is by first covering its centre by a circular piece of paper, as much
as one half of its diameter, and adjusting it for distinct vision of a
given object, such as the disk above mentioned, when the central rays
are intercepted--and then trying if the focal length remains unaltered
when the paper is taken away, and an aperture of the same size applied,
so that the extreme rays may in their turn be cut off. If the vision
remains equally distinct in both cases, without any new adjustment
for focal distance, the figure is good, and the spherical aberration
cured, and it may be seen by viewing a star of the first magnitude
successively in both cases, whether the irradiation is produced more
by the extreme or by the central parts of the glass. Or, in case the
one half be faulty and the other good, a semicircular aperture, by
being turned gradually round in trial, will detect what semicircle
contains the defective portion of the glass; and if such portion should
be covered, the only inconvenience that would ensue, would be the loss
of so much light as is thus excluded. When an object-glass produces
radiations in a large star, it is unfit for the nicer observations of
astronomy, such as viewing double stars of the first class. The smaller
a large star appears in any telescope, the better is the figure of the
object-glass, but if the image of the star be free from wings, the size
of its disk is not an objection in practical observations.[30]
Some opticians are in the habit of inserting a diaphragm into the
body of the large tube, to cut off the extreme rays coming from the
object-glass when the figure is not good, instead of lessening the
aperture by a cap. When this is the case, a deficiency of light will
be the consequence beyond what the apparent aperture warrants. It
is therefore proper to examine that the diaphragm be not placed too
near the object-glass, so as to intercept any of the useful rays.
Sometimes a portion of the object-glass is cut off by the stop in
the eye-tube. To ascertain this, adjust the telescope to distinct
vision, then take out the eye-glasses, and put your finger on some
other object on the edge of the outside of the object-glass, and look
down the tube; if you can see the tip of your finger, or any object
in its place, just peeping over the edge of the object-glass, no part
is cut off. I once had a 3-1/2 feet telescope whose object-glass
measured 3 inches diameter, which was neither so bright, nor did it
perform in other respects nearly so well as another of the same length
whose object-glass was only 2-3/4 inches diameter; but I found that a
diaphragm was placed about a foot within the end of the large tube,
which reduced the aperture of the object-glass to less than 2-1/2
inches; and when it was removed the telescope was less distinct than
before. The powers given along with this instrument were much lower
than usual--none of them exceeding 100 times. This is a trick not
uncommon with some opticians.
Dr. Pearson mentions that an old Dollond’s telescope of 63 inches focal
length, and 3-3/4 inches aperture, supposed to be an excellent one,
was brought to Mr. Tulley, when he was present, and the result of the
examination was that its achromatism was not perfect. The imperfection
was thus determined by experiment. A small glass globe was placed at 40
yards distance from the object-end of the telescope when the sun was
shining, and the speck of light seen reflected from this globe formed a
good substitute for a large star, as an object to be viewed. When the
focal length of the object-glass was adjusted to this luminous object,
no judgment could be formed of its prismatic aberrations, till the
eye-piece had been pushed in beyond the place of correct vision; but
when the telescope was shortened a little, the luminous disk occasioned
by such shortening was strongly tinged with _red_ rays at its
circumference. On the contrary, when the eye-piece was drawn out, so as
to lengthen the telescope too much, the disk thus produced was tinged
with a small circle of _red_ at its _centre_, thereby denoting that
the convex lens had too short a focal length; and Mr. Tulley observed,
that if one or both of the curves of the convex lens were flattened
till the total focal length should be about 4 inches increased, it
would render the telescope quite achromatic, provided in doing this the
aberration should not be increased.
The following general remarks may be added. 1. To make anything
like an accurate comparison of telescopes, they must be tried not
only at the same place, but as nearly as possible at the same time,
and, if the instruments are of the same length and construction,
if possible, with the same eye-piece. 2. A difference of 8 or 10
times in the magnifying power, will sometimes, on certain objects,
give quite a different character to a telescope. It has been found
by various experiments that object-glasses of two or three inches
longer focus will produce different vision with the same eye-piece.
3. Care must be taken to ascertain that the eye-glasses are perfectly
clean and free from defects. The defects of glass are either from
veins--specks--scratches--colour, or an incorrect figure. To discover
veins in an eye or an object-glass, place a candle at the distance of
4 or 5 yards; then look through the glass, and move it from your eye
till it appear full of light--you will then see every vein, or other
imperfection in it which may distort the objects and render vision
imperfect. Specks or scratches, especially in object-glasses, are not
so injurious as veins, for they do not distort the object, but only
intercept a portion of the light. 4. We cannot judge accurately of the
excellence of any telescope by observing objects with which we are
not familiarly acquainted. Opticians generally try an instrument at
their own marks, such as the dial-plate of a watch, a finely engraved
card, a weather-cock, or the moon and the planet Jupiter, when near
the meridian. Of several telescopes of the same length, aperture and
magnifying power, that one is generally considered the best with which
we can read a given print at the greatest distance, especially if the
print consists of _figures_, such as a table of logarithms, where the
eye is not apt to be deceived by the imagination, in _guessing_ at the
sense of a passage, when two or three words are distinguished.
There is a circumstance which I have frequently noticed, in reference
to achromatic telescopes, particularly those of a small size, and which
I have never seen noticed by any optical writer. It is this,--if the
telescope, when we are viewing objects, be gradually turned round its
axis, there is a certain position in which the objects will appear
distinct and accurately defined; and if it be turned round exactly
a semicircle from this point, the same degree of distinctness is
perceived; but in all other positions, there is an evident want of
clearness and defining power. This I find to be the case in more than
ten 1 foot and 2 feet telescopes now in my possession; and therefore
I have put marks upon the object-end of each of them, to indicate the
positions in which they should be used for distinct observation.--This
is a circumstance which requires, in many cases, to be attended to
in the choice and the use of telescopical instruments, and in fixing
and adjusting them on their pedestals. In some telescopes this defect
is very striking, but it is in some measure perceptible in the great
majority of instruments which I have had occasion to inspect. Even in
large and expensive achromatic telescopes this defect is sometimes
observable. I have an achromatic whose object-glass is 4-1/10 inches
diameter, which was much improved in its defining power, by being
unscrewed from its original position, or turned round its axis--about
one-eighth part of its circumference. This defect is best detected by
looking at a large printed bill, or a sign-post at a distance, when, on
turning round the telescope or object-glass, the letters will appear
much better defined in one position than in another. The position in
which the object appears least distinct is when the upper part of the
telescope is a quadrant of a circle different from the two positions
above-stated, or at an equal distance from each of them.
