends of a tube, and lost no time
in presenting the new instrument to Prince Maurice of Nassau.
According to another version, Lippershey’s children were playing in
their father’s shop, and were looking through two lenses, one convex and
the other concave, when they found to their surprise that the vane on
the clock-tower of Middleburg Church was greatly magnified and
apparently brought nearer. The surprise expressed by the children having
awakened the attention of Lippershey, he tried the experiment of fixing
the lenses on a piece of board; afterwards he tried it again by fixing
them at the ends of two pieces of tube, sliding in each other, and
succeeded in making the first telescope on record.
The principal documents from which the above facts touching Lippershey
have been extracted, are to be found in a memoir on the subject by
Olbers, printed in Schumacher’s _Astronomical Annual_ for 1843.
It was said in the time of Galileo that he had in his possession a
telescope by the aid of which he could see the birds flying at Fiesole
from the window of his palace in Florence. This story does not in the
least detract from the merit of the illustrious astronomer, who not only
constructed a telescope for himself, but was the first to direct it
heavenwards, and that too by purely theoretical researches; for in spite
of all the documents adduced above, there is little or no proof that he
had ever seen or heard of the Dutchman’s telescope. It is only right,
therefore, that the instrument constructed on this principle should be
called the Galilean telescope. He afterwards increased its power from
four to thirty times, beyond which he could not get with the means at
his command. With his imperfect instruments Galileo discovered the
satellites of Jupiter, the mountains of the moon, and the spots on the
sun, and earned for himself the name of Lynceus, who according to the
ancients was one of the Argonauts, possessed of the power of seeing
through a wall. Towards the end of his life, when the old man was blind,
and the Academy of the Lincei treated his hypotheses with disdain, he
would laugh sadly at the name bestowed on him, and the obstinate
Academy. Fig. 41 (see next page) shows the path of the rays in a
Galilean telescope. The object-glass _O_ is double convex, and the
eye-piece o bi-concave. The image is formed between these lenses, and
the eye appears to see it at that point. The States-General complained
of being obliged to shut one eye when looking through a telescope, but
in 1671 a good Capuchin monk, whose name was Cherubino, placed two
telescopes together, little thinking that the moderns would imitate him
in that very worldly instrument, the opera-glass.
[Illustration:
Fig. 41.—The Galilean Telescope.
]
Everybody has noticed that when objects are close to us they appear
larger than when they are at a distance; it accordingly amounts to the
same thing whether, in speaking of the power of telescopes, we say they
magnify twice, four times, or a hundred times, or that they are brought
within half, a quarter, or a hundredth of their distance. Thus there is
a telescope at Lord Rosse’s Observatory, at Parsonstown in Ireland,
which is the finest yet constructed. Its highest magnifying power is
6,000, therefore every object we look at with it is brought within the
6,000th of its distance from us. Looking at the moon, for instance, we
know that our satellite is distant some 240,000 miles from us; we have,
therefore, only to divide that number by 6,000 to find that by means of
this wonderful instrument the moon is brought within 40 miles of the
earth. This statement, however, is not strictly true, for it supposes
the whole of the apparatus used to be theoretically perfect.
Kepler, whose great name is now-a-days always associated with that of
Galileo, but who during their life-time was somewhat his rival,
substituted for the single lens forming the eye-piece a combination
consisting of two convex lenses, in order to obtain a larger field for
observation than that given by the single bi-concave. This combination
is commonly known as the astronomical eye-piece. It reverses the object
looked at, but for astronomical purposes this defect is of no
consequence.
[Illustration:
FIG. 42.—The Astronomical Telescope.
]
The instrument shown in the above figure represents an astronomical
telescope reduced to its simplest form.
Fixed parallel to the axis of the larger telescope is the finder, a
small telescope of low power and large field, used for finding celestial
objects not easily visible to the naked eye. It is so arranged, that
when the object is found and carried to its centre, it is also in the
centre of the field of the larger instrument. The handle and the two
toothed wheels serve to raise or lower the telescope, which is movable
on the horizontal axis, which supports it in front, so that it may be
directed to any part of the heavens the observer may desire.
