ON THE DIFFERENT KINDS OF REFRACTING TELESCOPES.
There are two kinds of telescopes, corresponding to two modes of
vision, namely, those which perform their office by _refraction_
through lenses, and those which magnify distant objects by _reflection_
from mirrors. The telescope which is constructed with lenses, produces
its effects solely by refracted light, and is called a Dioptric, _or
refracting telescope_. The other kind of telescope produces its effects
partly by reflection, and partly by refraction, and is composed both
of mirrors and lenses; but the mirrors form the principal part of the
telescope; and therefore such instruments are denominated _reflecting
telescopes_. In this chapter I shall describe the various kinds of
_refracting_ telescopes.
SECT 1.--THE GALILEAN TELESCOPE.
This telescope is named after the celebrated Galileo, who first
constructed, and probably _invented_ it in the year 1609. It consists
of only two glasses, a _convex_ glass next the object, and a _concave_
next the eye. The convex is called the _object-glass_, and the concave
to which the eye is applied, is called the _eye-glass_. Let C (fig.
43.) represent the convex object-glass, presented to any object in the
direction DEI, so that the rays fall parallel upon it;--if these rays,
after passing through it, were not intercepted by the concave lens
K, they would pass on, and cross each other in the focus F, where an
inverted image of the object would be formed. But the concave lens K,
the virtual focus of which is at F, being interposed, the rays are not
suffered to converge to that point, but are made less convergent,[19]
and enter the pupil almost parallel, as GH, and are converged by the
humours of the eye to their proper foci on the retina. The object,
through this telescope, is seen upright, or in its natural position,
because the rays are not suffered to come to a focus, so as to form an
inverted picture. The concave eye-glass is placed as far within the
focus of the object-glass, as is equal to its own virtual focus; and
the magnifying power is as the focal length of the object-glass to
that of the eye-glass, that is, as CF to BF. Thus, suppose the focus of
the object-glass to be 10 inches, and the focus of the eye-glass to be
1 inch, the magnifying power will be 10 times--which is always found by
dividing the focal length of the object-glass by that of the eye-glass.
The interval between the two glasses, in this case, will be 9 inches,
which is the length of the telescope, and the objects seen through it
will appear under an angle ten times greater than they do to the naked
eye. These propositions might be proved mathematically; but the process
is somewhat tedious and intricate, and might not fully be understood
by general readers. I shall therefore only mention some of the general
properties of this telescope, which is now seldom used, except for the
purpose of _opera-glasses_.
[Illustration: _figure 43_]
1. The focal distance of the object-glass must be greater than that of
the eye-glass, otherwise it would not magnify an object: if the focal
distance of the eye-glass were greater than that of the object-glass,
it would diminish objects, instead of magnifying them. 2. The visible
area of the object is greater, the nearer the eye is to the glass; and
it depends on the diameter of the pupil of the eye, and on the breadth
of the object-glass; consequently the field of view in this telescope
is very small. 3. The distinctness of vision in this construction
of a telescope exceeds that of almost any other. This arises from
the rays of light proceeding from the object directly through the
lenses, _without crossing_ or intersecting each other; whereas in the
combination of convex lenses, they intersect one another to form an
image in the focus of the object-glass, and this image is magnified by
the eye-glass with all its imperfections and distortions. The thinness
of the centre of the concave lens also contributes to distinctness.
4. Although the field of view in this telescope is very small, yet
where no other telescope can be procured, it might be made of such a
length as to show the spots on the Sun, the crescent of Venus, the
satellites of Jupiter, and the ring of Saturn; and, requiring only two
glasses, it is the cheapest of all telescopes. It has been found that
an object-lens 5 feet focal distance, will bear a concave eye-glass of
only 1 inch focal distance, and will consequently magnify the diameters
of the planets 60 times, and their surfaces 3600 times, which is
sufficient to show the phenomena now stated. And, although only a small
portion of the sun and moon can be seen at once, yet Jupiter and all
his satellites may sometimes be seen at one view; but there is some
difficulty in finding objects with such telescopes. 5. Opera-glasses,
which are always of this construction, have the object-lens generally
about 6 inches focus and 1 inch diameter, with a concave eye-glass of
about 2 inches focus. These glasses magnify about 3 times in diameter,
have a pretty large field, and produce very distinct vision. When
adjusted to the eye, they are about 4 inches in length. To the object
end of an opera-glass there is sometimes attached a plane mirror,
placed at an angle of 45 degrees, for the purpose of viewing objects on
either side of us. By this means, in a theatre or assembly, we can take
a view of any person without his having the least suspicion of it, as
the glass is directed in quite a different direction. The instrument
with this appendage is sometimes called a _Polemoscope_.
SECT. 2.--THE COMMON ASTRONOMICAL REFRACTING TELESCOPE.
The astronomical telescope is the most simple construction of a
telescope, composed of convex lenses only, of which there are but
two essentially necessary, though a third is sometimes added to
the eye-piece for the purpose of enlarging the field of view. Its
construction will be easily understood from a description of the
following figure. Its two essential parts are, an object-glass AD,
and an eye-glass EY, so combined in a tube that the focus F of the
object-glass is exactly coincident with the focus of the eye-glass.
Let OB (fig. 44.) represent a distant object, from which rays nearly
parallel proceed to the object-lens AD. The rays passing through this
lens will cross at F, and form an image of the object at IM. This image
forms as it were an object to the eye-glass EY, which is of a short
focal distance, and the eye is thus enabled to contemplate the object
as if it were brought much nearer than it is in reality. For the rays,
which after crossing proceed in a divergent state, fall upon the lens
EY, as if they proceeded from a real object situated at F. All that
is effected therefore, by such a telescope is, to form an image of
a distant object by means of the object-lens, and then to give the
eye such assistance as is necessary for viewing that image as near as
possible, so that the angle it shall subtend at the eye shall be very
large compared with the angle which the object itself would subtend in
the same situation.
[Illustration: _figure 44._]
Here it may be expedient to explain, 1. how this arrangement of glasses
shows distant objects distinctly, and 2. the reason why objects appear
magnified when seen through it. As to the first particular, it may be
proved as follows:--The rays OA and BD, which are parallel before they
fall upon the object-glass, are by this glass refracted and united
at its focus: In order, then, to distinct vision, the eye-glass must
re-establish the parallelism of the rays,--which is effected by placing
the eye-glass so that its focus may be at F, and consequently the rays
will proceed from it parallel to each other and fall upon the eye in
that direction. For distinct vision is produced by _parallel_ rays. 2.
The reason why the object appears magnified will appear, if we consider
that, if the eye viewed the object from the centre of the object-glass,
it would see it under the angle OCB; let OC and BC then be produced
to the focus of the glass, they will then limit the image IM formed
in the focus. If then, two parallel rays are supposed to proceed to
the eye-glass EY, they will be converged to its focus H, and the eye
will see the image under the angle EHY. The apparent magnitude of the
object, therefore, as seen by the naked eye, is to the magnitude of the
image as seen through the telescope, as OCB to EHY, or as the distance
CF to the distance FG, in other words, _as the focal length of the
object-glass to that of the eye-glass_.
It is obvious from the figure, that, through this telescope, all
objects will appear _inverted_; since the object OB is depicted by the
object-glass in an inverted position at IM, and in this position is
viewed by the eye-glass EY; and, therefore this kind of telescope is
not well adapted for viewing terrestrial objects, since it exhibits
the tops of trees, houses, and other objects as undermost, and the
heads of people as pointing downwards. But this circumstance is of
no consequence with respect to the heavenly bodies, since they are
round, and it can make little difference to an observer which side
of a globular body appears uppermost or undermost. All astronomical
refracting telescopes invert objects; but they are preferred to any
other telescopes, because they have few glasses, and consequently more
light. This telescope however, can be transformed into a common day
telescope for land objects, by the addition of two other eye-glasses,
as we shall afterwards explain; but in this case a quantity of light is
lost by refraction at each lens; for there is scarcely any transparent
substance that transmits all the rays of light that fall upon it.
The _magnifying power_ of this telescope is found _by dividing the
focal distance of the object-glass by the focal distance of the
eye-glass_: the quotient gives the magnifying power, or the number of
times that the object seen through the telescope, appears larger or
nearer than to the naked eye. Thus, for example, if the focal distance
of the object-glass be 28 inches, and the focal distance of the
eye-glass 1 inch, the magnifying power will be 28 times. If we would
enlarge the telescope and select an object-glass 10 feet, or 120 inches
focus, an eye-glass of 2 inches focal length might be applied, and
then the diameter of objects would be magnified 60 times, and their
surfaces 3600 times. If we would use an object-glass of 100 feet, it
would be necessary to select an eye-glass about 6 inches focus, and
the magnifying power would be 200 times, equal to 1200 inches divided
by 6. Since, then, the power of magnifying depends on the proportion
of the focal length of the object and eye-glasses, and this proportion
may be varied to any degree, it may seem strange to some that a short
telescope of this kind will not answer that purpose as well as a long
one. For instance, it may be asked why an object-glass of 10 feet
focus, may not be made to magnify as much, as one of 100 feet focal
length, by using an eye-glass of half an inch focus, in which case,
the magnifying power would be 240 times? But it is to be considered,
that if the power of magnifying be increased, while the length of the
telescope remains the same, it is necessary to diminish the focal
length of the eye-glass in the same proportion, and this cannot be
done on account of the great distortion and colouring which would then
appear in the image, arising both from the deep convexity of the lens
and the different refrangibility of the rays of light. It is found
that the length of common refracting telescopes must be increased in
proportion to the square of the increase of their magnifying power; so
that in order to magnify twice as much as before, with the same light
and distinctness, the telescope must be lengthened four times; to
magnify 3 times as much, 9 times; and to magnify four times as much,
sixteen times; that is--suppose a telescope of 3 feet to magnify 33
times,--in order to procure a power four times as great, or 132 times,
we must extend the telescope to the length of 48 feet, or 16 times the
length of the other. Much likewise depends upon the breadth or aperture
of the object-glass. If it be too small, there will not be sufficient
light to illuminate the object; and if it be too large, the redundance
of light will produce confusion in the image.
The following table, constructed originally by Huygens, and which I
have re-calculated and corrected, shows the linear aperture, the focal
distance of the eye-glass, and the magnifying power of astronomical
telescopes of different lengths, which may serve as a guide to those
who wish to construct telescopes of this description.
---------------+----------------------------------+-----------
Focal distance | Linear aperture Focal distance | Magnifying
of the | of the of the | power.
object-glass. | object-glass. eye-glass. |
---------------+-----------------+----------------+-----------
Feet. | Inch. Dec. | Inch. Dec. |
1 | 0. 545 | 0. 605 | 20
2 | 0. 76 | 0. 84 | 28.5
3 | 0. 94 | 1. 04 | 34.6
4 | 1. 08 | 1. 18 | 40
5 | 1. 21 | 1. 33 | 45
6 | 1. 32 | 1. 45 | 50
7 | 1. 43 | 1. 58 | 53
8 | 1. 53 | 1. 69 | 56.8
9 | 1. 62 | 1. 78 | 60.6
10 | 1. 71 | 1. 88 | 63.8
15 | 2. 10 | 2. 30 | 78
20 | 2. 43 | 2. 68 | 89.5
30 | 3. 00 | 3. 28 | 109
40 | 3. 43 | 3. 76 | 127
50 | 3. 84 | 4. 20 | 142
60 | 4. 20 | 4. 60 | 156
70 | 4. 55 | 5. 00 | 168
80 | 4. 83 | 5. 35 | 179
90 | 5. 15 | 5. 65 | 190
100 | 5. 40 | 5. 95 | 200
120 | 5. 90 | 6. 52 | 220
In the above table, the first column expresses the focal length of
the object-glass in feet; the second column, the diameter of the
aperture[20] of the object-glass, the third column, the focal distance
of the eye-glass, and the fourth, the magnifying power, which is found
by reducing the feet in the first column to inches, and dividing by the
numbers in the third column. From this table it appears that, in order
to obtain a magnifying power of 168 times, by this kind of telescope,
it is requisite to have an object-glass of 70 feet focal distance,
and an eye-glass five inches focus, and that the aperture of the
object-glass ought not to be more than about 4-1/2 inches diameter. To
obtain a power of 220 times requires a length of 120 feet.