7. _On the mode of determining the magnifying power of Telescopes._
In regard to refracting telescopes, we have already shown that, when a
single eye-glass is used, the magnifying power may be found by dividing
the focal distance of the object-glass by that of the eye-glass. But
when a Huygenian eye-piece, or a four-glass terrestrial eye-piece such
as is now common in achromatic telescopes, is used, the magnifying
power cannot be ascertained in this manner; and in some of the delicate
observations of practical astronomy, it is of the utmost importance
to know the exact magnifying power of the instrument with which the
observations are made, particularly when micrometrical measurements
are employed to obtain the desired results.--The following is a
general method of finding the magnifying powers of telescopes when the
instrument called a _dynameter_ is not employed; and it answers for
refracting and reflecting telescopes of every description.
Having put up a small circle of paper, an inch or two in diameter, at
the distance of about 100 yards, draw upon a card 2 black parallel
lines, whose distance from each other is equal to the diameter of the
paper circle. Then view through the telescope the paper circle with one
eye, and the parallel lines with the other; and let the parallel lines
be moved nearer to or further from, the eye, till they seem exactly
to cover the small circle viewed through the telescope. The quotient
obtained by dividing the distance of the paper circle by the distance
of the parallel lines from the eye, will be the magnifying power of the
telescope. It requires a little practice before this experiment can
be performed with accuracy. The one eye must be accustomed to look at
an object near at hand, while the other is looking at a more distant
object through the telescope. Both eyes must be open at the same time,
and the image of the object seen through the telescope must be brought
into apparent contact with the real object near at hand. But a little
practice will soon enable any observer to perform the experiment with
ease and correctness, if the telescope be mounted on a firm stand, and
its elevation or depression produced by rack-work.
The following is another method, founded on the same
principle:--Measure the space occupied by a number of the courses, or
rows of bricks in a modern building--which, upon an average, is found
to have 8 courses in 2 feet, so that each course or row, is 3 inches.
Then cut a piece of paper 3 inches in height, and of the length of a
brick--which is about 9 inches--so that it may represent a brick, and
fixing the paper against the brick wall, place the telescope to be
examined at the distance of about 80 or 100 yards from it. Now, looking
through the telescope at the paper with one eye, and at the same time,
with the other eye, looking past the telescope, observe what extent of
wall the magnified image of the paper appears to cover, then count the
courses of bricks in that extent, and it will give the magnifying power
of the telescope. It is to be observed, however, that the magnifying
power determined in this way, will be a fraction greater than for very
distant objects, as the focal distance of the telescope is necessarily
lengthened in order to obtain distinct vision of near objects.
In comparing the magnifying powers of two telescopes, or of the same
telescope, when different magnifying powers are employed, I generally
use the following simple method. The telescopes are placed at 8 or 10
feet distant from a window, with their eye-ends parallel to each other,
or at the same distance from the window. Looking at a distant object,
I fix upon a portion of it whose magnified image will appear to fill
exactly two or three panes of the window. Then putting on a different
power, or looking through another telescope, I observe the same
object, and mark exactly the extent of its image on the window-panes,
and compare the extent of the one image with the other. Suppose for
example, that the one telescope has been previously found to magnify
90 times, and that the image of the object fixed upon exactly fills
three panes of the window, and that with the other power or the other
telescope, the image fills exactly two panes, then the magnifying power
is equal to two thirds of the former, or 60 times; and were it to
fill only one pane, the power would be about 30 times. A more correct
method is to place at one side of the window, a narrow board, two or
three feet long, divided into 15 or 20 equal parts, and observe how
many of these parts appear to be covered by the respective images, of
the different telescopes. Suppose, in the one case, 10 divisions to be
covered by the image, in a telescope magnifying 90 times, and that the
image of the same object in another telescope, measures 6 divisions,
then its power is found by the following proportion, 10 : 90 : 6 : 54 :
that is, this telescope magnifies 54 times.
Another mode which I have used for determining, to a near
approximation, the powers of telescopes, is as follows:--Endeavour
to find the focus of a single lens which is exactly equivalent to
the magnifying power of the eye-piece, whether the Huygenian or the
common terrestrial eye-piece. This may be done by taking a small lens,
and using it as an object-glass to the eye-piece. Looking through
the eye-piece to a window and holding the lens at a proper distance,
observe whether the image of one of the panes exactly coincides with
the pane, as seen by the naked eye; if it does, then the magnifying
power of the eye-piece is equal to that of the lens. If the lens be
1/2 inch focal length, the eye-piece will produce the same magnifying
power, as a single lens when used as an eye-glass to the telescope, and
the magnifying power will then be found by dividing the focal distance
of the object-glass by that of the eye-glass. But if the image of
the pane of glass does not exactly coincide with the pane as seen by
the other eye, then proportional parts may be taken by observing the
divisions of such a board as described above, or we may try lenses of
different focal distances. Suppose, for example, that a lens 2 inches
focal length had been used, and that the image of a pane covered
exactly the space of two panes, the power of the eye-piece is then
equal to that of a single lens 1 inch focal distance.
The following is another mode depending on the same general principle.
If a slip of writing-paper one inch long, or a disk of the same
material of one inch diameter, be placed on a black ground at from
30 to 50 yards distance from the object-end of the telescope, and a
staff painted white, and divided into inches and parts by strong black
lines, be placed vertically near the said paper or disk; the eye that
is directed through the telescope when adjusted for vision, will see
the magnified disk, and the other eye, looking along the outside of
the telescope, will observe the number of inches and parts that the
disk projected on it will just cover, and as many inches as are thus
covered will indicate the magnifying power of the telescope--at the
distance for which it is adjusted for distinct vision. The solar power,
or powers for very distant objects, may be obtained by the following
proportion:--As the terrestrial focal length, at the given distance:
is to the solar focal length :: so is the terrestrial power, to the
solar power. For example, a disk of white paper one inch in diameter,
was placed on a black board, and suspended on a wall contiguous to a
vertical black staff that was graduated into inches by strong white
lines, at a distance of 33 yards 2-1/2 feet, and when the adjustment
for vision was made with a 42 inch telescope, the left eye of the
observer viewed the disk projected on the staff, while the right eye
observed that the enlarged image of the disk covered just 58-1/2 inches
on the staff, which number was the measure of the magnifying power,
at the distance answering to 33 yards 2-1/2 feet--which in this case
exceeded the solar focus by an inch and a half. Then according to the
above analogy, we have, as 43.5 : 42 :: 58.5 : 56.5 nearly. Hence the
magnifying power due to the solar focal length of the telescope in
question is 56.5, and the distance 33 yards 2-1/2 feet, is that which
corresponds to an elongation of the solar focal distance an inch and a
half.[31] If we multiply the terrestrial and the solar focal distances
together, and divide the product by their difference, we shall again
obtain the distance of the terrestrial object from the telescope. Thus,
(43.5 + 42)/1.5 = 1218 inches = 101.5 feet, or 33 yards 2-1/2 feet.