The following figure shows the arrangement of the lenses, and the path
of the rays through them, in telescopes of this form.
[Illustration:
FIG. 43.—Section of an Astronomical Telescope.
]
The convex lens which serves as an object-glass, gives at _a b_ a
reversed image of the star A B. The small convex lens which acts the
part of an eye-piece, enlarged this reversed image without changing its
position, and causes it to be seen in the line A´ B´. This eye-piece is
fixed at the extremity of a tube, which is smaller than that containing
the object-glass, and slides easily backwards and forwards from the spot
where the image _a b_ is found. The latter is an indispensable
condition, for it is rare to meet two persons whose eyes are of the same
focus; besides, the image _a b_ will fall at a different spot for
objects at different distances: thus, if you are looking at the moon,
and suddenly turn the instrument on to a distant nebula, you will find
that the eye-piece requires adjusting. In showing ordinary observers an
object in the telescope, it is well to insist on their moving the
eye-piece backwards and forwards until distinct vision is obtained, for
it often happens that people will say they see an object quite
distinctly, when it is in reality misty, and will generally refuse to
allow the focus to be altered. It is very singular how human vanity or
complaisance will step in when some persons are looking through a
telescope. They seem to think that there is some disgrace or rudeness
involved in their not being able to see what their predecessors at the
instrument have seen. Poor John Leech leaves us an amusing instance of
this in a comic cut inserted in one of the early numbers of our old
friend _Punch_. A gentleman is endeavouring to show a lady a distant
steamboat through a telescope, but she has it accidentally pointed at
two swans that are swimming on the margin of the lake below;
consequently when he asks her if she sees the steamer, she replies that
“she sees it most distinctly, and there are two of them,” a pretty good
proof that the instrument was not only pointed at the wrong object, but
was out of focus as well.
In constructing a telescope similar to the one described above, the
object-glass ought to be of considerable diameter and of long focus; the
eye-piece, on the contrary, should be comparatively small and of short
focus. A little consideration will show the reason of this. An
object-glass of long focus will form a large image at the point _a b_,
and the eye-piece of short focus will magnify this image more than
another lens of less convexity. It is, however, on the size, length of
focus, and perfection of workmanship of the objective that the
excellence of the telescope depends; large object-glasses are
consequently rare, and are only to be found in observatories of the
first class. The object-glass of the large telescope at the Observatory
at Paris is nearly fifteen inches in diameter, and the highest
magnifying power capable of being employed with it is 3,000. The
Observatory of Pulkowa, near St. Petersburg, possesses a similar
instrument, and the Observatory at Chicago, United States, a still
larger one, measuring between eighteen and nineteen inches in diameter.
But the largest of all is an objective in the possession of Mr.
Buckingham, an amateur astronomer, who has an observatory near London,
which is twenty inches in diameter, and twenty-eight feet in focal
length.
The eye-pieces of astronomical telescopes are of different powers, and
are changed according to the class of object to be observed. Thus, in
taking a general view of the moon, a low power would be used. If you
wished to examine any particular mountain, you would raise the
magnifying power by inserting a stronger eye-piece. The power used also
depends on the state of the atmosphere. For instance, on warm evenings,
when the air is charged with moisture, the tremulousness of the
atmosphere is so great, that it is often only possible to use the very
lowest power. By combining four convex lenses together, we obtain what
is called a terrestrial or erecting eye-piece, which has the property of
_re-reversing_ the image formed by the objective. The eye-pieces of all
telescopes for use on land or at sea are made on this principle. The
same effect may be obtained, as we have already shown in fig. 41, by
using a concave lens, but in this the field of view is much diminished.