The following is a summary view of the properties of this telescope. 1.
The object is always inverted. 2. The magnifying power is always in the
proportion of the focal distance of the object-glass to the eye-glass.
3. As the rays emerging from the eye-glass, should be rendered parallel
for every eye, there is a small sliding tube next the eye, which should
be pushed out or in till the object appears distinct. When objects
are pretty near, this tube requires to be pulled out a little. These
circumstances require to be attended to in all telescopes. 4. The
apparent magnitude of an object is the same wherever the eye be placed,
but the visible area, or field of view, is the greatest when the eye
is nearly at the focal distance of the eye-glass. 5. The visual angle
depends on the breadth of the eye-glass; for it is equal to the angle
which the eye-glass subtends at the object-glass; but the breadth of
the eye-glass cannot be increased beyond a certain limit, without
producing colouring and distortion.
If the general principles on which this telescope is constructed
be thoroughly understood, it will be quite easy for the reader to
understand the construction of all the other kinds of telescopes,
whether refracting or reflecting. A small astronomical telescope can be
constructed in a few moments, provided one has at hand the following
lenses:--1. A common reading-glass, eight or ten inches focal distance;
2. A common magnifying lens, such as watchmakers or botanists use, of
about 1-1/2 or 2 inches focus. Hold the reading-glass--suppose of ten
inches focus--in the left hand opposite any object, and the magnifying
lens of two inches focus, in the right hand near the eye, at twelve
inches distance from the other in a direct line, and a telescope is
formed which magnifies five times. I have frequently used this plan,
when travelling, when no other telescope was at hand.
SECT. 3.--THE AERIAL TELESCOPE.
The Aerial is a refracting telescope of the kind we have now described,
intended to be used without a tube in a dark night; for the use of a
tube is not only to direct the glasses, but to make the place dark
where the images are formed. It appears from the preceding table
inserted above, that we cannot obtain a high magnifying power, with
the common astronomical telescope, without making it of an extreme
length, in which case the glasses are not manageable in tubes--which
are either too slight and apt to bend, or too heavy and unwieldy if
made of wood, iron or other strong materials. The astronomers of the
seventeenth century, feeling such inconveniences in making celestial
observations with long tubes, contrived a method of using the glasses
without tubes. Hartsocker, an eminent optician, contrived to fix
them at the top of a tree, a high wall, or the roof of a house; but
the celebrated Huygens, who was not only an astronomer, but also an
excellent mechanic, made considerable improvements in the method of
using an object-glass without a tube. He placed it at the top of a
very long pole, having previously enclosed it in a short tube, which
was made to turn in all directions by means of a ball and socket. The
axis of this tube he could command with a fine silken string, so as
to bring it into a line with the axis of another short tube which he
held in his hand, and which contained the eye-glass. The following is
a more particular description of one of these telescopes. On the top
of a long pole or mast _ab_ (fig. 45), is fixed a board moveable up
and down in the channel _cd_: _e_ is a perpendicular arm fixed to it,
and _ff_ is a transverse board that supports the object glass enclosed
in the tube _i_, which is raised or lowered by means of the silk cord
_rl_; _gg_ is an endless rope with a weight _h_, by which the apparatus
of the object-glass is counterpoised; _kl_ is a stick fastened to the
tube _i_; _m_ the ball and socket, by means of which the object-glass
is moveable every way: and to keep it steady, there is a weight _n_
suspended by a wire; _l_ is a short wire to which the thread _rl_ is
tied; _o_ is the tube which holds the eye-glass; _q_ the stick fixed
to this tube, _s_ a leaden bullet, and _t_ a spool to wind the thread
on; _u_ is pins for the thread to pass through; _x_ the rest for the
observer to lean upon, and _y_ the lantern. Fig. 46 is an apparatus
contrived by M. de la Hire for managing the object-glass; but which it
would be too tedious particularly to describe. To keep off the dew from
the object-glass, it was sometimes included in a pasteboard tube, made
of spongy paper, to absorb the humidity of the air. And to find an
object more readily, a broad annulus of white pasteboard was put over
the tube that carried the eye-glass; upon which the image of the object
being painted, an assistant who perceived it, might direct the tube of
the eye-glass into its place.
[Illustration: _figure 45._]
[Illustration: _fig 46._]
Such was the construction of the telescopes with which Hevelius,
Huygens, Cassini, and other eminent astronomers of the seventeenth
century made their principal discoveries. With such telescopes,
Huygens discovered the fourth satellite of Saturn, and determined
that this planet was surrounded with a ring; and with the same kind
of instrument Cassini detected the first, second, third, and fifth,
satellites of Saturn, and made his other discoveries. When the night
was very dark, they were obliged to make the object-glass visible,
by means of a lantern so constructed as to throw the rays of light
up to it in a parallel direction. In making such observations, they
must have taken incredible pains, endured much cold and fatigue,
and subjected themselves to very great labour and expense--which
almost makes us wonder at the discoveries they were instrumental in
bringing to light--and should make modern philosophers sensible of the
obligations they are under to such men as Newton and Dollond, through
whose inventions such unwieldy instruments are no longer necessary.
Telescopes of the description now stated were made of all sizes,
from 30 to above 120 feet in length. Divini at Rome, and Campani at
Bologna, were famed as makers of the object-glasses of the long focal
distance to which we have alluded, who sold them for a great price,
and took every method to keep the art of making them a secret. It was
with telescopes made by Campani, that Cassini made his discoveries.
They were made by the express order of Louis XIV, and were of 86, 100,
and 136 Paris feet in focal length. M. Auzout made one object-glass
of 600 feet focus; but he was never able to manage it, so as to make
any practical observations with it. Hartsocker is said to have made
some of a still greater focal length. The famous aerial telescope of
Huygens was 123 feet in focal length, with six inches of aperture. At
his death, he bequeathed it to the Royal Society of London, in whose
possession it still remains. It required a pole of more than a hundred
feet high, on which to place the object-glass for general observations.
It was with this glass, that Dr. Derham made the observations to which
he alludes in his preface to his ‘Astro-Theology.’ When this glass
was in the possession of Mr. Cavendish, it was compared with one of
Mr. Dollond’s forty-six inch treble object-glass Achromatics, and the
gentlemen who were present at the trial, said that ‘the Dwarf was
fairly a match for the Giant.’ It magnified 218 times, and the trouble
of managing it, was said to be extremely tiresome and laborious.
SECT. 4.--THE COMMON REFRACTING TELESCOPE FOR TERRESTRIAL OBJECTS.
[Illustration: _figure 47._]
This telescope is constructed on the same principle as the astronomical
telescope already described, with the addition of two or three glasses.
In fig. 47, OB represents a distant object, LN, the object glass, which
forms the image IM in its focus, which is, of course, in an inverted
position, and, if the eye were applied at the lens EE, the object would
appear, exactly as through the astronomical telescope, every object
being apparently turned upside down. To remedy this inconvenience,
there are added two other glasses FF and GG, by which a second image is
formed from the first, in the same position as the object. In order to
effect this, the first of these two glasses, namely FF, is placed at
twice its focal distance from the former glass EE, and the other lens
GG, next the eye, is placed at the same distance from FF. For all the
three glasses are supposed to be of the same focal distance. Now, the
lens FF, being placed at twice the focal distance for parallel rays
from EE, receives the pencils of parallel rays after they have crossed
each other at X, and forms an image at _i m_ similar to that at IM and
equal to it, but contrary in position, and consequently erect; which
last image is viewed by the lens GG, in the same manner as the first
image IM would be viewed by the lens EE. In this case, the image IM is
considered as an object to the lens FF of which it forms a picture in
its focus, in a reverse position from that of the first image, and of
course, in the same position as the object.
The magnifying power of this telescope is determined precisely in
the same way as that of the astronomical telescope. Suppose the
object-glass to be thirty inches focal distance, and each of the
eye-glasses 1-1/2 inch focal distance, the magnifying power is in the
proportion of 30 to 1-1/2, or 20 times, and the instrument is, of
course, considerably longer than an astronomical telescope of the same
power. The distance, in this case, between the object-glass and the
first eye-glass EE is 31-1/2 inches; the distance between EE, and the
second glass FF, is 3 inches, and the distance between FF and the glass
GG next the eye, 3 inches; in all 37-1/2 inches, the whole length of
the telescope. Although it is usual to make use of three eye-glasses in
this telescope, yet two will cause the object to appear erect, and of
the same magnitude. For suppose the middle lens FF taken away, if the
first lens EE be placed at X, which is double its focal distance from
the image IM, it will at the same distance X _m_, on the other side,
form a secondary image _i m_ equal to the primary image IM, and also in
a contrary position. But such a combination of eye-glasses produces a
great degree of colouring in the image, and therefore is seldom used.
Even the combination now described, consisting of three lenses of equal
focal distances, is now almost obsolete, and has given place to a much
better arrangement consisting of _four_ glasses, of different focal
distances--which shall be afterwards described.
The following figures, 48, 49, 50 represent the manner in which the
rays of light are refracted through the glasses of the telescopes we
have now described. Fig. 48 represents the rays of light as they pass
from the object to the eye in the Galilean telescope. After passing in
a parallel direction to the object-glass, they are refracted by that
glass, and undergo a slight convergence in passing towards the concave
eye-glass, where they enter the eye in a parallel direction, but no
image is formed previous to their entering the eye, till they arrive
at the retina. Fig. 49 represents the rays as they pass through the
glasses of the astronomical telescope. The rays, after entering the
object-glass, proceed in a converging direction, till they arrive at
its focus, about A, where an image of the object is formed; they then
proceed diverging to the eye-glass, where they are rendered parallel,
and enter the eye in that direction. Fig. 50 represents the rays as
they converge and diverge in passing through the four glasses of
the common day-telescope described above. After passing through the
object-glass, they converge towards B, where the first image is formed.
They then diverge towards the first eye-glass where they are rendered
parallel; and passing through the second eye-glass, they again converge
and form a second image at C; from which point they again diverge,
and passing through the first eye-glass enter the eye in a parallel
direction. If the glasses of these telescopes were fixed on long pieces
of wood, at their proper distances from each other, and placed in a
darkened room, when the sun is shining, the beam of the sun’s light
would pass through them in the same manner as here represented.
[Illustration: _fig. 48._]
[Illustration: _fig. 49._]
[Illustration: _fig. 50._]
SECT. 5.--TELESCOPE FORMED BY A SINGLE LENS.
This is a species of telescope altogether unnoticed by optical writers,
so far as I know; nor has the property of a single lens in magnifying
distant objects been generally adverted to or recognised. It may not
therefore be inexpedient to state a few experiments which I have made
in relation to this point. When we hold a spectacle-glass of a pretty
long focal distance--say, from 20 to 24 inches--close to the eye, and
direct it to distant objects, they do not appear sensibly magnified.
But if we hold the glass about 12 or 16 inches from our eye, we shall
perceive a sensible degree of magnifying power, as if distant objects
were seen at less than half the distance at which they are placed. This
property of a spectacle-glass I happened to notice when a boy, and, on
different occasions since that period have made several experiments on
the subject, some of which I shall here relate.