The magnifying power of a telescope is also determined, by measuring
the image which the object-glass or the large speculum of a telescope
forms at its solar focus. This is accomplished by means of an
instrument called a _Dynameter_. This apparatus consists of a strip of
mother-of-pearl, marked with equal divisions, from the 1/100th to the
1/1000th of an inch apart, according to the accuracy required. This
measure is attached to a magnifying lens in its focus, in order to
make the small divisions more apparent. When the power of a telescope
is required, the person must measure the clear aperture of the
object-glass, then holding the pearl _dynameter_ next the eye-glass,
let him observe how many divisions the small circle of light occupies,
when the instrument is directed to a bright object. Then by dividing
the diameter of the object-glass by the diameter of this circle of
light, the power will be obtained.[32] The most accurate instrument
of this kind is the _Double Image Dynameter_ invented by Ramsden,
and another on the same principle now made by Dollond, a particular
description of which may be found in Dr. Pearson’s ‘Introduction to
Practical Astronomy.’ The advantage attending these dynameters is that
they do not require any knowledge of the thickness and focal lengths
of any of the lenses employed in a telescope, nor yet of their number
or relative positions; neither does it make any difference whether
the construction be refracting or reflecting, direct or inverting.
One operation includes the result arising from the most complicated
construction.
I shall only mention farther the following method of discovering the
magnifying power, which is founded on the same general principle as
alluded to above. Let the telescope be placed in such a position
opposite the sun, that the rays of light may fall perpendicularly on
the object-glass; and the pencil of rays may be received on a piece of
paper, and its diameter measured. Then, as the diameter of the pencil
of rays is to that of the object-glass, so is the magnifying power of
the telescope.
8.--_On cleaning the lenses of telescopes._--
It is necessary, in order to distinct vision, that the glasses,
particularly the eye-glasses of telescopes be kept perfectly clean,
free of damp, dust, or whatever may impede the transmission of the
rays of light. But great caution ought to be exercised in the wiping
of them, as they are apt to be scratched, or otherwise injured by
a rough and incautious mode of cleaning them. They should never be
attempted to be wiped unless they really require it; and, in this
case, they should be wiped carefully and gently with a piece of new
and soft lamb’s-skin leather. If this be not at hand, a piece of fine
silk paper, or fine clean linen may be used as a substitute. The lens
which requires to be most particularly attended to is the second glass
from the eye, or the field-glass; for if any dust or other impediment
be found upon this glass, it is always distinctly seen, being magnified
by the glass next the eye. The next glass which requires attention is
the fourth from the eye, or that which is next the object. Unless the
glass next the eye be very dusty, a few small spots or grains of dust
are seldom perceptible. The object-glass of an achromatic should seldom
be touched, unless damp adheres to it. Care should be taken never to
use pocket handkerchiefs or dirty rags for wiping lenses. From the
frequent use of such articles, the glasses of seaman’s telescopes get
dimmed and scratched in in the course of a few years. If the glasses be
exceedingly dirty, and if greasy substances are attached to them, they
may be soaked in spirits and water, and afterwards carefully wiped.
In replacing the glasses in their socket, care should be taken not
to touch the surfaces with the fingers, as they would be dimmed with
the perspiration: they should be taken hold of by the edges only, and
carefully screwed into the same cells from which they were taken.
ON MEGALASCOPES, OR TELESCOPES FOR VIEWING VERY NEAR OBJECTS.
It appears to have been almost overlooked by opticians and others, that
telescopes may be constructed so as to exhibit a beautiful and minute
view of very near objects, and to produce even a microscopic effect,
without the least alteration in the _arrangement_ of the lenses of
which they are composed. This object is effected simply by making the
eye-tube of a telescope of such a length as to be capable of being
drawn out 12 or 13 inches beyond the point of distinct vision for
distant objects. The telescope is then rendered capable of exhibiting
with distinctness all kinds of objects, from the most distant to those
which are placed within 3 or 4 feet of the instrument--or not nearer
than double the focal distance of the object-glass. Our telescopes,
however, are seldom or never fitted with tubes that slide farther than
an inch or two beyond the point of distinct vision for distant objects,
although a tube of a longer size than usual, or an additional tube
would cost but a very trifling expence.
The following, among many others, are some of the objects on which I
have tried many amusing experiments with telescopes fitted up with
the long tubes to which I allude. The telescope to which I shall
more particularly advert is an achromatic, mounted on a pedestal,
having an object-glass about 19 inches focal length, and 1-5/8 inch
diameter, with magnifying powers for distant objects of 13 and 20
times. When this instrument is directed to a miniature portrait,
3-1/2 inches in length, placed in a good light, at the distance of
about 8 or 10 feet, it appears as large as an oil-painting four or
five feet long, and represents the individual as large as life. The
features of the face appear to stand out in bold relief: and perhaps
there is no representation of the human figure that more resembles
the living prototype, than in this exhibition, provided the miniature
is finely executed. In this case the tube requires to be pulled out
four or five inches from the point of distinct vision for distant
objects, and consequently the magnifying power is proportionally
increased. Another class of objects to which such a telescope may be
applied is _Perspective prints_, either of public buildings, streets
or landscapes. When viewed in this way they present a panoramic
appearance, and seem nearly as natural as life--just in the same manner
as they appear in the Optical Diagonal Machine, or when reflected in
a large concave mirror--with this advantage, that, while in these
instruments the left hand side of the print appears where the right
should be,--the objects seen through the telescope appear exactly in
their natural position. In this case, however, the telescope should
have a small magnifying power, not exceeding 5 or 6 times, so as to
take in the whole of the landscape. If an astronomical eye-piece be
used, the print will require to be inverted.