Hitherto we have only spoken of refracting telescopes or those
instruments provided with a convex object-glass, to collect and refract
the rays of light given off by the object we are desirous of examining;
but there is another and very important class of instruments, in which
the object-glass is replaced by a reflecting mirror. The first
reflecting telescope was invented by Dr. Gregory, an English
philosopher, about 1650. It consisted of a brass tube, at the lower
extremity of which was fixed a concave mirror made of metal, and
provided with a hole in its centre for the insertion of the small tube
containing the eye-glass. Towards the other end of the telescope was
placed a second and smaller mirror, which reflected the image formed by
the large mirror, through the eye-piece to the eye. The following figure
will show the path of the rays in the Gregorian telescope.
[Illustration:
FIG. 44.—Section of the Gregorian Telescope.
]
The rays A B, proceeding from the object at which the instrument is
pointed, are first reflected from the surface of the principal mirror M
M on to the small mirror _m_, whence they proceed to form a magnified
image at _a b_, which is then again enlarged by the eye-piece appearing
to the eye as if placed at A´ B´. The focus in the Gregorian is altered,
not by sliding the eye-piece backwards and forwards but by moving the
mirror _m_, which is provided with a long screw, to which is attached a
handle. At first sight a reflecting telescope has the appearance of a
very stumpy-looking refracting instrument, but one instant’s examination
will show the observer that the usual object-glass is absent at the end
of the tube. In fig. 45 we have a Gregorian telescope, mounted on a
tripod stand.
[Illustration:
FIG. 45.—Gregorian Telescope.
]
Whilst experimenting on the Gregorian telescope, Newton made certain
improvements in its construction, which we shall proceed to describe. A
glance at fig. 46 will show that the path of the rays is much more
simple than in the instrument we have just noticed.
[Illustration:
FIG. 46.—Section of a Newtonian Telescope.
]
The rays of light A B are first reflected from the concave mirror M on
to the surface of the small plane mirror _m_, which is placed at an
angle of 45°, and reflects them as far as the point A´ B´, where they
form the image to be magnified by the eye-glass. It is therefore at the
_side_ of the instrument, and not at the end, as hitherto, that the
observer is placed, and at right angles to the path of the rays.
Observers looking at an object through a Newtonian telescope for the
first time are generally sufficiently astonished to find that there is
really no difficulty after all in seeing round a corner. We shall
presently return to the subject of Newtonian telescopes, which were
abandoned by astronomers for many years, until they were brought into
use again by M. Foucault, a distinguished French philosopher.
Towards the end of the last century Sir William Herschel invented and
constructed the reflecting telescope which bears his name. His great
object was to avoid the loss of light consequent on the double
reflection which took place in all instruments constructed up to that
time, and he succeeded at last in making a telescope in which the
observer looked directly through the eye-piece at the image formed by
the mirror, which was inclined in such a manner that the rays were
reflected to the lower edge of the open end of the tube. In using this
kind of telescope the observer is placed with his back to the object he
wishes to examine, a position that is even more astonishing to those
unaccustomed to the use of a Herschellian telescope than the one assumed
when employing an instrument of the Newtonian construction. This
position has the defect of causing a small portion of the rays
proceeding from the object to be intercepted by the head of the
observer, but the amount of light lost is so small in comparison to the
size of the mirror that in practice it amounts to nothing.
The dimensions of the telescope constructed by Herschel were enormous
for that day. It measured 40 feet long, and the mirror was 4 feet in
diameter. It was supported by a complicated system of scaffolding,
pulleys, and cords, and was capable of magnifying an object 6,000 times.
It was by means of this splendid instrument that Sir William Herschel
made those wonderful discoveries in astronomy which are inseparably
associated with his name. With it he discovered the planet Uranus, many
of the double stars, and a large number of nebulæ, which up to that time
were unknown. His son, Sir John Herschel, inherits his father’s talents
as an astronomer, and has enriched science with numberless observations
and discoveries of the greatest importance made with this fine
instrument. Fig. 47 shows the construction of the Herschellian
telescope, and the path of the rays may be easily followed by the
student without any help from us.