With the object-glass of a common refracting telescope 4-1/2 feet focal
distance, and 2-1/2 inches diameter, I looked at distant objects--my
eye being at about 3-1/2 feet from the lens, or about 10 or 12 inches
within its focus--and it produced nearly the same effect as a telescope
which magnifies the diameters of objects 5 or 6 times. With another
lens 11 feet focal distance and 4 inches diameter--standing from it at
the distance of about 10 feet, I obtain a magnifying power of about 12
or 14 times, which enables me to read the letters on the sign-posts
of a village half a mile distant. Having some time ago procured a
very large lens 26 feet focal distance, and 11-1/2 inches diameter,
I have tried with it various experiments of this kind upon different
objects. Standing at the distance of about 25 feet from it, I can see
distant objects through it magnified about 26 times in diameter, and
consequently 676 times in surface, and remarkably clear and distinct,
so that I can distinguish the hour and minute hands of a public clock
in a village two miles distant. This single lens, therefore answers the
purpose of an ordinary telescope with a power of 26 times. In making
such experiments our eye must always be _within_ the focus of the
lens, at least 8 or 10 inches. The object will, indeed, be seen at any
distance from the glass within this limit; but the magnifying power is
diminished in proportion as we approach nearer to the glass. Different
eyes, too, will require to place themselves at different distances, so
as to obtain the greatest degree of magnifying power with distinctness,
according as individuals are long or short-sighted.
This kind of telescope stands in no need of a tube, but only of a small
pedestal on which it may be placed on a table, nearly at the height
of the eye, and that it be capable of a motion in a perpendicular
or parallel direction, to bring it in a line with the eye and the
object. The principle on which the magnifying power, in this case, is
produced, is materially the same as that on which the performance of
the Galilean telescope depends. The eye of the observer serves instead
of the concave lens in that instrument; and as the concave lens is
placed as much within the focus of the object-glass, as is equal to
its own focal distance, so the eye, in these experiments, must be
placed at least its focal distance within the focus of the lens with
which we are experimenting; and the magnifying power will be nearly
in the proportion of the focal distance of the lens to the focal
distance of the eye. If, for example, the focal distance of the eye,
or the distance at which we see to read distinctly, be 10 inches, and
the focal distance of the lens, 11 feet, the magnifying power will
be as 11 feet, or 132 inches to 10, that is, about 13 times. Let A
(fig. 51.) represent the lens placed on a pedestal; the rays of light
passing through this lens from distant objects will converge towards a
focus at F. If a person then, place his eye at E, a certain distance
within the focal point, he will see distant objects magnified nearly in
the proportion of the focal distance of the lens to that of the eye;
and when the lens is very broad--such as the 26 feet lens mentioned
above--two or three persons may look through it at once, though they
will not all see the same object. I have alluded above to a lens made
by M. Azout of 600 feet focal distance. Were it possible to use such a
lens for distant objects, it might represent them as magnified 5 or 600
times, without the application of any eye-glass. In this way the aerial
telescope of Huygens would magnify objects above 100 times, which is
about half the magnifying power it produced with its eye-piece. Suppose
Azout’s lens had been fitted up as a telescope, it would not have
magnified above 480 times, as it would have required an eye-glass of 14
or 15 inches focal distance, whereas, without an eye-glass, it would
have magnified objects considerably above 500 times. It is not unlikely
that the species of telescope to which I have now adverted, constituted
one of those instruments for magnifying distant objects which were said
to have been in the possession of certain persons long before their
invention in Holland, and by Galileo in Italy--to which I have referred
in p. 182. Were this kind of telescope to be applied to the celestial
bodies, it would require to be elevated upon a pole in the manner
represented, fig. 45, p. 226.
[Illustration: _figure 51._]
SECT. 6.--THE ACHROMATIC TELESCOPE.
This telescope constitutes the most important and useful improvement
ever made upon telescopic instruments; and, it is probable, it will,
ere long, supersede the use of all other telescopes. Its importance and
utility will at once appear when we consider, that a good achromatic
telescope of only 4 or 5 feet in length will bear a magnifying power
as great, as that of a common astronomical telescope 100 feet long,
and even with a greater degree of distinctness, so that they are now
come into general use both for terrestrial and celestial observations.
There are, indeed, certain obstructions which prevent their being made
of a very large size; but from the improvement in the manufacture
of achromatic glass which is now going forward, it is to be hoped
that the difficulties which have hitherto impeded the progress of
opticians will soon be removed. In order to understand the nature
of this telescope, it will be necessary to advert a little to the
_imperfections_ connected with common refracting telescopes.
[Illustration: _figure 52._]
The first imperfection to which I allude is this, that _spherical
surfaces do not refract the rays of light accurately to a point_;
and hence the image formed by a single convex lens is not perfectly
accurate and distinct. The rays which pass near the extremities of such
a lens meet in foci nearer to the lens than those which pass nearly
through the centre, which may be illustrated by the following figure.
Let PP (fig. 52) be a convex lens and E_e_ an object, the point E of
which corresponds with the axis, and sends forth the rays EM, EN, EA,
&c., all of which reach the surface of the glass, but in different
parts. It is manifest that the ray EA which passes through the middle
of the glass, suffers no refraction. The rays EM, EM, likewise, which
pass through near to EA, will be converged to a focus at F, which we
generally consider as the focus of the lens. But the rays EN, EN, which
are nearer to the edge of the glass will be differently refracted, and
will meet about G, nearer to the lens, where they will form another
image G_g_. Hence, it is evident, that the first image F_f_, is formed
only by the union of those rays which pass very near the centre of the
lens; but as the rays of light proceeding from every point of an object
are very numerous, there is a succession of images formed, according
to the parts of the lens where they penetrate, which necessarily
produces indistinctness and confusion. This is the imperfection which
is distinguished by the name of _spherical aberration_, or the error
arising from the spherical form of lenses.
The second and most important imperfection of single lenses, when used
for the object-glasses of telescopes, is, that the rays of compounded
light being differently refrangible, come to their respective foci at
different distances from the glass; the more refrangible rays, as the
_violet_, converging sooner than those which are less refrangible, as
the _red_. I have had occasion to illustrate this circumstance, when
treating on the colours produced by the prism, (see p. 128, and figures
32 and 33,) and it is confirmed by the experiment of a paper painted
red, throwing its image, by means of a lens, at a greater distance
than another paper painted blue. From such facts and experiments, it
appears, that the image of a white object consists of an indefinite
number of coloured images, the violet being nearest, and the red
farthest from the lens, and the images of intermediate colours at
intermediate distances. The aggregate, or image itself, must therefore
be in some degree confused; and this confusion being much increased
by the magnifying power, it is found necessary to use an eye glass of
a certain limited convexity to a given object glass. Thus, an object
glass of 34 inches focal length will bear an eye-glass of only 1 inch
focus, and will magnify the diameters of objects 34 times; one of 50
feet focal distance will require an eye-glass of 4-1/2 inches focus,
and will magnify only 142 times; whereas, could we apply to it an
eye-glass of only 1 inch focus, as in the former case, it would magnify
no less than 600 times. And were we to construct an object-glass of 100
feet focal length, we should require to apply an eye-glass, not less
than 6 inches focus, which would produce a power of about 200 times;
so that there is no possibility of producing a great power by single
lenses, without extending the telescope to an immoderate length.
Sir Isaac Newton, after having made his discoveries respecting the
colours of light, considered the circumstance we have now stated as
an insuperable barrier to the improvement of refracting telescopes;
and therefore turned his attention to the improvement of telescopes
by _reflection_. In the telescopes which he constructed and partly
invented, the images of objects are formed by reflection from speculums
or mirrors; and being free from the irregular convergency of the
various coloured rays of light, will admit of a much larger aperture
and the application of a much greater degree of magnifying power. The
reflector which Newton constructed was only 6 inches long, but it was
capable of bearing a power equal to that of a 6 feet refractor. It was
a long time, however, after the invention of these telescopes before
they were made of a size fitted for making celestial observations.
After reflecting telescopes had been some time in use, Dollond made his
famous discovery of the principle which led him to the construction
of the _achromatic_ telescope. This invention consists of a compound
object glass formed of two different kinds of glass, by which both
the spherical aberration and the errors arising from the different
refrangibility of the rays of light are, in a great measure corrected.
For the explanation of the nature of this compound object glass and of
the effects it produces; it may be expedient to offer the following
remarks respecting the dispersion of light and its refraction by
different substances.
The _dispersion_ of light is estimated by the variable angle formed by
the red and violet rays which bound the solar spectrum;--or rather,
it is the excess of the refraction of the most refrangible ray above
that of the least refrangible ray. The dispersion is not proportional
to the refraction--that is, the substances which have an equal mean
refraction, do not _disperse_ light in the same ratio. For example,
if we make a prism with plates of glass, and fill it with oil of
Cassia, and adjust its refracting angle ACB, (fig. 31, p. 127,) so
that the middle of the spectrum which it forms falls exactly at the
same place where the green rays of a spectrum formed by a glass prism
would fall--then we shall find that the spectrum formed by the _oil
of Cassia_ prism will be two or three times _longer_ than that of the
_glass_ prism. The oil of Cassia, therefore, is said to _disperse_ the
rays of light more than the glass, that is, to separate the extreme
red and violet rays at O and P more than the mean ray at _green_, and
to have a greater _dispersive power_. Sir I. Newton appears to have
made use of prisms composed of different substances, yet, strange to
tell, he never observed that they formed spectrums, whose lengths were
different, when the refraction of the green ray was the same; but
thought that the dispersion was proportional to the refraction. This
error continued to be overlooked by philosophers for a considerable
time, and was the cause of retarding the invention of the achromatic
telescope for more than 50 years.
Dollond was among the first who detected this error. By his experiments
it appears, that the different kinds of glass differ extremely with
respect to the divergency of colours produced by equal refractions.
He found that two prisms, one of white flint glass, whose refracting
angle was about 25 degrees, and another of crown glass whose refracting
angle was about 29 degrees, refracted the beam of light nearly alike;
but that the divergency of colour in the white flint was considerably
more than in the crown glass; so that when they were applied together,
to refract contrary ways, and a beam of light transmitted through
them, though the emergent continued parallel to the incident part,
it was, notwithstanding, separated into component colours. From this
he inferred, that, in order to render the emergent beam white, it is
necessary that the refracting angle of the prism of crown glass should
be _increased_, and by repeated experiments he discovered the exact
quantity. By these means he obtained a theory in which refraction was
performed without any separation or divergency of colour; and thus the
way was prepared for applying the principle he had ascertained to the
construction of the object glasses of refracting telescopes. For the
edges of a convex and concave lens, when placed in contact with each
other, may be considered as two prisms which refract contrary ways;
and if the excess of refraction in the one be such as precisely to
destroy the divergency of colour in the other, a colourless image will
be formed. Thus, if two lenses are made of the same focal length, the
one of flint glass and the other of crown, the length or diameter of
the coloured image in the first will be to that produced by the crown
glass, as 3 to 2 nearly. Now, if we make the focal lengths of the
lenses in this proportion, that is, as 3 to 2, the coloured spectrum
produced by each will be equal. But if the flint lens be concave, and
the crown convex--when placed in contact--they will mutually correct
each other, and a pencil of white light refracted by the compound lens
will remain colourless.
[Illustration: _figure 53._]
The following figure may perhaps illustrate what has been now stated.
Let LL (fig. 53.) represent a convex lens of _crown glass_, and _ll_
a concave lens of _flint glass_. A ray of the sun S, falls at F on
the convex lens which will refract it exactly as the prism ABC, whose
faces touch the two surfaces of the lens at the points where the ray
enters and quits it. The solar ray, SF, thus refracted by the lens LL,
or prism ABC, would have formed a spectrum PT on the wall, had there
been no other lens, the violet ray F crossing the axis of the lens
at V, and going to the upper end P of the spectrum; and the red ray
FR, going to the lower end T. But as the flint-glass lens _ll_, or the
prism A_a_C which receives the rays FV, FR, at the same points, is
interposed, these rays will be united at _f_, and form a small circle
of white light; the ray SF of the sun being now refracted without
colour from its primitive direction SFY into the new direction F_f_.