Other kinds of objects which may be viewed with this instrument, are
trees, flowers, and other objects in gardens immediately adjacent to
the apartment in which we make our observations. In this way we may
obtain a distinct view of a variety of rural objects, which we cannot
easily approach, such as the buds and blossoms on the tops of trees,
and the insects with which they may be infested. There are certain
objects on which the telescope may be made to produce a powerful
microscopical effect, such as the more delicate and beautiful kinds
of flowers, the leaves of trees, and similar objects. In viewing
such objects, the telescope may be brought within little more than
double the focal distance of the object-glass from the objects to be
viewed, and then the magnifying power is very considerably increased.
A nosegay composed of a variety of delicate flowers, and even a
single flower, such as the sea-pink, makes a splendid appearance in
this way. A peacock’s feather, or even the fibres on a common quill,
appear very beautiful, when placed in a proper light. The leaves of
trees, particularly the leaf of the plane-tree, when placed against a
window-pane, so that the light may shine through them--appear, in all
their internal ramifications, more distinct, beautiful and interesting,
than when viewed in any other way; and in such views a large portion
of the object is at once exhibited to the eye. In this case, the
eye-piece of such a telescope as that alluded to requires to be drawn
out 12 or 14 inches beyond the point of distinct vision for objects
at a distance--and the distance between these near objects and the
object-end of the telescope, is only about 3-1/2 feet.
A telescope having a diagonal eye-piece presents a very pleasant view
of near objects in this manner. With an instrument of this kind, I have
frequently viewed the larger kind of small objects alluded to above,
such as the leaves of shrubs and trees, flowers consisting of a variety
of parts, the fibres of a peacock’s feather and similar objects. In
this case the object-glass of the instrument, which is 10-1/2 inches
focal length, was brought within 22 inches of the object, and the eye
looked down upon it, in the same manner, as when we view objects in
a compound microscope. A common pocket achromatic telescope may be
used for the purposes now stated, provided the tube in the eye-piece
containing the two lenses next the object, be taken out, in which case
the two glasses next the eye form an astronomical eye-piece, and the
tubes may be drawn out 5 or 6 inches beyond the focal point for distant
objects, and will produce distinct vision for objects not farther
distant than about 20 or 24 inches. But, in this case, the objects to
be viewed must be inverted, in order that they may be seen in their
natural positions when viewed through the instrument. Telescopes of
a large size and high magnifying powers may likewise be used with
advantage for viewing very near objects in gardens adjacent to the room
in which the instruments are placed, provided the sliding-tube next the
eye has a range of two or three inches beyond the point of vision for
distant objects. In this case, a magnifying power of 100 times on a
3-1/2 or a 5 feet achromatic produces a very pleasant effect. In making
the observations to which I have now alluded, it is requisite in order
to distinct vision, and to obtain a pleasing view of the objects, that
the instrument should be placed on a pedestal, and capable of a motion
in every direction. The adjustment for distinct vision may be made
either by the sliding-tube, or by removing the telescope nearer to or
farther from the object.
REFLECTIONS ON LIGHT AND VISION--AND ON THE NATURE AND UTILITY OF
TELESCOPES.
Light is one of the most wonderful and beneficial, and at the same time
one of the most mysterious agents in the material creation. Though the
sun from which it flows to this part of our system is nearly a hundred
millions of miles from our globe, yet we perceive it as evidently, and
feel its influence as powerfully, as if it emanated from no higher a
region than the clouds. It supplies life and comfort to our physical
system, and without its influence and operations on the various objects
around us, we could scarcely subsist and participate of enjoyment for a
single hour. It is diffused around us on every hand from its fountain
the sun; and even the stars, though at a distance hundreds of thousands
of times greater than that of the solar orb, transmit to our distant
region a portion of this element. It gives beauty and fertility to the
earth, it supports the vegetable and animal tribes, and is connected
with the various motions which are going forward throughout the
system of the universe. It unfolds to us the whole scenery of external
nature--the lofty mountains and the expansive plains, the majestic
rivers and the mighty ocean; the trees, the flowers, the crystal
streams, and the vast canopy of the sky adorned with ten thousands of
shining orbs. In short there is scarcely an object within the range of
our contemplation, but what is exhibited to our understanding through
the medium of light, or at least bears a certain relation to this
enlivening and universal agent. When we consider the extreme minuteness
of the rays of light, their inconceivable velocity, the invariable laws
by which they act upon all bodies, the multifarious phenomena produced
by their inflections, refractions and reflections, while their original
properties remain the same; the endless variety of colours they
produce on every part of our terrestrial creation, and the facility
with which millions of rays pass through the smallest apertures, and
pervade substances of great density, while every ray passes forward in
the crowd without disturbing another, and produces its own specific
impression--we cannot but regard this element as the most wonderful,
astonishing and delightful part of the material creation. When we
consider the admirable beauties and the exquisite pleasures of which
light is the essential source, and how much its nature is still
involved in mystery, notwithstanding the profound investigations of
modern philosophers, we may well exclaim with the Poet:--
“How then shall I attempt to sing of HIM
Who, light himself, in uncreated light
Invested deep, dwells awfully retired
From mortal eye or angel’s purer ken;
Whose single smile has, from the first of time,
Filled, overflowing, all yon lamps of heaven,
That beam for ever through the boundless sky.”--THOMSON.
The eye is the instrument by which we perceive the beautiful and
multifarious effects of this universal agent. Its delicate and
complicated structure, its diversified muscles, its coats and
membranes, its different humours possessed of different refractive
powers, and the various contrivances for performing and regulating
its external and internal motions, so as to accomplish the ends
intended--clearly demonstrate this organ to be a master-piece of
Divine mechanism--the workmanship of Him whose intelligence surpasses
conception, and whose Wisdom is unsearchable. ‘Our sight (says Addison)
is the most perfect and delightful of all our senses. It fills the
mind with the largest variety of ideas, converses with its objects at
the greatest distance, and continues the longest in action, without
being tired or satiated with its proper enjoyments. The sense of
feeling can indeed give us a notion of extension, shape, and all other
ideas that enter the eye, except colours; but at the same time it is
very much strained, and confined in its operation to the number, bulk
and distance of its particular objects. Our sight seems designed to
supply all these defects, and may be considered as a more delicate and
diffusive kind of touch that spreads itself over an infinite multitude
of bodies, comprehends the largest figures, and brings into our reach
some of the more remote parts of the universe.’