The vulgar, ever prone to make mountains out of molehills, magnified the
power of Sir William Herschel’s telescope beyond all bounds. Stories
were circulated about his having given a dinner in the interior of the
tube to a select party of friends, but as the diameter of the telescope
was only a little more than 4 feet, the entertainment, to say the least
of it, would have proved somewhat inconvenient to the guests. Another
story, which was credited by great numbers of people, was that he had
discovered inhabitants in the moon, but that he hesitated to make the
matter public for fear he should be prosecuted for spreading atheistical
notions. In fact, the tales told of Sir William Herschel’s telescope
were endless, and caused the astronomer great inconvenience by
attracting crowds of idle people to the neighbourhood of Slough, where
he vainly endeavoured to carry on his investigations in peace and
quietness. It was in vain that these silly assertions were disproved
again and again. Having once believed them, people were slow to reject
them, and the story of the dinner was told over and over again for many
years.
[Illustration:
FIG. 47.—The Herschellian Telescope.
]
The instrument above described is one of those known as _front view
telescopes_, on account of the image of the star being reflected from
the surface of the mirror, which was placed obliquely at the bottom of
the tube in front of the observer, who examined it by means of the
eye-piece without any other reflection taking place, thereby effecting a
saving of light, which fully compensated for any loss caused by the
mirror being placed askew. The concave mirror made by Herschel alone
weighed a ton, to say nothing of the enormous tube and its fittings.
Herschel had consequently to invent a special apparatus for holding and
moving this gigantic instrument. The moving gear consisted of a mass of
beams, pulleys and cords, reminding one more of the rigging of a ship
than of a philosophical instrument. The apparatus for moving the
telescope appeared so complicated to the casual observer, although in
reality it was very simple, that it doubtless contributed in no small
degree to the propagation of the fanciful stories we have already spoken
of.
The performances of this splendid instrument hardly came up to the
expectations of those who saw it in progress. Herschel, it is true, was
enabled by its means to use a power of from 3 to 6,000, but he could
only use these amplifications on a few objects—the planets, for
instance, giving so little light under a high power as to become
indistinct and misty. In 1802 Baron von Zach, in his _Monthly
Astronomical Compendium_, went so far as to say that this colossal
instrument was not of the slightest utility, that no discovery had ever
been made with it, and that it ought to be considered merely as an
optical curiosity. Subsequent events, however, proved very conclusively
that Baron von Zach was utterly wrong in his statements and prophecies.
The telescope constructed by Herschel, although very wonderful for the
day in which it was made, has long since been eclipsed by that belonging
to Lord Rosse, and erected by his late father at Birr Castle, near
Parsonstown in Ireland. It is superior to Herschel’s instrument both in
point of size, and workmanship. The late Lord Rosse, not fearing that
his dignity would be compromised by such an act, went boldly to work,
and learned to polish mirrors like an ordinary workman, the consequence
of which was that he could bestow unusual pains upon the finishing of
the speculum. His Lordship not only learnt the mere handicraft of
speculum polishing, but went deeply into the engineering difficulties of
the operation, and succeeded in inventing many improvements for
diminishing labour and rendering the form of the surface more perfect.
The specula ground and polished under Lord Rosse’s method are almost
entirely free from what is called spherical aberration,—that is to say,
all rays proceeding from a single point of light, such as a star, are
collected into a single point instead of being scattered in a round
mass. This freedom from spherical aberration is of course necessary to
produce perfectly distinct images. In his _Life of Newton_ Sir David
Brewster calls it one of the most marvellous combinations of art and
science yet seen in the world.
The tube of Lord Rosse’s instrument is 55 feet long, and weighs 6½ tons.