In like manner the corresponding ray SM will be refracted to _f_,
and a white and colourless image of the sun will be there formed by
the two lenses. In this combination of lenses it is obvious that the
spherical aberration of the flint lens corrects to a considerable
degree that of the crown-glass, and by a proper adjustment of the radii
of the surfaces, it may be almost wholly removed. This error is still
more completely corrected in the _triple_ achromatic object-glass,
which consists of three lenses--a concave flint lens placed between
convexes of crown glass. Fig. 54 shows the _double_ achromatic lens,
and fig. 55, the _triple_ object-glass, as they are fitted up in their
cells, and placed at the object end of the telescope. In consequence
of their producing a focal image free of colour they will bear a
much larger aperture and a much greater magnifying power than common
refracting telescopes of the same length. While a common telescope
whose object-glass is 3-1/2 feet focal distance will bear an aperture
of scarcely 1 inch, the 3-1/2 feet Achromatic will bear an aperture
of 3-1/4 inches, and consequently transmits 10-1/2 times the quantity
of light. While the one can bear a magnifying power of only about 36
times, the other will bear a magnifying power for celestial objects of
more than 200 times.
[Illustration: _figure 54._]
[Illustration: _figure 55._]
The theory of the achromatic telescope is somewhat complicated and
abstruse, and would require a more lengthened investigation than my
limits will permit. But what has been already stated may serve to
give the reader a general idea of the _principle_ on which it is
constructed, which is all I intended. The term _achromatic_ by which
such instruments are now distinguished was first given to them by Dr.
Bevis. It is compounded of two Greek words which signify, ‘free of
colour.’ And, were it not that even philosophers are not altogether
free of that pedantry which induces us to select Greek words which
are unintelligible to the mass of mankind, they might have been
contented with selecting the plain English word _colourless_, which
is as significant and expressive as the Greek word _achromatic_. The
_crown-glass_, of which the convex lenses of this telescope are made,
is the same as good common window-glass; and the _flint-glass_ is that
species of glass of which wine-glasses, tumblers, decanters and similar
articles are formed, and is sometimes distinguished by the name of
crystal-glass. Some opticians have occasionally formed the concave
lens of an achromatic object-glass from the bottom of a broken tumbler.
This telescope was invented and constructed by Mr. John Dollond, about
the year 1758. When he began his researches into this subject, he was
a silk weaver in Spitalfields, London. The attempt of the celebrated
Euler to form a colourless telescope, by including water between two
meniscus glasses, attracted his attention, and, in the year 1753, he
addressed a letter to Mr. Short, the optician, which was published in
the Philosophical Transactions of London, ‘concerning a mistake in
Euler’s theorem for correcting the aberrations in the object glasses
of refracting telescopes.’ After a great variety of experiments on
the refractive and dispersive powers of different substances, he
at last constructed a telescope in which an exact balance of the
opposite dispersive powers of the crown and flint lenses made the
colours disappear, while the predominating refraction of the crown
lens disposed the achromatic rays to meet at a distant focus. In
constructing such object glasses, however, he had several difficulties
to encounter. In the first place, the focal distance as well as the
particular surfaces must be very nicely proportioned to the densities
or refractive powers of the glasses, which are very apt to vary in the
same sort of glass made at different times. In the next place, the
centers of the two glasses must be placed truly in the common axis of
the telescope, otherwise the desired effect will be in a great measure
destroyed. To these difficulties is to be added--that there are four
surfaces (even in double achromatic object glasses) to be wrought
perfectly spherical; and every person practised in optical operations
will allow, that there must be the greatest accuracy throughout the
whole work. But these and other difficulties were at length overcome by
the judgment and perseverance of this ingenious artist.
It appears, however, that Dollond was not the only person who had
the merit of making this discovery--a private gentleman, Mr. Chest,
of Chest-hall, a considerable number of years before, having made a
similar discovery, and applied it to the same purpose. This fact was
ascertained in the course of a process raised against Dollond at the
instance of Watkins, optician at Charing-cross, when applying for a
patent. But as the other gentleman had kept his invention a secret,
and Dollond had brought it forth for the benefit of the public, the
decision was given in his favour. There was no evidence that Dollond
borrowed the idea from his competitor, and both were, to a certain
extent, entitled to the merits of the invention.
One of the greatest obstructions to the construction of large
achromatic telescopes is, the difficulty of procuring large discs of
flint glass of an uniform refractive density--of good colour, and free
from veins. It is said that, fortunately for Mr. Dollond, this kind
of glass was procurable when he began to make achromatic telescopes,
though the attempts of ingenious chemists have since been exerted to
make it without much success. It is also said, that the glass employed
by Dollond in the fabrication of his best telescopes, was of the same
melting, or made at the same time, and that, excepting this particular
treasure, casually obtained, good dense glass for achromatic purposes,
was always as difficult to be procured as it is now. The dispersion of
the flint glass, too, is so variable, that, in forming an achromatic
lens, trials on each specimen require to be made before the absolute
proportional dispersion of the substances can be ascertained. It is
owing, in a great measure, to these circumstances, that a large and
good achromatic telescope cannot be procured unless at a very high
price. Mr. Tulley of Islington--who has been long distinguished as a
maker of excellent achromatic instruments--showed me, about six years
ago, a rude piece of flint glass about five inches diameter, intended
for the concave lens of an achromatic object glass, for which he paid
eight guineas. This was before the piece of glass was either figured
or polished, and, consequently, he had still to perform the delicate
operation of figuring, polishing, and adjusting this concave to the
convex lenses with which it was to be combined; and during the process
some veins or irregularities might be detected in the flint glass which
did not then appear. Some years before, he procured a disc of glass
from the continent about seven or eight inches diameter, for which he
paid about thirty guineas, with which an excellent telescope, twelve
feet focal length, was constructed for the Astronomical Society of
London. It is obvious therefore, that large achromatic telescopes must
be charged at a pretty high price.
In order to stimulate ingenious chemists and opticians to make
experiments on this subject, the Board of Longitude, more than half
a century ago, offered a considerable reward for bringing the art
of making good flint glass for optical purposes to the requisite
perfection. But considerable difficulties arise in attempting
improvements of this kind; as the experiments must all be tried on a
very large scale, and are necessarily attended with a heavy expence.
And although government has been extremely liberal in voting money for
warlike purposes, and in bestowing pensions on those who stood in no
need of them, it has hitherto thrown an obstruction in the way of such
experiments, by the heavy duty of excise, which is rigorously exacted,
whether the glass be manufactured into saleable articles or not; and
has thus been instrumental in retarding the progress of improvement
and discovery. It would appear that experiments of this kind have
been attended with more success in France, Germany, and other places
on the continent, than in Britain; as several very large achromatic
telescopes have been constructed in those countries by means of flint
glass which was cast for the purpose in different manufactories, and
to which British artists have been considerably indebted; as the
London opticians frequently purchase their largest discs of flint
glass from Parisian agents. Guinaud, a continental experimenter, and
who was originally a cabinet maker, appears to have had his labours
in this department of art crowned with great success. Many years were
employed in his experiments, and he too frequently, notwithstanding all
his attention, discovered his metal to be vitiated by striæ, spects
or grains, with cometic tails. He constructed a furnace capable of
melting two _cwt_ of glass in one mass, which he sawed vertically,
and polished one of the sections, in order to observe what had taken
place during the fusion. From time to time, as he obtained blocks,
including portions of good glass, his practice was to separate them by
sawing the blocks into horizontal sections, or perpendicular to their
axes. A fortunate accident conducted him to a better process. While
his men were one day carrying a block of this glass, on a hand-barrow,
to a saw mill which he had erected at the Fall of the Doubs, the
mass slipped from its bearers, and, rolling to the bottom of a steep
and rocky declivity, was broken to pieces. Guinaud having selected
those fragments which appeared perfectly homogeneous, softened them
in circular moulds, in such a manner, that on cooling, he obtained
discs that were afterwards fit for working. To this method he adhered,
and contrived a way for clearing his glass while cooling, so that the
fractures should follow the most faulty parts. When flaws occurred in
the large masses, they were removed by cleaving the pieces with wedges;
then smelting them again in moulds, which give them the form of discs.
The Astronomical Society of London have made trial of discs made by
Guinaud, and have found them entirely homogeneous and free from fault.
Of this ingenious artist’s flint glass, some of the largest achromatic
telescopes on the continent have been constructed. But, it is more
than twenty years since this experimenter took his flight from this
terrestrial scene, and it is uncertain whether his process be still
carried on with equal success.
_Notices of some large Achromatic telescopes on the Continent and in
Great Britain._
1. _The Dorpat Telescope._--This is one of the largest and most
expensive Refracting telescopes ever constructed. It was made by
the celebrated Fraunhofer of Munich for the observatory of the
Imperial University of Dorpat, and was received into the observatory
by Professor Struve in the year 1825. The aperture of the object
glass of this telescope is 9-1/2 English inches, and its solar focal
length about fourteen feet, the main tube being thirteen French feet
exclusive of the tube which holds the eye pieces. The smallest of the
four magnifying powers it possesses, is 175, and the largest 700,
which, in favourable weather, is said to present the object with the
utmost precision. ‘This instrument,’ says Struve, ‘was sold to us by
Privy-Counsellor VON UTZCHNEIDER, the chief of the optical
establishment at Munich, for 10,500 florins, (about £950 sterling), a
price which only covers the expenses which the establishment incurred
in making it.’ The frame work of the stand of this telescope is of oak
inlaid with pieces of mahogany in an ornamental manner, and the tube is
of deal veneered with mahogany and highly polished. The whole weight
of the telescope and its counterpoises is supported at one point, at
the common center of gravity of all its parts; and though these weigh
3000 Russian pounds, yet, we are told that this enormous telescope may
be turned in every direction towards the heavens with more ease and
certainty than any other hitherto in use. When the object end of the
telescope is elevated to the zenith, it is sixteen feet four inches,
Paris measure, above the floor, and its eye end in this position is
two feet nine inches high. This instrument is mounted on an Equatorial
stand, and clock work is applied to the Equatorial axis, which gives it
a smooth and regular sidereal motion, which, it is said, keeps a star
in the exact center of the field of view, and produces the appearance
of a state of rest in the starry regions, which motion can be made
solar, or even lunar, by a little change given to the place of a
pointer, that is placed as an index on the dial plate. Professor Struve
considers the optical powers of this telescope superior to those of
Schröeter’s twenty-five feet reflector, from having observed σ Orionis
with fifteen companions, though Schröeter observed only twelve, that
he could count with certainty. Nay, he seems disposed to place it in
competition with the late Sir W. Herschel’s forty feet reflector. The
_finder_ of this telescope has a focal distance of 30 French inches,
and 2-42 aperture.
2. _Sir James South’s Telescope._--About the year 1829, Sir J.
South, President of the London Astronomical Society, procured of M.