Could we suppose an order of beings endued with every human faculty but
that of sight, it would appear incredible to such beings--accustomed
only to the slow information of touch--that by the addition of an
organ consisting of a ball and socket, of an inch diameter, they might
be enabled, in an instant of time, without changing their place, to
perceive the disposition of a whole army, the order of a battle, the
figure of a magnificent palace, or all the variety of a landscape. If
a man were by feeling to find out the figure of the Peak of Teneriffe,
or even of St. Peter’s church at Rome, it would be the work of a
lifetime. It would appear still more incredible to such beings as we
have supposed, if they were informed of the discoveries which may
be made by this little organ in things far beyond the reach of any
other sense--that, by means of it we can find our way in the pathless
ocean--that we can traverse the globe of the earth, determine its
figure and dimensions, and delineate every region of it--yea, that we
can measure the planetary orbs, and make discoveries in the sphere of
the fixed stars. And, if they were farther informed that, by means of
this same organ, we can perceive the tempers and dispositions, the
passions and affections of our fellow-creatures, even when they want
most to conceal them--that when the tongue is taught most artfully to
lie and dissemble, the hypocrisy should appear in the countenance to a
discerning eye--and that by this organ we can often perceive what is
straight and what is crooked in the mind as well as in the body--would
it not appear still more astonishing to beings such as we have now
supposed?[33]
Notwithstanding these wonderful properties of the organ of vision, the
eye, when unassisted by art, is comparatively limited in the range
of its powers. It cannot ascertain the existence of certain objects
at the distance of three or four miles, nor perceive what is going
forward in nature or art beyond such a limit. By its natural powers
we perceive the moon to be a globe about half a degree in diameter,
and diversified with two or three dusky spots, and that the sun is
a luminous body of apparently the same size--that the planets are
luminous points, and that about a thousand stars exist in the visible
canopy of the sky. But the ten thousandth part of those luminaries,
which are within the reach of human vision, can never be seen by the
unassisted eye. Here the TELESCOPE interposes, and adds a new
power to the organ of vision, by which it is enabled to extend its
views to regions of space immeasurably distant, and to objects, the
number and magnitude of which could never otherwise have been surmised
by the human imagination. By its aid we obtain a sensible demonstration
that space is boundless--that the universe is replenished with
innumerable suns and worlds--that the remotest regions of immensity,
immeasurably beyond the limits of unassisted vision, display the
energies of Creating Power, and that the Empire of the Creator extends
far beyond what eye hath seen or the human imagination can conceive.
The telescope is an instrument of a much more wonderful nature than
what most people are apt to imagine. However popular such instruments
now are, and however common a circumstance it is to contemplate objects
at a great distance which the naked eye cannot discern, yet, prior
to their invention and improvement, it would have appeared a thing
most mysterious, if not impossible, that objects at the distance of
ten miles could be made to appear as if within a few yards of us, and
that some of the heavenly bodies could be seen as distinctly as if we
had been transported by some superior power, hundreds of millions of
miles beyond the bounds of our terrestrial habitation. Who could ever
have imagined--reasoning _a priori_--that the refraction of light in
glass--the same power by which a straight rod appears crooked in water,
by which vision is variously distorted, and by which we are liable to
innumerable deceptions--that that same power, or law of nature, by the
operation of which the objects in a landscape appear distorted when
seen through certain panes of glass in our windows, that that power
should ever be so modified and directed as to extend the boundaries
of vision, and enable us clearly to distinguish scenes and objects at
a distance a thousand times beyond the natural limits of our visual
organs? Yet such are the discoveries which science has achieved,
such the powers it has brought to light, that by glasses ground into
different forms, and properly adapted to each other, we are enabled as
it were to contract the boundaries of space, to penetrate into the most
distant regions, and to bring within the reach of our knowledge the
most sublime objects in the universe.
When Pliny declared in reference to Hipparchus, the ancient astronomer,
‘_Ausus rem Deo improbam annumerare posteris stellas_,’--that ‘he dared
to enumerate the stars for posterity, an undertaking forbidden by God,’
what would that natural historian have said, had it been foretold that
in less than 1600 years afterwards, a man would arise who should enable
posterity to perceive, and to enumerate ten times more new stars than
Hipparchus ever beheld--who should point out higher mountains on the
moon than on the earth, who should discover dark spots, as large as our
globe, in the sun, the fountain of light--who should descry four moons
revolving in different periods of time around the planet Jupiter, and
could show to surrounding senators the varying phases of Venus? and
that another would soon after arise who should point out a double ring
of six hundred thousand miles in circumference, revolving around the
planet Saturn, and ten hundreds of thousands of stars which neither
Hipparchus nor any of the ancient astronomers could ever descry? Yet
these are only a small portion of the discoveries made by Galileo
and Herschel, by means of the telescope. Had any one prophetically
informed Archimedes, the celebrated geometrician of Syracuse, that
vision would, in after ages, be thus wonderfully assisted by art--and
further, that one manner of improving vision would be to place a dark
_opake_ body directly between the object and the eye--and that another
method would be, not to look at the object, but to keep the eye quite
in a different, and even in an _opposite_ direction, or to stand with
the back directly opposed to it, and to behold all the parts of it,
invisible to the naked eye, most distinctly in this way--he would,
doubtless have considered the prophet as an enthusiastic fool or a
raving madman. Yet these things have been realized in modern times in
the fullest extent. In the Gregorian reflecting telescope an opake
body, namely the small speculum near the end of the tube, interposes
_directly_ between the eye and the object. In the Newtonian Reflector,
and in the diagonal eye-pieces formerly described, the eye is directed
in a line at right angles to the object, or a deviation of 90 degrees
from the direct line of vision. In Herschel’s’ large telescopes, and in
the _Aerial Reflector_ formerly described (in pp. 311-325) the back is
turned to the object, and the eye in an opposite direction.
These circumstances should teach us humility and a becoming diffidence
in our own powers; and they should admonish us not to be too dogmatical
or peremptory in affirming what is possible or impossible in regard
either to nature or art, or to the operations of the Divine Being.