In form it may be compared to the chimney of a steamboat of enormous
size. At one end it terminates in a kind of square box, within which is
contained the mirror, whose diameter is 6 feet, and which weighs nearly
4 tons. The weight of the whole apparatus is consequently nearly 10½
tons, or four times as much as Herschel’s. It is erected on an oblong
mass of masonry, 75 feet in length from north to south, between two
solid walls nearly 50 feet high, which serve as supports for the
mechanism intended to move this enormous tube in all directions. To the
walls are also fixed movable staircases with platforms that can be
brought up to the eye-piece with the greatest facility, no matter in
what position the telescope may be placed. This noble instrument has
penetrated space to a distance perfectly unattempted before its
existence, and has resolved numerous nebulæ into masses of stars that
until then were supposed to be mere clouds of luminous matter. The exact
forms of other nebulæ have also been accurately determined by this
telescope, which fully deserves the glowing eulogium passed upon it by
the Duke of Argyle in his presidential address at the meeting of the
British Association at Glasgow, in 1855. “This instrument,” said his
Grace, “in extending the range of astronomical science as it has done,
has been the means of throwing certain doubts upon the laws that govern
the motions of the heavenly bodies, and render it possible that certain
of the far-distant nebulæ are regulated in their movements by other laws
than those to which the members of our own system are subjected.”
The clearness with which this telescope exhibits every object within its
range is so great that the most distant nebulæ are seen with as great
distinctness as the nearest planet. On directing it towards the moon,
which is only distant from us about 240,000 miles, the surface of our
satellite may be explored with a facility almost as great as that with
which we examine the details of a landscape with an ordinary telescope.
Maedler, a German astronomer, who has measured nearly every mountain and
valley on the moon’s surface with the greatest exactitude, stated some
years before Lord Rosse’s telescope was perfected that if a monument as
large as one of the Pyramids existed on the surface of the moon it could
have been readily distinguished by the instruments then in use. With
Lord Rosse’s telescope we can see the surface of our satellite so much
enlarged that a space 220 feet square could be readily perceived by a
good observer. This enormous eye, measuring 6 feet in diameter, would
hardly show us a lunar elephant; but it is certain that if a troop of
buffaloes, or animals analogous to them, crossed the field of vision,
they would undoubtedly be perceptible. Masses of troops marching
backwards and forwards would also be plainly visible, and we may assert
with something like absolute certainty that there are neither towns nor
villages in the moon, nor any buildings as large as St. Paul’s of London
or the colossal railway stations of that metropolis.
This telescope, as we have said before, is the largest hitherto
constructed, and cost its noble constructor more than 25,000_l._ It must
also be recollected that it was not a mere scientific toy belonging to
an amateur philosopher, but a real working instrument in the possession
of a true man of science, who did work with it that will render his name
famous while civilization lasts. The present Lord Rosse seems worthy in
every way of his father’s great name, and has already enriched
astronomical science with numerous valuable observations.
We shall finish this chapter by a description of the Newtonian telescope
constructed by M. Léon Foucault. The mirror, instead of being made of
speculum metal, which is an alloy of tin and copper, is made of glass
from the famous manufactory of St. Gobain. The first rough grinding
having been finished, it passed into the workshops of M. Secrétan, the
optician to the Paris Observatory, to receive its final polish and
finishing touches from the hand of M. Foucault himself, the most careful
optical tests being applied to it before the commencement of each
operation.
The glass mirror having reached the degree of perfection desired, was
then silvered on its concave surface by being plunged into a bath of
nitrate of silver, dissolved in water, and mixed with certain
proportions of gum galbanum, nitrate of ammonia, and oil of cloves. Half
an hour in this bath was sufficient for the deposition of a film of
silver of sufficient thickness to bear polishing. When finished, the
mirror was found to reflect 92 per cent. of the light incident on its
surface, the loss in the case of achromatic object-glasses and metal
specula being 20 and 35 per cent. respectively. The substitution of a
parabolic glass mirror for the ordinary metal speculum offers the triple
advantage of greater lightness, increased distinctness, and more
brilliant images. Fig. 48 represents the large silvered glass telescope
constructed under M. Foucault’s direction for the observatory at
Marseilles. It measures 32 inches in diameter, and has a focal length of
a little more than 16 feet, and is put in motion by clockwork of a very
perfect description, so that when once pointed at a star or planet it
follows the object, which would otherwise disappear on account of the
rotation of the earth. The path taken by the rays is precisely the same
as in Newton’s telescope, the eye-piece being placed at the side of the
tube, which is provided with a movable platform and staircase for the
observer.