Cauchoix of Paris, an achromatic object glass of 11-2/10 inches, clear
aperture, and of 19 feet focal length. The flint glass employed in its
construction was the manufacture of the late Guinaud le Pere, and was
found to be absolutely perfect. The first observation was made with
this telescope, while on a temporary stand, on Feb. 13, 1830, when Sir
J. Herschel discovered with it a _sixth_ star in the trapezium in the
nebula of Orion, whose brightness was about one third of that of the
fifth star discovered by Struve, which is as distinctly seen as the
companion to Polaris is in a five feet achromatic. Sir James gives the
following notices of the performance of this instrument on the morning
of May 14, 1830. ‘At half past two, placed the 20 feet achromatic on
the Georgium Sidus, saw it with a power of 346, a beautiful planetary
disc; not the slightest suspicion of any ring, either perpendicular or
horizontal; but the planet three hours east of the meridian, and the
moon within three degrees of the planet.’ At a quarter before three,
viewed _Jupiter_ with 252 and 346, literally covered with belts, and
the diameters of his satellites might have been as easily measured
as himself. One came from behind the body, and the contrast of the
colour with that of the planet’s limb was striking. At three o’clock
viewed _Mars_. The contrast of light in the vicinity of the poles very
decided. Several spots on his body well and strongly marked--that about
the south pole seems to overtake the body of the planet, and gives an
appearance not unlike that afforded by the new moon, familiarly known
as ‘the old moon in the new moon’s arms.’ _Saturn_ has been repeatedly
seen with powers from 130 to 928 under circumstances the most
favourable; but not any thing anomalous about the planet or its ring
could even be suspected. This telescope is erected on an Equatorial
stand at Sir J. South’s observatory, Kensington.
3. _Captain Smyth’s Telescope in his private observatory at
Bedford._--This Achromatic telescope is 8-1/2 feet focal length, with a
clear aperture of 5-9/10 inches worked by the late Mr. Tulley, Senior,
from a disk purchased by Sir James South at Paris. It is considered
by Captain Smyth to be the finest specimen of that eminent optician’s
skill, and, it is said, will bear with distinctness, a magnifying
power of 1200. Its distinctness has been proved by the clear vision
it gives of the obscure nebulæ, and of the companions of Polaris,
Rigel, α Lyræ, and the most minute double stars---the lunar mountains,
cavities and shadows under all powers--the lucid polar regions of
Mars--the sharpness of the double ring of Saturn--the gibbous aspect
of Venus--the shadows of Jupiter’s satellites across his body, and
the splendid contrast of colours in α Hercules, γ Andromedæ and other
superb double stars.
_Other large Achromatics._--Besides the above, the following, belonging
to public observatories and private individuals, may be mentioned. In
the Royal observatory at Greenwich, there is an Achromatic of 10 feet
focal distance, having a double object glass 5 inches diameter, which
was made by Mr. Peter Dollond, and the only one of that size he ever
constructed. There is also a 46 inch achromatic, with a triple object
glass 3-3/4 inches aperture, which is said to be the most perfect
instrument of the kind ever produced. It was the favourite instrument
of Dr. Maskelyne, late Astronomer Royal, who had a small room fitted
up in the observatory for this telescope. The observatory, some years
ago erected near Cambridge, is perhaps the most splendid structure of
the kind in Great Britain. It is furnished with several very large
achromatic telescopes on Equatorial machinery: but the Achromatic
telescope, lately presented to it by the Duke of Northumberland, is
undoubtedly the largest instrument of this description which is to be
found in this country. The object glass is said to be 25 feet focal
distance, and of a corresponding diameter, but as there was no access
to this instrument at the time I visited this observatory, nearly six
years ago, I am unable to give a particular description of it. In
the Royal Observatory at Paris, which I visited in 1837, I noticed,
among other instruments, two very large Achromatic telescopes which,
measuring them rudely by the eye--I estimated to be from 15 to 18
feet long, and the aperture at the object end, from 12 to 15 inches
diameter. They were the largest achromatics I had previously seen; but
I could find no person in the observatory at that time, who could give
me any information as to their history, or to their exact dimensions,
or powers of magnifying.[21]
The Rev. Dr. Pearson, Treasurer to the Astronomical Society of London,
is in possession of the telescope formerly alluded to, made by Mr.
Tulley, of twelve feet focal distance and seven inches aperture, which
is said to be a very fine one. The small star which accompanies the
pole star, with a power of a 100, appears through this telescope, as
distinct and steady as one of Jupiter’s satellites. With a single lens
of 6 inches focus, which produced a power of 24 times, according to
the testimony of an observer who noticed it--the small star appeared
as it does in an achromatic of 3 inches aperture, which shows the
great effect of illuminating power in such instruments. Mr. Lawson, a
diligent astronomical observer in Hereford, possesses a most beautiful
achromatic telescope of about 7 inches aperture, and 12 feet focal
distance, which was made by one of the Dollonds, who considered it
as his _chief d’oeuvre_. It is said to bear powers as high as 1100
or 1400; and has been fitted up with mechanism devised by Mr. Lawson
himself, so as to be perfectly easy and manageable to the observer,
and which displays this gentleman’s inventive talent. In several of
his observations with this instrument, he is said to have had a view
of some of the more minute subdivisions of the ring of Saturn. A very
excellent achromatic telescope was fitted up some years ago by my
worthy friend William Bridges, Esq., Blackheath. Its object glass is
5-1/2 inches diameter, and about 5-1/2 feet focal length. It is erected
upon Equatorial machinery, and placed in a circular observatory which
moves round with a slight touch of the hand. The object glass of this
instrument cost about 200 Guineas, the equatorial machinery on which it
is mounted cost 150 Guineas, and the circular observatory in which it
is placed about 100 Guineas; in all 450 Guineas. Its powers vary from
50 to 300 times.[22]
_Achromatic telescopes of a moderate size._
Such telescopes as I have alluded to above, are among the largest which
have yet been made on the achromatic principle; they are, of course,
comparatively rare, and can be afforded only at a very high price. Few
of the _object glasses_ in the telescopes to which I have referred,
would be valued at less than 200 Guineas, independently of the tubes,
eye pieces and other apparatus with which they are fitted up. It is so
difficult to procure large discs of flint glass for optical purposes,
to produce the requisite curves of the different lenses, and to combine
them together with that extreme accuracy which is requisite, that when
a good compound lens of this description is found perfectly achromatic,
the optician must necessarily set a high value upon it; since it may
happen that he may have finished half a dozen before he has got one
that is nearly perfect. The more common sizes of achromatic telescopes
for astronomical purposes, which are regularly sold by the London
opticians, are the following:--
1. _The 2-1/2 feet Achromatic._--This telescope has an object glass 30
inches in focal length, and 2 inches clear aperture. It is generally
furnished with two eye pieces, one for terrestrial objects, magnifying
about 30 or 35 times, and one for celestial objects with a power of 70
or 75 times. It might be furnished with an additional astronomical
eye-piece--if the object glass be a good one, so as to produce a power
of 90 or 95 times. With such a telescope, the belts and satellites of
Jupiter, the phases of Venus and the ring of Saturn may be perceived;
but not to so much advantage as with larger telescopes. It is generally
fitted up either with a mahogany or a brass tube, and is placed upon a
tripod brass stand, with a universal joint which produces a horizontal
and vertical motion. It is packed, along with the eye-pieces, and
whatever else belongs to it, in a neat mahogany box. Its price varies,
according as it is furnished with an elevating rack or other apparatus.
The following are the prices of this instrument as marked in the
catalogue of Mr. Tulley, Terrett’s Court, Islington, London.
£ s. d.
2-1/2 feet telescopes, brass mounted on plain pillar and claw
stand, with one eye piece for astronomical purposes, and
one for land objects, to vary the magnifying power,
packed in a mahogany box 10 10 0
Ditto, ditto, brass mounted on pillar and claw stand, with
elevating rack, 1 eye piece for astronomical purposes, and
1 for land objects, to vary the magnifying power, packed
in a mahogany box 12 12 0
The following prices of the same kind of telescope are from the
catalogue of Messrs. W. and, S. Jones, 30, Lower Holborn, London.
£ s. d.
The improved 2-1/2 feet achromatic refractor, on a brass
stand, mahogany tube, with three eye pieces, two magnifying
about 40 and 50 times for terrestrial objects, and the
other about 75 times for astronomical purposes, in a
mahogany case 10 10 0
Ditto, ditto, the tube all brass, with three eye pieces 11 11 0
Ditto, ditto, with vertical and horizontal rack work
motions 15 15 0
2. _The 3-1/2 feet Achromatic Telescope._--The object glass of this
telescope is from 44 to 46 inches focal length, and 2-3/4 inches
diameter. It is generally furnished with four eye-pieces, two for
terrestrial and two for celestial objects. The lowest power for land
objects is generally about 45, which affords a large field of view, and
exhibits the objects with great brilliance. The other terrestrial power
is usually from 65 to 70. The astronomical powers are about 80 and 130;
but such a telescope should always have another eye-piece, to produce
a power of 180 or 200 times, which it will bear with distinctness,
in a serene state of the atmosphere, if the object glass be truly
achromatic. The _illuminating power_ in this telescope is nearly double
that of the 2-1/2 feet telescope, or in the proportion of 7, 56 to
4; and therefore it will bear about double the magnifying power with
nearly equal distinctness. This telescope is fitted up in a manner
somewhat similar to the former, with a tripod stand which is placed
upon a table. Sometimes, however, it is mounted on a long mahogany
stand which rests upon the floor, (as in fig. 58.), and is fitted with
an equatorial motion; and has generally a small telescope fixed near
the eye end of the large tube, called a _finder_, which serves to
direct the telescope to a particular object in the heavens when the
higher powers are applied. It is likewise eligible that it should have
an elevating rack and sliding tubes, for supporting the eye end of the
instrument, to keep it steady during astronomical observations, and it
would be an advantage, for various purposes which shall be afterwards
described, to have fitted to it a _Diagonal Eye Piece_ magnifying 40
times or upwards.
The prices of this instrument, as marked in Mr. Tulley’s Catalogue, are
as follows:--
£ s. d.
The 3-1/2 feet achromatic telescope 2-3/4 inches aperture, on
plain pillar and claw stand, 2 eye pieces for astronomical
purposes, and 1 for land objects to vary the magnifying
power, packed in a mahogany box 21 0 0
Ditto, ditto, with elevating rack and achromatic finder,
2 eye pieces for astronomical purposes, and 1 for day objects
to vary the magnifying power, packed in a mahogany
box 26 5 0
The following are the prices as marked in Messrs. W. and S. Jones’
Catalogue.
£ s. d.
The 3-1/2 feet achromatic, plain mahogany tube 18 18 0
Ditto, ditto, brass tube 21 0 0
Ditto, all in brass, with rack work motions, &c. 26 5 0
Ditto, the object glass of the largest aperture, and the
rack motions on an improved principle _from_ 37l. 16s. to 42 0 0
Ditto, fitted up with Equatorial motion, framed mahogany
stand, divided altitude, and azimuth arches, or declination
and right ascension circles, &c. &c. _from_ 60l to 80 0 0
This is the telescope which I would particularly recommend to
astronomical amateurs, whose pecuniary resources do not permit them
to purchase more expensive instruments. When fitted up with the eye
pieces and powers already mentioned, and with a finder and elevating
rack,--price 25 guineas--it will serve all the purposes of general
observation. By this telescope, satisfactory views may be obtained of
most of the interesting phenomena of the heavens, such as the spots of
the sun--the mountains, vales, and caverns on the lunar surface--the
phases of Mercury and Venus--the spots on Mars--the satellites and
belts of Jupiter--the ring of Saturn--many of the more interesting
nebulæ, and most of the double stars of the second and third classes.
When the object glass of this telescope is accurately figured and
perfectly achromatic, a power of from 200 to 230 maybe put upon it, by
which the division of Saturn’s ring might occasionally be perceived. It
is more easily managed and represents objects considerably brighter
than reflecting telescopes of the same price and magnifying power,
and it is not so apt to be deranged as reflectors generally are. A
telescope of a less size would not in general be found satisfactory for
viewing the objects I have now specified, and for general astronomical
purposes. It may not be improper for the information of some readers,
to explain what is meant in Mr. Tulley’s catalogue, when it is stated
that this instrument has ‘one eye piece for day objects, _to vary the
magnifying power_.’ The eye piece alluded to is so constructed, that
by drawing out a tube next the eye, you may increase the power at
pleasure, and make it to vary, say from 40 to 80 or 100 times; so that
such a construction of the terrestrial eye piece (to be afterwards
explained) serves in a great measure, the purpose of separate
eye-pieces. The whole length of the 3-1/2 feet telescope, when the
terrestrial eye piece is applied, is about 4-1/2 feet from the object
glass to the first eye glass.