Art has accomplished, in modern times, achievements, in regard to
locomotion, marine and aërial navigation, the improvement of vision,
the separation and combinations of invisible gases, and numerous
other objects, of which the men of former ages could not have formed
the least conception. And even yet, we can set no boundaries to the
future discoveries of science and the improvements of art; but have
every reason to indulge the hope that, in the ages to come, scenes of
Divine mechanism in the system of nature will be unfolded, and the
effects of chemical and mechanical powers displayed, of which the human
mind, in its present state of progress, cannot form the most imperfect
idea. Such circumstances likewise should teach us not to reject any
intimations which have been made to us in relation to the character,
attributes, and dispensations of the Divine Being, and the moral
revelations of his will given in the Sacred Records, because we are
unable to comprehend every truth and to remove every difficulty, which
relates to the moral government of the Great Ruler of the universe.
For, if we meet with many circumstances in secular science, and even in
the common operations of nature, which are difficult to comprehend--if
even the construction of such telescopes as we now use, would have
appeared an incomprehensible mystery to ancient philosophers--we must
expect to find difficulties almost insurmountable to such limited
minds as ours, in the eternal plans and moral arrangements of the
“King Immortal and Invisible,” as delineated only in their outlines,
in the Sacred Oracles--particularly those which relate to the origin
of physical and moral evil, the ultimate destiny of man, and the
invisible realities of a future world.
The UTILITY of the telescope may be considered in relation to
the following circumstances.
In the first place, it may be considered as an instrument or machine
which virtually transports us to the distant regions of space. When
we look at the moon through a telescope which magnifies 200 times,
and survey its extensive plains, its lofty peaks, its circular ranges
of mountains, throwing their deep shadows over the vales, its deep
and rugged caverns, and all the other varieties which appear on the
Lunar surface, we behold such objects in the same manner as if we were
standing at a point 238,800 miles from the earth in the direction of
the moon, or only twelve hundred miles from that orb, reckoning its
distance to be 240,000 miles. When we view the planet Saturn with a
similar instrument, and obtain a view of its belts, and satellites, and
its magnificent rings, we are transported, as it were, through regions
of space, to a point in the heavens more than _nine hundred millions
of miles_ from the surface of our globe, and contemplate those august
objects, as if we were placed within five millions of miles of the
surface of that planet.[34] Although a supernatural power, sufficient
to carry us in such a celestial journey, a thousand miles every day,
were exerted--it would require more than two thousand four hundred and
sixty years, before we could arrive at such a distant position; yet
the telescope, in a few moments, transports our visual powers to that
far distant point of space. When we view, with such an instrument, the
minute and very distant clusters of stars in the Milky Way, we are
carried in effect through the regions of space to the distance of _five
hundred thousand millions of miles_ from the earth; for we behold those
luminaries through the telescope nearly as if they were actually viewed
from such a distant point in the spaces of the firmament. These stars
cannot be conceived as less than _a hundred billions_ of miles from
our globe, and the instrument we have supposed brings them within the
two hundredth part of this distance. Suppose we were carried forward
by a rapid motion towards this point, at the rate of a thousand miles
_every hour_, it would require more than _fifty-seven thousand years_,
before we could reach that very distant station in space to which the
telescope, _in effect_, transports us. So that this instrument is far
more efficient in opening to our view the scenes of the universe than
if we were invested with powers of locomotion to carry us through the
regions of space, with the rapidity of a cannon ball at its utmost
velocity; and all the while we may sit at ease in our terrestrial
apartments.
In the next place, the telescope has been _the means of enlarging
our views of the sublime scenes of creation_, more than any other
instrument which art has contrived. Before the invention of this
instrument the universe was generally conceived as circumscribed within
very narrow limits. The earth was considered as among the largest
bodies in creation; the planets were viewed as bodies of a far less
size than what they are now found to be; no bodies similar to our moon
were suspected as revolving around any of them; and the stars were
supposed to be little more than a number of brilliant lamps hung up
to emit a few glimmering rays, and to adorn the canopy of our earthly
habitation. Such a wonderful phenomenon as the Ring of Saturn was
never once suspected, and the sun was considered as only a large ball
of fire. It was suspected, indeed, that the moon was diversified with
mountains and vales, and that it might possibly be a habitable world;
but nothing certainly could be determined on this point, on account of
the limited nature of unassisted vision. But the telescope has been
the means of expanding our views of the august scenes of creation to
an almost unlimited extent. It has withdrawn the veil which formerly
interposed to intercept our view of the distant glories of the sky.
It has brought to light five new planetary bodies, unknown to former
astronomers, one of which is more than eighty times larger than the
earth--and seventeen _secondary_ planets which revolve around the
primary. It has expanded the dimensions of the solar system to double
the extent which was formerly supposed. It has enabled us to descry
hundreds of comets which would otherwise have escaped our unassisted
vision, and to determine some of their trajectories and periods of
revolution.
It has explored the profundities of the Milky Way, and enabled us to
perceive hundreds of thousands of those splendid orbs, where scarcely
one is visible to the naked eye. It has laid open to our view thousands
of _Nebulæ_, of various descriptions, dispersed through different
regions of the firmament--many of them containing thousands of separate
stars. It has directed our investigations to thousands of double,
treble and multiple stars--suns revolving around suns, and systems
around systems, and has enabled us to determine some of the periods of
their revolutions. It has demonstrated the immense distances of the
starry orbs from our globe, and their consequent magnitudes; since
it shows us that, having brought them nearer to our view by several
hundreds or thousands of times, they still appear only as so many
shining _points_. It has enabled us to perceive that mighty changes are
going forward throughout the regions of immensity--new stars appearing,
and others removed from our view, and motions of incomprehensible
velocity carrying forward those magnificent orbs through the spaces of
the firmament. In short, it has opened a vista to regions of space so
immeasurably distant, that a cannon ball impelled with its greatest
velocity, would not reach tracts of creation so remote in two thousand
millions of years, and even light itself, the swiftest body in nature,
would require more than a thousand years before it could traverse this
mighty interval. It has thus laid a foundation for our acquiring an
approximate idea of the infinity of space, and for obtaining a glimpse
of the far distant scenes of creation, and the immense extent of the
universe.