[Illustration:
FIG. 48.—Foucault’s Large Telescope.
]
The optician to whose talent in his art this fine instrument is due, has
recently executed several small telescopes upon the same model, at such
a price as to bring them within the reach of amateurs with slender
purses. The principal part of these telescopes, one of which is
represented in fig. 49, (see next page), is the mirror, which is about 4
inches in diameter, and 24 inches’ focal length. The body, which is
cylindrical, is made of brass, and revolves on two pivots placed
horizontally at about one-third of its length from the bottom. The
bearings on which the pivots move consist of two upright standards of
metal, which are connected at the bottom, and revolve on a pin in the
middle of the plate of the tripod stand. They are made of such a height
that the lower portion of the instrument may pass between them, when it
is necessary to observe objects in the zenith. By the turn of a screw
the whole of the upper portion of the instrument may be dismounted and
fixed on a lower standard, so that the observer may work sitting down if
necessary. The body of the telescope is provided with a finder. One of
the great advantages of this form of instrument is that it can be used
for observations on the zenith without giving the observer those
unpleasant cricks in the neck so inseparable from the use of ordinary
telescopes in a nearly upright condition. The mirror will bear a power
of 220 diameters, and shows the mountains of the moon, the phases of
Mercury and Venus, Saturn and his ring, Jupiter and his satellites, and
a large number of double stars and nebulæ. It is provided with a set of
eye-pieces, so that any power almost from 50 to 220 diameters may be
used at will. The figure on the opposite page will give the amateur a
good idea of the form and size of this instrument.
[Illustration:
FIG. 49.—Foucault’s Small Telescope.
]
------------------------------------------------------------------------
PART III.
NATURAL MAGIC.
--------------
CHAPTER I.
THE MAGIC LANTERN.
THE illusions of which we have spoken in the first part of this work
depended principally on the nature of man’s vision, who, we found, was
the constant and heedless victim of his own powers of sight. We shall
now examine a series of illusions that are still more extraordinary, but
which have nothing to do with the deceptions practised on us by our
visual organs. Instead of being deceived by ourselves, we shall find
that we are led astray by others whose knowledge of the laws of optics
is greater than our own, enabling them to construct instruments capable
of amusing us or imposing on us, according to our ignorance of natural
laws. Let us hope, however, that the science of optics has now become so
familiar to most educated people, that no such thing as a real
imposition can take place, although at the present day there are so many
exhibitions of the marvellous that ordinary observers have the greatest
difficulty in accounting for them. In former ages, when the knowledge of
science was confined to a certain class, the commonest optical facts of
the present day were taken advantage of to delude the ignorant. The
deceptions practised by the ancient priests of Egypt, Greece, and Rome
were undoubtedly many of them of this description. It is a well known
fact that both plane and concave metallic mirrors were used by the
ancients, and a passage in Pliny gives an account of certain glass
mirrors that were made at Sidon. Aulus Gellius, quoting Varro, speaks of
the reflecting properties of hollow mirrors, and we shall see, as we go
on, what a number of illusions may be practised by means of a series of
plane mirrors arranged in a particular way. But we will first devote a
short time to the curious historical facts connected with the principle
of the magic lantern which took place long before the modern invention
of this instrument by Father Kircher.