When the aperture of the object glass of this telescope exceeds 2-3/4
inches its price rapidly advances.
The following is Mr. Tulley’s scale of prices, proportionate to the
increase of aperture:--
£ s. d.
3-1/2 feet telescopes 3-1/4 inches aperture, with vertical and
horizontal rack work motions, achromatic finder, 3 eye
pieces for astronomical purposes, and one for day objects
to vary the magnifying power, packed in a mahogany box 42 0 0
Ditto, ditto, 3-3/4 inches diameter, mounted as above 68 5 0
Ditto, with universal Equatorial, instead of pillar and
claw stand 84 0 0
Here, in the one case, the increase of half an inch in the diameter
of the object-glass, adds about £16. to the expense; and in the
other case no less than £26. 5s. The proportion of light in those
two telescopes, compared with that of 2-3/4 inches aperture, is as
follows:--The square of the 2-3/4 object-glass is 7.56; that of 3-1/4,
10.56, and that of the 3-3/4, 14.06; so that the light admitted by
the 3-1/4 compared with the 2-3/4 aperture, is nearly as 10 to 7; and
the light admitted by the 3-3/4 object-glass is nearly double that of
the 2-3/4 aperture, and will bear nearly a proportional increase of
magnifying power.
3. _The 5 feet Achromatic telescope._ The focal length of the
object-glass of this telescope is 5 feet 3 inches, and the diameter
of its aperture 3-8/10 inches. The usual magnifying powers applied
to it are, for land objects 65 times; and for celestial objects,
110, 190, 250, and sometimes one or two higher powers. The quantity
of light it possesses is not much larger than that of the 3-1/2 feet
telescope, with 3-3/4 inches aperture; but the larger focal length of
this telescope is considered to be an advantage; since the longer the
focus of the object-glass, the less will be its chromatic and spherical
aberrations, and the larger may be the eye-glasses, and the flatter the
field of view.
The following are the prices of these telescopes as marked in Mr.
Tulley’s catalogue.
£ s. d.
5 feet telescopes 3-3/4 inches aperture, on an universal
equatorial stand, with achromatic finder, 4 eye pieces for
astronomical purposes, and 1 for day objects to vary the
power, packed in a mahogany box 100 guineas to 157 10 0
7 feet ditto, 5 inches aperture, on a newly improved
universal equatorial stand, 6 eye pieces for astronomical
purposes, and 1 for day objects to vary the magnifying
power, with achromatic finder, and Troughton’s Micrometer 207 5 0
The above are all the kinds of achromatic telescopes _generally_
made by the London opticians. Those of the larger kind, as 5 and 7
feet telescopes, and the 3-1/2 feet with 3-3/4 inches aperture,
are generally made to order, and are not always to be procured. But
the 2-1/2 and 3-1/2 feet achromatics of 2-3/4 inches aperture, are
generally to be found ready-made at most of the optician’s shops in the
metropolis. The prices of these instruments are nearly the same in most
of the optician’s shops in London. Some of them demand a higher price,
but few of them are ever sold lower than what has been stated above,
unless in certain cases, where a discount is allowed.
[Illustration: _figure 57._]
The stands for these telescopes, and the manner in which they are
fitted up for observation are represented in figures 57, 58, and 59.
Fig. 57 represents either the 2-1/2 or the 3-1/2 feet telescopes
mounted on a plain brass stand, to be placed on a table. A is the long
eye-piece for land objects, and B the small eye-piece for astronomical
observation, which is composed of two lenses, and represents the object
in an inverted position. These eye-pieces are screwed on, as occasion
requires, at E, the eye-end of the telescope. The shorter of the two
astronomical eye-tubes which accompany this telescope, produces the
highest magnifying power. For adjusting the telescope to distinct
vision, there is a brass knob or button at _a_, which moves a piece of
rack-work connected with the eye-tube, which must be turned either one
way or the other till the object appears distinctly; and different eyes
frequently require a different adjustment.
Fig. 58, represents a 5 feet telescope fitted up for astronomical
observations. It is mounted on a mahogany stand, the three legs of
which are made to close up together by means of the brass frame _aaa_,
which is composed of three bars, connected with three joints in the
centre, and three other joints, connected with the three mahogany bars.
It is furnished with an apparatus for equatorial motions. The brass pin
is made to move round in the brass socket _b_, and may be tightened by
means of the finger screw _d_, when the telescope is directed nearly to
the object intended to be viewed. This socket may be set perpendicular
to the horizon, or to any other required angle; and the quantity of
the angle is ascertained by the divided arc, and the instrument made
fast in that position by the screw _e_. If this socket be set to the
latitude of the place of observation, and the plane of this arc be
turned so as to be in the plane of the meridian, the socket _b_ being
fixed to the inclination of the pole of the earth, the telescope
when turned in this socket, will have an equatorial motion, so that
celestial objects may be always kept in view, when this equatorial
motion is performed. The two handles at _k_ are connected with
rack-work, intended to move the telescope in any required direction.
The two sets of brass sliding rods _ii_ are intended to render the
telescope as steady as possible, and to elevate and depress it at
pleasure, and are so constructed as to slide into each other with the
utmost ease.
[Illustration: _figure 58._]
The _Finder_ is placed at AE, either on the top or the left side of
the tube of the telescope. When high magnifying powers are applied to
any telescope, it is sometimes difficult, on account of the smallness
of the field of view, to direct the main tube of the telescope to the
object. But the Finder, which is a telescope with a small power, and
consequently has a large field of view--when directed to any object, it
is easily found, and being brought to the centre of the field, where
two cross hairs intersect each other, it will then be seen in the
larger telescope. B is the eye-tube for terrestrial objects, containing
4 glasses, and C, one of the astronomical eye-pieces. A socket is
represented at _g_, containing a stained glass, which is screwed to any
of the eye-pieces, to protect the eye from the glare of light, when
viewing the spots of the sun. The brass nut above _f_, is intended for
the adjustment of the eye-piece to distinct vision. The 3-1/2 feet
telescope is sometimes mounted in this form.
Fig. 59, represents a 5 or 6 feet telescope, mounted on a stand of a
new construction by Dollond. It possesses the advantage of supporting
the telescope in two places, which renders it extremely steady--a
property of great importance when viewing celestial objects with high
magnifying powers. It possesses likewise, the advantage of enabling the
observer to continue seated at the same height from the floor, although
the telescope be raised to any altitude--_the elevation being entirely
at the object end_, although it may be changed from the horizon to
the zenith. The frame-work is composed of bars of mahogany, and rests
on three castors, two of which are made fast to their respective legs
in the usual way, and the third stands under the middle of the lower
horizontal bar that connects the two opposite legs, so that the frame
has all the advantages of a tripod. As it becomes very inconvenient to
stoop to the eye end of a telescope, when the altitude of an object is
considerable, and the centre of motion at the middle of the tube, this
construction of a stand serves to remedy such inconvenience.
[Illustration: _figure 59._]
_Proportions of curvature of the lenses which form an achromatic
object-glass._
As some ingenious mechanics may feel a desire to attempt the
construction of a compound achromatic object-glass, I shall here state
some of the proportions of curvature of the concave and convex lenses,
which serve to guide opticians in their construction of achromatic
instruments. These proportions are various; and even when demonstrated
to be mathematically correct, it is sometimes difficult to reduce them
to practice, on account of the different powers of refraction and
dispersion possessed by different discs of crown and flint-glass, and
of the difficulty of producing by mechanical means, the exact curves
which theory requires. The following table shows the radii of curvature
of the different surfaces of the lenses necessary to form _a double
achromatic object-glass_--it being supposed that the sine of refraction
in the crown-glass is as 1.528 to 1, and in the flint as 1.5735 to 1;
the ratio of their dispersive powers being as 1 to 1.524. It is also
assumed that the curvatures of the concave lens are as 1 to 2, that
is, that the one side of this lens is ground on a tool, the radius
of which is double that of the other. The 1st column expresses the
compound focus of the object-glass in _inches_; the 2nd column states
the radius of the _anterior_ surface of the _crown_, and column 3rd,
its _posterior_ side. Column 4th expresses the radius of the anterior
surface of the _concave_ lens, and column 5th its posterior surface,
which, it will be observed, is exactly double that of the other.
-------+-----------+-----------+-----------+-----------
Focus |Radius of |Radius of |Radius of |Radius of
in |anterior |posterior |anterior |posterior
inches.|surface, |surface. |surface, |surface.
|convex. | |concave. |
-------+-----------+-----------+-----------+-----------
|Inc. Dec. |Inc. Dec. |Inc. Dec. |Inc. Dec.
12 | 3 | 4. 652 | 4. 171 | 8. 342
24 | 6 | 9. 304 | 8. 342 | 16. 684
30 | 7. 5 | 11. 063 | 10. 428 | 20. 856
36 | 9 | 13. 956 | 12. 513 | 25. 027
48 | 12 | 18. 608 | 16. 684 | 33. 369
60 | 15 | 23. 260 | 20. 856 | 41. 712
120 | 30 | 46. 520 | 41. 712 | 83. 424
From the above table it will be seen, that to construct, for example,
a 30 inch compound object-glass, the radius of the anterior side of
the crown must be 7-1/2 inches, and that of the posterior side 11.63
inches; the radius of the anterior surface of the concave 10.428, and
that of the posterior 20.856 inches. It may be proper to observe,
that in these computations, the radius of the anterior surface of the
concave is less than the posterior side of the convex, and consequently
admits of its approach, without touching in the centre--a circumstance
which always requires to be guarded against in the combination of
achromatic glasses. The following table shows the radii of curvature of
the lenses of a _triple_ object-glass, calculated from formula deduced
by Dr. Robison of Edinburgh.
-------+--------------------+---------------------+-------------------
Focal | Convex lens of | Concave lens of | Convex lens of
length.| crown glass. | flint glass. | crown glass.
-------+--------------------+---------------------+-------------------
Inches |Inc. Dec. Inc. Dec. |Inc. Dec. Inc. Dec. |Inc. Dec Inc. Dec.
6 | 4. 54 3. 03 | 3. 03 6. 36 | 6. 36 0. 64
9 | 6. 83 4. 56 | 4. 56 9. 54 | 9. 54 0. 92
12 | 9. 25 6. 17 | 6. 17 12. 75 | 12. 75 1. 28
18 | 13. 67 9. 12 | 9. 12 19. 08 | 19. 08 1. 92
24 | 18. 33 12. 25 | 12. 25 25. 50 | 25. 50 2. 56
30 | 22. 71 15. 16 | 15. 16 31. 79 | 31. 79 3. 20
36 | 27. 33 18. 25 | 18. 25 38. 17 | 38. 17 3. 84
42 | 31. 87 21. 28 | 21. 28 44. 53 | 44. 53 4. 48
48 | 36. 42 24. 33 | 24. 33 50. 92 | 50. 92 5. 12
54 | 40. 96 27. 36 | 27. 36 57. 28 | 57. 28 5. 76
60 | 45. 42 30. 33 | 30. 33 63. 58 | 63. 58 6. 40
The following table contains the proportions of curvature, said to be
employed by the London opticians.
--------+-----------------------+-------------+------------------------
| | |
| | Radius of |
| | both the |
Focal | Convex of crown | surfaces of | Convex lens of
length. | glass. | the concave | crown glass.
| | of flint |
| | glass |
--------+-----------------------+-------------+------------------------
| | | | |
Inches. | Inc. Dec. | Inc. Dec. | Inc. Dec. | Inc. Dec. | Inc. Dec.