Again, the telescope, in consequence of the discoveries it has enabled
us to make, has tended _to amplify our conceptions of the attributes
and the Empire of the Deity_. The amplitude of our conceptions of
the Divine Being bears a certain proportion to the expansion of our
views in regard to his works of creation, and the operations he is
incessantly carrying forward throughout the universe. If our views of
the works of God, and of the manifestations he has given of himself to
his intelligent creatures, be circumscribed to a narrow sphere, as to
a parish, a province, a kingdom, or a single world, our conceptions of
that Great Being, will be proportionably limited. For it is chiefly
from the manifestation of God in the material creation that our
ideas of his Power, his Wisdom, and his other natural attributes,
are derived. But in proportion to the ample range of prospect we
are enabled to take of the operations of the Most High, will be
our conceptions of his character, attributes, and agency. Now, the
telescope--more than any other invention of man--has tended to open to
our view the most magnificent and extensive prospects of the works of
God. It has led us to ascertain that, within the limits of the solar
system, there are bodies which, taken together, comprise a mass of
matter nearly two thousand five hundred times greater than that of the
earth--that these bodies are all constituted and arranged in such a
manner as to fit them for being habitable worlds--and that the sun, the
centre of this system, is five hundred times larger than the whole.
But, far beyond the limits of this system, it has presented to our view
a universe beyond the grasp of finite intelligences, and to which human
imagination can assign no boundaries. It has enabled us to descry suns
clustering behind suns, rising to view in boundless perspective, in
proportion to the extent of its magnifying and illuminating powers--the
numbers of which are to be estimated, not merely by thousands, and
tens of thousands, and hundreds of thousands, but by scores of
_millions_--leaving us no room to doubt that hundreds of millions more,
beyond the utmost limits of human vision, even when assisted by art,
lie hid from mortal view’s in the unexplored and unexplorable regions
of immensity.
Here, then, we are presented with a scene which gives us a display
of _Omnipotent Power_ which no other objects can unfold, and which,
without the aid of the telescope, we should never have beheld--a
scene which expands our conceptions of the Divine Being, to an extent
which the men of former generations could never have anticipated--a
scene which enables us to form an approximate idea of Him who is the
“King Eternal, Immortal, and Invisible,” who “created all worlds, and
for whose pleasure they are, and were created.” Here we behold the
operations of a Being whose power is illimitable and uncontrollable,
and which far transcends the comprehension of the highest created
intelligences--a power, displayed not only in the vast extension of
material existence, and the countless number of mighty globes which
the universe contains--but in the astonishingly _rapid motions_ with
which myriads of them are carried along through the immeasurable spaces
of creation,--some of those magnificent orbs moving with a velocity
of one hundred and seventy thousand miles an hour. Here, likewise, we
have a display of the infinite _Wisdom_ and Intelligence of the Divine
Mind, in the harmony and order with which all the mighty movements of
the universe are conducted--in proportionating the magnitudes, motions
and distances of the planetary worlds--in the nice adjustment of the
projectile velocity to the attractive power--in the constant proportion
between the times of the periodical revolution of the planets and
the cubes of their mean distances--in the _distances_ of the several
planets from the central body of the system, compared with their
respective _densities_--and in the constancy and regularity of their
motions, and the exactness with which they accomplish their destined
rounds--all which circumstances evidently show that He who contrived
the universe is “the only Wise God,” who is “wonderful in counsel
and excellent in working.” Here, in fine, is a display of _boundless
benevolence_. For we cannot suppose, for a moment, that so many myriads
of magnificent globes, fitted to be the centres of a countless number
of mighty worlds, should be nothing else than barren wastes, without
the least relation to intelligent existence. And if they are peopled
with intellectual beings of various orders--how vast must be their
numbers, and how overflowing that Divine Beneficence which has provided
for them all, every thing requisite to their existence and happiness!
In these discoveries of the telescope, we obtain a glimpse of the
grandeur and the unlimited extent of God’s universal empire. To this
empire no boundaries can be perceived. The larger, and the more
powerful our telescopes are, the further are we enabled to penetrate
into those distant and unknown regions; and however far we penetrate
into the abyss of space, new objects of wonder and magnificence still
continue rising to our view--affording the strongest presumption,
that were we to penetrate ten thousand times farther into those
remote spaces of immensity, new suns, and systems, and worlds would
be disclosed to our view. Over all this vast assemblage of material
existence, and over all the sensitive and intellectual beings it
contains, God eternally and unchangably presides; and the minutest
movements, either of the physical or the intelligent system, throughout
every department of those vast dominions, are at every moment “naked
and open” to his Omniscient eye. What _boundless Intelligence_ is
implied in the _Superintendence_ and _arrangement_ of the affairs of
such an unlimited empire! and what a lofty and expansive idea does it
convey of Him who sits on the throne of Universal Nature, and whose
greatness is unsearchable! But without the aids of the telescopic tube,
we could not have formed such ample conceptions of the greatness,
either of the Eternal Creator himself, or of the universe which he hath
brought into existence.
Besides the above, the following uses of the telescope, in relation to
science and common life, may be shortly noticed:--
In the business of astronomy, scarcely any thing can be done with
accuracy without the assistance of the telescope. 1. It enables the
astronomer to determine with precision _the transits of the planets
and stars_, across the meridian; and on the accuracy with which
these transits are obtained, a variety of important conclusions and
calculations depend. The computation of astronomical and nautical
tables for aiding the navigator in his voyages round the globe, and
facilitating his calculations of latitude and longitude, is derived
from observations made by the telescope, without the use of which
instrument, they cannot be made with precision. 2. The _apparent
diameters of the planets_ can only be measured by means of this
instrument, furnished with a micrometer. By the naked eye no accurate
measurements of the diameters of these bodies can be taken; and
without knowing their apparent diameters, in minutes or seconds, their
real bulk cannot be determined, even although their exact distances
be known. The differences, too, between their polar and equatorial
diameters cannot be ascertained without observations made by powerful
telescopes. For example, the equatorial diameter of Jupiter is found
to be in proportion to the polar as 14 to 13, that is, the equatorial
is more than 6000 miles longer than the polar diameter, which could
never have been determined by observations made by the naked eye.
3. The _parallaxes_ of the heavenly bodies can only be accurately
ascertained by the telescope; and it is only from the knowledge of
their parallaxes, that their distances from the earth or from the
sun can be determined. In the case of the fixed stars, nothing of
the nature of a parallax could ever be expected to be found without
the aid of a telescope. It was by searching for the parallax of a
certain fixed star, that the important fact of the _Aberration of
light_ was discovered. The observations, for this purpose, were made
by means of a telescope 24 feet long, fixed in a certain position. 4.