Brewster says, when treating of this subject, that there can be little
doubt that the concave mirror was the principal instrument used in
connexion with the pretended apparitions of the gods and goddesses in
the ancient temples. In the meagre history of these apparitions that has
come down to us, we can easily perceive the traces of an optical
illusion. In the ancient temple of Hercules at Tyre, there existed a
certain seat made of consecrated stone, out of which the gods rose,
apparently at the will of the priests. Æsculapius appeared frequently to
his worshippers in his temple in Tarsus, and the temple of Eugenium was
famous for the number of gods and goddesses which were constantly
visiting its sacred precincts. Iamblicus tells us that the priests
showed the gods to the people in the midst of smoke; and when the great
magician Marinus terrified his auditory by suddenly showing them the
statue of Hercules in the midst of a cloud of incense, it was
undoubtedly a woman who performed the part, dressed up in man’s robes
for the occasion.
The character of these spectacles in the ancient temples is admirably
described by Damasius, and there is no difficulty in seeing that optical
illusions were the means employed to delude the audience. He describes
the apparition on the wall of a large spot of white, which at first
appeared at a distance, but gradually came nearer and nearer until at
last it assumed the form of a divine or supernatural being, of severe
yet mild aspect and of great personal beauty. This being the
Alexandrians immediately honoured as Osiris or Adonis.
Amongst more modern examples of this illusion may be mentioned that of
the Emperor Basil of Macedonia. Inconsolable at the loss of his son,
this potentate had recourse to the prayers of the Pontiff Theodore
Lantabaren, who was celebrated for his power of working miracles. The
conjurer showed the Emperor the image of his dead son magnificently
attired and mounted on a splendid war-horse. The young man dismounted,
and, going up to his father, threw himself into his arms and
disappeared. Salvertius, in speaking of this story, observes
judiciously, that the deception could only take place through the agency
of some person who closely resembled the Emperor’s son, and that the
trick would have been easily discovered when the person embraced the
Emperor. A better explanation of the affair is, however, afforded by
supposing that the Emperor saw an aërial image of a person resembling
his son, and that when he rushed forward to embrace him it disappeared.
The accounts of the operations of the ancient magicians are too meagre
to give us any idea of the splendour of some of these ancient
ceremonies. A system of deception such as this, employed as a means of
government, must have brought into requisition not only the talents of
all the learned men of the day, but a crowd of accessories calculated to
astonish and confound the judgment, fascinate the senses, and facilitate
imposture.
An account of an instance of modern necromancy has been left us by
Benvenuto Cellini, who played a prominent part in a case of this sort.
He accidentally made the acquaintance of a Sicilian priest, a man of
great genius and acquirements, and well versed in Greek and Latin
classical lore. One day the conversation turned on necromancy, and the
great goldsmith told him that he had the greatest desire to know
something about this wonderful art, and that he had felt all his life a
great curiosity to penetrate its mysteries.
The priest replied, that a man ought to have a very resolute and
fearless character to study this art; but Benvenuto answered he had both
resolution and courage. The priest went on to say, that if he had the
heart to try, he would be the means of obtaining the fulfilment of his
wishes. They consequently agreed upon a plan of necromantic study. One
evening, Benvenuto invited one of his companions, Vincenzio Romoli, to
take part in some experiments that were to be made amongst the ruins of
the Coliseum. They there met the Sicilian priest, who after the manner
of the ancients began to describe a number of circles in the air in the
most imposing manner. He had brought with him various gums and perfumes,
and had made a fire, into which his assistant necromancer was to throw
them at the proper time. He commenced his conjurations, the ceremony
continuing about an hour, when there appeared legions of demons, in such
numbers that the whole of the ruins seemed filled with them. Benvenuto
was nearly fainting with the perfumes, when the priest roused him by
telling him to ask for something. He replied, that he wished to be
transported to the side of his Sicilian mistress; but the demons were
evidently unpropitious, for nothing came of it. His instructor, however,
told him that they must repeat their experiments a second time, and that
Benvenuto must bring with him a child that had never committed sin. The
next time Benvenuto took with him a boy of twelve years old whom he had
in his service, and his friends Romoli and Guddi. When they arrived at
the place of meeting, they found the priest had made the same
preparations as before. This time, however, he used more powerful
conjurations, calling on a number of demons by their names, in Hebrew,
Greek, and Latin; so that the ruin was filled with a still greater mass
of them than on the other occasion. The fire and perfumes were put under
the charge of Guddi and Romoli, and he gave Benvenuto a magic picture to
hold in a certain direction, the boy being placed underneath it. The
priest told him again to wish to be in the company of his lady love, but
on his expressing the wish, the magician told him that the demons still
refused to do his bidding in this way, but that he should visit her once
more in a month’s time. The poor boy underneath the magic picture was
seized with a terrible fright, and exclaimed, that he saw millions of
ferocious spirits and four giants, all endeavouring to break through the
magic circle the priest had formed. All there were evidently in a most
abject state of terror, and remained in the place until the church bells
began to ring for morning prayers, when they returned home, the boy
declaring that two of the demons preceded them, dancing and gambolling
before them, and sometimes running along the housetops.