6 | 3. 77 | 4. 49 | 3. 47 | 3. 77 | 4. 49
9 | 5. 65 | 6. 74 | 5. 21 | 5. 65 | 6. 74
12 | 7. 54 | 8. 99 | 6. 95 | 7. 54 | 8. 99
18 | 11. 30 | 13. 48 | 10. 42 | 11. 30 | 13. 48
24 | 15. 08 | 17. 98 | 13. 90 | 15. 08 | 17. 98
36 | 22. 61 | 26. 96 | 20. 84 | 22. 61 | 26. 96
42 | 26. 38 | 31. 45 | 24. 31 | 26. 38 | 31. 45
48 | 30. 16 | 35. 96 | 27. 80 | 30. 16 | 35. 96
54 | 33. 91 | 40. 45 | 31. 27 | 33. 91 | 40. 45
60 | 37. 68 | 44. 94 | 34. 74 | 37. 68 | 44. 94
From this table it appears, that the two convex lenses, have the same
radii of their respective sides and that the concave flint lens has
its two surfaces equally concave, so that a triple object-glass formed
according to these proportions, would require only three pair of
grinding tools. The following are the curves of the lenses of one of
the best of Dollond’s achromatic telescopes, the focal length of the
compound object-glass being 46 inches. Reckoning from the surface next
the object--the radii of the crown-glass were 28 and 40 inches: the
concave lens 20.9 inches, and the inner crown-glass lens, 28.4 and 28.4
inches. This telescope carried magnifying powers of from 100 to 200
times.
Although I have inserted the above tables, which might in some measure
guide an ingenious artist, yet on the whole, a private amateur has
little chance in succeeding in such attempts. The diversity of glasses,
and the uncertainty of an unpractised workman’s producing the precise
curvatures he intends, is so great, that the object-glass, for the
most part, turns out different from his expectations. The great
difficulty in the construction is to find the exact proportion of the
dispersive powers of the crown and flint glass. The crown is pretty
constant, but there are hardly two pots of flint glass which have the
same dispersive power. Even if constant, it is difficult to measure
it accurately; and an error in this greatly affects the instrument;
because the focal distances of the lenses must be nearly as their
dispersive powers. In the two preceding tables, the sine of incidence,
in the crown glass, is supposed to be to the sine of refraction as
1.526 to 1; and in the flint glass, as 1.604 to 1. Opticians who make
great numbers of lenses both of flint and crown glass, acquire, in
time, a pretty good guess of the nature of the errors which may remain
after they have finished an object-glass; and having many lenses
intended to be of the same form, but unavoidably differing a little
from it, they try several of the concaves with the two convexes, and
finding one better than the rest, they make use of it to complete the
set. In this way some of the best achromatic telescopes are frequently
formed. I have sometimes found, when supplying a concave flint glass
to a telescope where it happened to be wanting, that, of four or five
concave lenses which appeared to be the same as to curvature and other
properties, only one was found to produce a distinct and colourless
image. Should any one, however, wish to attempt the construction of
an achromatic lens, the best way for preventing disappointments in
the result is, to procure a _variety_ of tables of the respective
curvatures founded on _different conditions_, and which, of course,
require the surfaces of the several lenses to be of different curves.
Having lenses of different radii at his command, and having glass of
different refractive or dispersive powers, when one combination does
not exactly suit, he may try another, and ultimately may succeed in
constructing a good achromatic telescope; for, in many cases, it has
been found that chance, or a happy combination of lenses by trial, has
led to the formation of an excellent object-glass.
_Achromatic telescopes composed of fluid lenses._
The best achromatic telescopes, when minutely examined, are found to be
in some respects defective, on account of that slight degree of colour
which, by the aberration of the rays, they give to objects, unless
the object-glass be of small diameter. When we examine with attention
a good achromatic telescope we find that it does not show white or
luminous objects perfectly free from colour, their edges being tinged
on one side with a claret-coloured fringe, and on the other with a
green fringe. This telescope, therefore, required farther improvement,
to get rid of these secondary colours, and Father Boscovich, to whom
every branch of optics is much indebted, displayed much ingenuity in
his attempts to attain this object. But it is to Dr. Blair, professor
of astronomy in Edinburgh, that we are chiefly indebted for the first
successful experiments by which this end was accomplished. By a
judicious set of experiments, he proved that the quality of dispersing
the rays in a greater degree than crown-glass, is not confined to a few
mediums; but is possessed by a great variety of fluids, and by some of
these in a most extraordinary degree. Having observed that when the
extreme red and violet rays were perfectly united, the green were left
out, he conceived the idea of making an achromatic concave lens which
should refract the green less than the united red and violet, and an
achromatic convex lens which should do the same, and as the concave
lens refracted the outstanding green _to_ the axis, while the concave
one refracted them _from_ the axis, it followed, that, by a combination
of these two opposite effects, the green would be united with the red
and violet.
By means of an ingenious prismatic apparatus, he examined the optical
properties of a great variety of fluids. The solutions of metals and
semi-metals proved in all cases more dispersive than crown glass. Some
of the salts, such as sal-ammoniac, greatly increased the dispersive
power of water. The marine acid disperses very considerably, and this
quality increases with its strength. The most dispersive fluids were
accordingly found to be those in which this acid and the metals were
combined. The chemical preparation called _causticum antimoniale_, or
butter of antimony, in its most concentrated state, when it has just
attracted sufficient humidity to render it fluid, possesses the quality
of dispersing the rays in an astonishing degree. The great quantity
of the semi-metal retained in solution, and the highly concentrated
state of the marine acid, are considered as the cause of this striking
effect. Corrosive sublimate of mercury, added to a solution of
_sal-ammoniacum_ in water, possesses the next place to the butter of
antimony among the dispersive fluids, which Dr. Blair examined. The
essential oils were found to hold the next rank to metallic solutions,
among fluids which possess the dispersive quality, particularly those
obtained from bituminous minerals, as native petrolea, pit coal, and
amber. The dispersive power of the essential oil of sassafras, and
the essential oil of lemons, when genuine, were found to be not much
inferior to any of these. But of all the fluids fitted for optical
purposes, Dr. Blair found that _the muriatic acid mixed with a metallic
solution_, or, in other words, a fluid in which the marine acid and
metalline particles, hold a due proportion, most accurately suited his
purpose. In a spectrum formed by this fluid the green were among the
most refrangible rays, and when its dispersion was corrected by that
of glass, there was produced an inverted secondary spectrum, that is,
one in which the green was above, when it would have been below with a
common medium. He therefore placed a concave lens of muriatic acid with
a metallic solution between the two lenses, as in fig. 60, where AB is
the concave fluid lens, CF a plano-convex lens, with its plane side
next the object, and ED, a meniscus. With this object-glass the rays of
different colours were bent from their rectilineal course with the same
equality and regularity as in reflection.
[Illustration: _figure 60._]
Telescopes constructed with such object-glasses were examined by the
late Dr. Robison and professor Playfair. The focal distance of the
object-glass of one of these did not exceed 17 inches, and yet it
bore an aperture of 3-1/2 inches. They viewed some single and double
stars and some common objects with this telescope; and found, that,
in magnifying power, brightness, and distinctness, it was manifestly
superior to one of Mr. Dollond of 42 inches focal length. They had
most distinct vision of a star, _when using an erecting eye-piece_,
which made this telescope magnify more than a 100 times; and they found
the field of vision as uniformly distinct as with Dollond’s 42 inch
telescope magnifying 46 times; and were led to admire the nice figuring
and centering of the very deep eye-glasses which were necessary for
this amplification. They saw double stars with a degree of perfection
which astonished them. These telescopes, however, have never yet come
into general use; and one reason perhaps, is, that they are much more
apt to be deranged, than telescopes constructed of object-glasses
which are solid. If any species of glass, or other solid transparent
substance could be found with the same optical properties, instruments
might perhaps be constructed of a larger size, and considerably
superior to our best achromatic telescopes.[23] It is said that Mr.
Blair, the son of Dr. Blair, some years ago, was engaged in prosecuting
his father’s views, but I have not heard any thing respecting the
result of his investigations.
_Barlow’s refracting telescope with a fluid concave lens._
Professor Barlow, not many years ago, suggested a new fluid telescope,
which is deserving of attention; and, about the year 1829 constructed
one of pretty large dimensions. The fluid he employs for this purpose
is the _sulphuret of Carbon_, which he found to be a substance which
possessed every requisite he could desire. Its index is nearly the
same as that of the best flint glass, with a dispersive power more
than double. It is perfectly colourless, beautifully transparent,
and although very expansible, possesses the same, or very nearly
the same optical properties under all circumstances to which it is
likely to be exposed in astronomical observations--except perhaps,
direct observations on the solar disc, which will probably be found
inadmissible. Mr. Barlow first constructed an object-glass with this
fluid of 3 inches aperture, with which he could see the small star in
Polaris with a power of 46, and with the higher powers several stars
which are considered to require a good telescope, for example 70, ρ
Ophinchi, 39 Bootis, the quadruple star ε Lyræ, ζ Aquarii, α Herculis,
&c. He next constructed a 6 inch object-glass. With this instrument
the small star in Polaris is so distinct and brilliant, with a power
of 143, that its transit might be taken with the utmost certainty. As
the mode of constructing these telescopes is somewhat novel, it may be
expedient to enter somewhat into detail.
In the usual construction of achromatic telescopes, the two or three
lenses composing the object-glass are brought into immediate contact;
and in the fluid telescope of Dr. Blair, the construction was the same,
the fluid having been enclosed in the object-glass itself. But in Mr.
Barlow’s telescope, the fluid correcting lens is placed at a distance
from the plate lens equal to half its focal length; and it might be
carried still farther back, and yet possess dispersive power to render
the object-glass achromatic. By this means the fluid lens--which is the
most difficult part of the construction--is reduced to one half or
to less than one half of the size of the plate lens; consequently, to
construct a telescope of 10 or 12 inches aperture involves no greater
difficulty in the manipulation, than in making a telescope of the usual
description of 5 or 6 inches aperture, except in the simple plate lens
itself; and, hence, a telescope of this kind, of 10 or 12 feet length,
will be equivalent in its focal power to one of 16 or 20 feet. By this
means, the tube may be shortened several feet and yet possess a focal
power more considerable than could be conveniently given to it on the
usual principle of construction. This will be better understood from
the annexed diagram. (fig. 61.)
[Illustration: _figure 61._]
In this figure ABCD represent the tube of the 6 inch telescope, CD, the
plate object-glass, F the first focus of rays, _de_ the fluid concave
lens, distant from the former 24 inches. The focal length MF being
48, and consequently, as 48 : 6 :: 24 : 3 inches, the diameter of the
fluid lens. The resulting compound focus is 62.5 inches. It is obvious,
therefore, that the rays _df_, _ef_, arrive at the focus under the same
convergency, and with the same light as if they proceeded from a lens
of 6 inches diameter, placed at a distance beyond the object-glass
CD (as GH,) determined by producing those rays till they meet the
sides of the tube in GH, namely at 62.5 inches beyond the fluid lens.
Hence, it is obvious, the rays will converge as they would do from an
object-glass GH of the usual kind with a focus of 10 feet 5 inches. We
have thus, therefore, shortened the tube 38.5 inches, or have at least
the advantage of a focus 38.5 inches longer than our tube; and the
same principle may be carried much farther, so as to reduce the usual
length of refracting telescopes nearly one half without increasing
the aberration in the first glass beyond the least that can possibly
belong to a telescope of the usual kind of the whole length. It should
likewise be observed that the adjustment for focus may be made either
in the usual way, or by a slight movement of the fluid lens, as in the
Gregorian Reflectors, by means of the small speculum.