The motions and revolutionary periods of _Sidereal systems_, can only
be determined by observations made by telescopes of great magnifying
and illuminating powers. Without a telescope the small stars which
accompany double or treble stars cannot be perceived, and much less
their motions or variation of their relative positions. Before the
invention of the telescope such phenomena--now deemed so wonderful
and interesting--could never have been surmised. 5. The accurate
determination of the longitude of places on the earth’s surface is
ascertained by the telescope, by observing with this instrument the
immersions and emersions of the satellites of Jupiter. From such
observations, with the aid of a chronometer, and having the time at any
known place, the situation of any unknown place is easily determined.
But the eclipses of Jupiter’s moons can be perceived only by telescopic
instruments of considerable power. 6. By means of a telescope, with
cross hairs in the focus of the eye-glass, and attached to a Quadrant,
the altitude of the sun or of a star, particularly the pole-star, may
be most accurately taken; and, from such observations, the _latitude_
of the place may be readily and accurately deduced.
Again, in the _Surveying of land_, the telescope is particularly
useful; and for this purpose it is mounted on a stand with a horizontal
and vertical motion, pointing out by divisions the degrees and minutes
of inclination of the instrument. For the more accurate reading
of these divisions, the two limbs are furnished with a Nonius, or
_Vernier’s scale_. The object here is to take the angular distances
between distant objects on a plane truly horizontal; or else the
angular elevation or depression of objects above or below the plane
of the horizon. In order to obtain either of those kinds of angles to
a requisite degree of exactness, it is necessary that the surveyor
should have as clear and distinct a view as possible of the objects,
or station-staves, which he fixes up for his purpose, that he may
with the greater certainty determine the point of the object which
exactly corresponds with the line he is taking. Now, as such objects
are generally at too great a distance for the surveyor to be able
to distinguish with the naked eye, he takes the assistance of the
telescope, by which he obtains, 1. A distinct view of the object to
which his attention is directed, and 2. he is enabled to determine the
precise point of the object aimed at, by means of the cross hairs in
the focus of the eye-glass. A telescope mounted for this purpose is
called a _Theodolite_, which is derived from two Greek words θεομαι _to
see_, and οδος, _the way_ or _distance_.
In the next place, the telescope is an instrument of special
importance, in the conducting of _Telegraphs_, and in the conveyance
of _signals_ of all descriptions. Without its assistance telegraphic
dispatches could not be conveyed with accuracy to any considerable
distance, nor in quadruple the time in which they are now communicated,
and the different stations would need to be exceedingly numerous. But
by the assistance of the telescope information may be communicated, by
a series of telegraphs, with great rapidity. Twenty-seven telegraphs
convey information from Paris to Calais--a distance of 160 miles--in 3
minutes; twenty-two from Paris to Lisle in 2 minutes; forty-six from
Strasburg to Paris in 4-1/2 minutes; and eighty from Paris to Brest
in 10 minutes. In many other cases which occur both on land and on
sea, the telescope is essentially requisite for descrying signals.
The _Bell-Rock Light House_, for example, is situated 12 miles from
Arbroath, and from every other portion of land, so that the naked eye
could not discern any signal which the keepers of that light could have
it in their power to make; but by means of a large telescope in the
station-house in Arbroath, the hoisting of a ball every morning at 9
A.M.--which indicates that ‘All is well’--may be distinctly
recognised.
Many other uses of this instrument, in the ordinary transactions of
life, will readily occur to the reader; and therefore I shall only
mention the following purpose to which it may be applied, namely,--
_To measure the distance of an object from one station._ This depends
upon the increase of the focal distance of the telescope in the case
of near objects. Look through a telescope at the object whose distance
is required, and adjust the focus till it appear quite distinct;
then slide in the drawer, till the object begins to be obscure, and
mark that place of the tube precisely. Next draw out the tube till
the object begins to be again obscured, and then make another mark
as before. Then take the middle point between these two marks, and
that will be the point where the image of the object is formed most
distinctly; which is to be nicely measured from the object lens, and
compared with the solar focus of the lens or telescope, so as to
ascertain their difference. And the rule for finding the distance
is,--‘As the difference between the focal distance of the object, and
the solar focal distance : Is to the solar focal distance :: So is the
focal distance of the object : To its true distance from the object
lens.’ An example will render this matter more perspicuous.
[Illustration: _figure 84._]
Let AB (fig. 84.) be the object lens, EY the eye-glass, FC the radius,
or focus of the lens AB, and C_f_ the focal distance of the object OB,
whose distance is to be measured. Now suppose CF = 48 inches, or 4
feet, and that we find by the above method that C_f_ is 50 inches, then
F_f_ is 2 inches; and the analogy is:--As F_f_ = 2, is to CF = 48, so
is C_f_ = 50, to CQ = 1200 inches, or 100 feet. Again, suppose C_f_ =
49 inches, then will F_f_ = 1 inch; and the proportion is, 1 : 48 :: 49
: 2352 = QC, or 196 feet. A telescope of this focal length, however,
will measure only small distances. But, suppose AB a lens whose solar
focus is 12 feet, or 144 inches; and that we find, by the above method,
that C_f_, or the focal distance of the object, is 146 inches; then
will F_f_ be 2 inches, and the proportion will be, as 2 : 144 :: 146
: 21024 inches, or 1752 feet = the distance QC. If with such a large
telescope, we view an object OB, and find F_f_ but 1/10th of an inch,
this will give the distance of the object as 17292 feet or nearly 3-1/3
miles.
Since the difference between the radius of the object lens and the
focal distance of the object is so considerable as 2 inches in a tube
of 4 feet, and more than 12 inches in one of 12 feet, a method might
be contrived for determining the distance of near objects by the
former, and more distant objects by the latter, by inspection only.
This may be done by adjusting or drawing a spiral line round the drawer
or tube, through the _two inch space_ in the small telescope, and by
calculation, graduate it for every 100 feet, and the intermediate
inches, and then, at the same time we view an object, we may see its
distance on the tube. In making such experiments, a common object-glass
of a long focal length, and a single eye-glass, are all that is
requisite; since the inverted appearance of the object can cause no
great inconveniency.