The priest then advised him to try another spiritual _séance_, and
endeavour to induce the demons to point out sundry pots of buried gold,
so that they all might become rich, but it does not appear that the
priest’s advice was followed.
It is impossible to read the foregoing description of what happened,
without being convinced that the whole affair was an optical illusion,
and not the mere result of the imagination of those who took part in it.
The smoke was evidently caused in order to afford a field for the
exhibition of painted images reflected by concave mirrors, and the
circle was formed in order that those within it might be within range of
the images formed on the smoke. The mirrors reflecting the images of the
demons had undoubtedly already been arranged so that they would fall
just above the fire, and become visible when the gums began to burn with
a smoky flame. The perfumes were simply to help to stupify the
spectators, and aid in working on their imaginations for those
occurrences which were beyond the reach of optics, for the poor
unfortunate boy saw things that his companions did not, even to a couple
of demons dancing through the streets in broad daylight. In fact, it is
somewhat difficult to draw the line between reality and imagination in
this case. No doubt the story is considerably exaggerated by Cellini,
who was a fervid Italian, and prone to believe in wonders, as is
instanced by his wish to study the black art. The priest, too, whom he
describes as a man of genius, no doubt had a great influence over the
famous artist, and made him see a great deal more than was really there.
The introduction of the magic lantern provided the magicians of the
seventeenth century with a very powerful instrument with which to
continue their deceptions. The use of the concave mirror, which does not
appear to have had any accessories worth speaking of, required a
separate apartment, or at least a hiding-place of some sort that was
difficult to discover under ordinary circumstances; but the magic
lantern, inclosing as it did the lamp, the optical apparatus, and the
figures in a comparatively small spice, was particularly appropriate to
the wants of the Homes and Davenports of the day, who until then had
never possessed anything so convenient and portable.
[Illustration:
FIG. 50.—Section of the Magic Lantern.
]
The magic lantern shown in figures 50 and 51 consists of a dark box,
containing a lamp and a concave metallic mirror, constructed in such a
way that the whole of the rays proceeding from the lamp are reflected
through the aperture holding the optical portion of the apparatus. In
front of the box is fixed a double tube C D, one-half of which (D)
slides in the other. A large plano-convex lens _c_ is fixed at the inner
extremity of the double tube, and a small one at its outer end. To the
fixed tube C E is attached a groove _b b_, which serves to hold the
painted glass. These glasses, or slides as they are generally called,
are painted with strong transparent colours.
The direct light of the lamp G, as well as that reflected by the mirror
and passing through the lens _c_, is so concentrated as to project a
brilliant beam of light through the painted slide, which being in the
conjugate focus of the large plano-convex lens _d_, the pictures on the
glass are refracted in a magnified form on the white cloth P Q.
The magic lantern, therefore, consists of a box to hold the lamp, a
concave mirror, and a convex lens to concentrate the light on the slide,
and a second convex lens to throw the image on the screen.
------------------------------------------------------------------------
[Illustration:
FIG. 51.—Magic Lantern.
]
------------------------------------------------------------------------