Mr. Barlow afterwards constructed another and a larger telescope on the
same principle, the clear aperture of which is 7.8 inches. Its tube is
11 feet, which, together with the eye-piece, makes the whole length
12 feet, but its effective focus is on the principle stated above, 18
feet. It carries a power of 700 on the closest double stars in South’s
and Herschel’s catalogue, and the stars are, with that power, round
and defined, although the field is not then so bright as could be
desired. The telescope is mounted on a revolving stand, which works
with considerable accuracy as an azimuth and altitude instrument. To
give steadiness to the stand it has been made substantial and heavy;
its weight by estimation being 400 pounds, and that of the telescope
130 pounds, yet its motions are so smooth, and the power so arranged,
that it may be managed by one person with the greatest ease, the star
being followed by a slight touch, scarcely exceeding that of the keys
of a piano-forte. The focal length of the plate lens is 78 inches,
and of the fluid lens 59.8 inches--which at the distance of 40 inches
produce a focal length of 104 inches, a total length of 12 feet, and
an equivalent focus of 18 feet. The curves of the parallel meniscus
checks for containing the fluid are--30 inches, and 144 inches, the
latter towards the eye. The curves for the plate lens are 56.4 and
144. There is an interior tube 5 inches diameter, and 3 feet 6 inches
long, which carries the cell in which the fluid is enclosed, and an
apparatus by which it may be moved backwards and forwards, so that
the proper adjustment may be made for colour, in the first instance,
and afterwards the focus is obtained by the usual rack-work motion.
The following is the mode by which the fluid was enclosed. After the
best position has been determined practically for the checks forming
the fluid lens, these, with the ring between them ground and polished
accurately to the same curves, are applied together, and taken into
an artificial high temperature, exceeding the greatest at which the
telescope is ever expected to be used. After remaining here with the
fluid some time, the space between the glasses is completely filled,
immediately closed, cooled down by evaporation, and removed into a
lower temperature. By this means a sudden condensation takes place, an
external pressure is brought on the checks, and a bubble formed inside,
which is of course filled with the vapour of the fluid; the excess of
the atmospheric pressure beyond that of the vapour being afterwards
always acting externally to prevent contact. The extreme edges are then
sealed with the serum of human blood, or by strong fish-glue, and some
thin pliable metal surface. By this process, Mr. Barlow says, ‘I have
every reason to believe the lens becomes as durable as any lens of
solid glass. At all events I have the satisfaction of stating, that my
first 3 inch telescope has now been completed more than fifteen months,
and that no change whatever has taken place in its performance, nor the
least perceptible alteration either in the quantity or the quality of
the fluid.’
The following are some of the observations which have been made with
this telescope, and the tests to which it has been subjected. The very
small star which accompanies the pole-star is generally one of the
first tests applied to telescopes. This small point of light appeared
brilliant and distinct; it was best seen with a power of 120, but was
visible with a power of 700. The small star in Aldebaran was very
distinct with a power of 120. The small star α Lyræ was distinctly
visible with the same power. The small star called by Sir J. Herschel
_Debilissima_, between 4 ε and 5 Lyræ, whose existence, he says, could
not be suspected in either the 5 or 7 feet equatorial, and invisible
also with the 7 and 10 feet reflectors of six and 9 inches aperture,
but seen double with the 20 feet reflector, is seen very satisfactorily
double with this telescope. η Persei, marked as double in South and
Herschel’s catalogue, at the distance of 28´´, with another small star
at the distance of 3´ 67´´, is seen distinctly sixfold, four of the
small stars being within a considerably less distance than the remote
one of η marked in the catalogue. And, rejecting the remote star, the
principal, and the four other stars, form a miniature representation
of Jupiter and his satellites, three of them being nearly in a line
on one side, and the other on the opposite. _Castor_, is distinctly
double with 120, and well opened and stars perfectly round with 360
and 700: γ Leonis and α Piscium are seen with the same powers equally
round and distinct. In ε Bootis, the small star is well separated
from the larger, and its blue colour well marked with a power of 360.
η Coronæ Borealis is seen double with a power of 360 and 700. 52
Orionis, ζ Orionis, and others of the same class are also well defined
with the same powers. In regard to the planets which happened to be
visible--Venus appeared beautifully white and well defined with a power
of 120, but showed some colour with 360. Saturn with the 120 power,
is a very brilliant object, the double ring and belts being well and
satisfactorily defined, and with the 360 power, it is still very fine.
The moon also is remarkably beautiful, the edges and the shadows being
well marked, while the quantity of light is such as to bring to view
every minute distinction of figure and shade.
The principal objections that may be made to this construction of a
telescope are such as these:--Can the fluid be permanently secured?
Will it preserve its transparency and other optical properties? Will
it not act upon the surface of the glass and partially destroy it? &c.
To such enquiries Mr. Barlow replies, that experience is the only test
we have; our spirit levels, spirit thermometers, &c., show that some
fluids at least may be preserved for many years, without experiencing
any change, and without producing any in the appearance of the glass
tubes containing them. But should any of these happen, except the last,
nothing can be more simple than to supply the means of replacing the
fluid at any time, and by any person, without disturbing the adjustment
of the telescope. He expresses his hope that, should these experiments
be prosecuted, an achromatic telescope may ultimately be produced
which shall exceed in aperture and power, any instruments of the kind
hitherto attempted. If the prejudice against the use of fluids could be
removed, he feels convinced that well-directed practice would soon lead
to the construction of the most perfect instruments, on this principle,
at a comparatively small expense. ‘I am convinced,’ he says, ‘judging
from what has been paid for large object-glasses, that my telescope,
telescope stand, and the building for observation, with every other
requisite convenience, have been constructed for a less sum than would
be demanded for the object-glass only, if one could be produced of the
same diameter of plate and flint-glass; and this is a consideration
which should have some weight, and encourage a perseverance in the
principle of construction.’[24]
ROGERS’ ACHROMATIC TELESCOPE ON A NEW PLAN.
The object of this construction is to render a small disc of
flint-glass available to perform the office of compensation to a much
larger one of crown-glass, and thus to render possible the construction
of telescopes of much larger aperture than are now common, without
hindrance from the difficulty at present experienced in procuring large
discs of flint-glass. It is well known to those who are acquainted
with telescopes, that in the construction of an ordinary achromatic
object-glass, in which a single crown lens is compensated by a single
one of flint, the two lenses admit of being separated only by an
interval too small to afford any material advantage, _in diminishing
the diameter of the flint lens_, by placing it in a narrower part
of the cone of rays--the actual amount of their difference in point
of dispersive power being such as to render the correction of the
chromatic aberration impossible, when their mutual distance exceeds
a certain limit. This inconvenience Mr. Rogers proposes to obviate,
by employing, as a correcting lens--not a single lens of flint, but a
compound one consisting of a convex crown and concave flint, whose foci
are such as to cause their combination to act as a plain glass on the
mean refrangible rays. Then it is evident, that by means of the greater
dispersive power of flint than of crown glass, this will act as a
concave on the violet, and as a convex on the red rays, and _that_ the
more powerfully, according as the lenses separately have greater powers
or curvature. If then, such a compound lens be interposed between the
object-glass of a telescope--supposed to be a single lens of plate or
crown-glass--and its focus, it will cause no alteration in the focus
for mean rays, while it will lengthen the focus for violet, and shorten
it for red rays. Now this is precisely what is wanted to produce an
achromatic union of all the rays in the focus; and as nothing in this
construction limits the powers of the individual correcting lenses,
they may therefore be applied any where that convenience may dictate;
and thus, theoretically speaking, a disc of flint-glass, however small,
may be made to correct the colour of one of crown however large.
This construction, likewise, possesses other and very remarkable
advantages. For, first, when the correcting lens is approximately
constructed on a calculation founded on its intended aperture, and on
the refractive and dispersive indices of its materials, the final and
complete dispersion of colour may be effected, not by altering the
lenses by grinding them anew, but by shifting the combination nearer
to, or farther from, the object-glass, as occasion may require, along
the tube of a telescope, by a screw motion, till the condition of
achromaticity is satisfied in the best manner possible. And secondly,
the spherical aberration may in like manner be finally corrected, by
slightly separating the lenses of the correcting glass, whose surfaces
should for this purpose be figured to curvatures previously determined
by calculation, to admit of this mode of correction--a condition which
Mr. Rogers finds to be always possible. The following is the rule
he lays down for the determination of the foci of the lenses of the
correcting glass:--‘The focal length of either lens of the correcting
lens is to that of the object-glass, in a ratio compounded of the ratio
of the square of the aperture of the correcting lens to that of the
object-glass, and of the ratio of the difference of the dispersive
indices of the crown and flint glass, to the dispersive index of
crown.’ For example, to correct the colour of a lens of crown or plate
glass of 9 inches aperture, and 14 feet focal length (the dimensions
of the telescope of Fraunhofer at Dorpat) by a disc of flint glass 3
inches in diameter, the focus of either lens of the correcting lens
will require to be about 9 inches. To correct it by a 4 inch disc will
require a focus of about 16 inches each.
Mr. Rogers remarks, that it is not indispensable to make the
correcting glass act as a plane lens. It is sufficient if it be so
adjusted as to have a shorter focus for red rays than for violet.
If, preserving this condition, it be made to act as a concave lens,
the advantage procured by Mr. Barlow’s construction of reducing the
length of the telescope with the same focal power, is secured, and he
considers, moreover, that by a proper adaptation of the distances,
foci, &c., of the lenses, we might hope to combine with all these
advantages that of the destruction of the secondary spectrum, and thus
obtain a perfect telescope.
The above is an abstract of a paper read to the ‘Astronomical Society
of London’ in April 1828, by A. Rogers, Esq.
The reader will easily perceive that the principle on which Mr.
Rogers proposes to construct his telescope is very nearly similar
to that of professor Barlow, described above, with this difference,
that the correcting lens of the Professor’s telescope is composed of
a transparent _fluid_, while that of Mr. Rogers is a _solid_ lens
consisting of a convex crown and concave flint. The general object
intended to be accomplished by both is the same, namely, to make a
correcting lens of a comparatively small diameter serve the purpose of
a large disc of flint glass, which has hitherto been very expensive,
and very difficult to be procured; and likewise to reduce the length of
the telescope while the advantage of a long focal power is secured.--A
telescope, on this principle, was constructed 7 or 8 years ago by
Mr. Wilson, lecturer on Philosophy and Chemistry, Glasgow, before he
was aware that Mr. Rogers had proposed a similar plan. I have had an
opportunity of particularly inspecting Mr. Wilson’s telescope, and
trying its effects on terrestrial objects with high powers, and was on
the whole highly pleased with its performance. It appeared to be almost
perfectly achromatic, and produced a distinct and _well-defined_ image
of minute distant objects, such as small letters on sign-posts, at 2,
3 and 4 miles distant. But I had no opportunity of trying its effects
on double stars or any other celestial objects. The instrument is above
6 feet long; the object lens is a plano-convex of crown glass 4 feet
focal distance, and 4 inches diameter, the plain side next the object.
At 26 inches distant from the object lens is the compound lens of 2
inches in diameter; and the two lenses of which it is composed are
both ground to a radius of 3-3/4 inches. That made of crown glass is
_plano-convex_, the other, made of flint glass, is plano-concave, and
are placed close together, the convex side being next the object, and
the concave side next the eye. The greater refractive power of the
flint glass renders the compound one slightly concave in its effect
(although the radius of curvature is similar in both), and lengthens
the focus to 6 feet from the object-glass; and this is consequently the
length of the instrument. The compound corrector so placed intercepts
all those rays which go to form the image in the field of view,
producing there an achromatic image. The concave power of the corrector
renders the image larger than if directly produced by a convex lens
of the same focus. The concavity of the corrector is valuable also in
this respect, that a very slight alteration in its distance from the
object-glass, changes the focal distance much more than if it were
plain, and enables us to adjust the instrument to perfect achromatism
with great precision.