ON REFLECTING TELESCOPES.
SECT. 1.--HISTORY OF THE INVENTION, AND A GENERAL DESCRIPTION OF THE
CONSTRUCTION OF THESE INSTRUMENTS.
Reflecting telescopes are those which represent the images of distant
objects by reflection, chiefly from concave mirrors.
Before the achromatic telescope was invented, there were two glaring
imperfections in refracting telescopes, which the astronomers of
the 17th century were anxious to correct. The first was its very
great length when a high power was to be applied, which rendered it
very unwieldy and difficult to use. The second imperfection was the
incorrectness of the image as formed by a single lens. Mathematicians
had demonstrated that a pencil of rays could not be collected in a
single point by a spherical lens, and also that the image transmitted
by such a lens would be in some degree incurvated. After several
attempts had been made to correct this imperfection by grinding lenses
to the figure of one of the conic sections, Sir I. Newton happened to
commence an examination of the colours formed by a prism; and having,
by the means of this simple instrument, discovered the different
refrangibility of the rays of light--to which we have several times
adverted in the preceding descriptions--he then perceived that the
errors of telescopes, arising from that cause alone, were some hundred
times greater than such as were occasioned by the spherical figure
of lenses; which induced this illustrious philosopher to turn his
attention to the improvement of telescopes by reflection.
It is generally supposed that Mr. James Gregory--a son of the Rev. John
Gregory, minister of Drumoak in the county of Aberdeen--was the first
who suggested the construction of a reflecting telescope. He was a
young man of uncommon genius, and an eminent mathematician; and in the
year 1663, at the age of only 24, he published in London, his treatise
entitled ‘Optica Promota,’ in which he explained the theory of that
species of reflecting telescope which still bears his name, and which
he stated as being his own invention. But as Gregory, according to his
own account, was endowed with no mechanical dexterity, and could find
no workman capable of realizing his invention--after some fruitless
attempts to form proper specula, he was obliged to give up the pursuit;
so that this telescope remained for a considerable time neglected.
It was several years after Gregory suggested the construction of
reflecting telescopes, till Newton directed his attention fully to the
subject. In a letter addressed to the secretary of the Royal Society,
dated in February, 1672, he says, ‘Finding reflections to be regular,
so that the angle of reflection of all sorts of rays was equal to
the angle of incidence, I understood that, by their mediation, optic
instruments might be brought to any degree of perfection imaginable,
providing a reflecting substance could be found which would polish as
finely as glass, and reflect as much light as glass transmits, and the
art of communicating to it a parabolic figure be also obtained. Amidst
these thoughts I was forced from Cambridge by the intervening plague,
and it was more than two years before I proceeded further.’
It was towards the end of 1668, or in the beginning of the following
year, when Newton, being obliged to have recourse to reflectors, and
not relying on any artificer for making the specula, set about the work
himself, and early in the year 1672, completed two small reflecting
telescopes. In these he ground the great speculum into a spherical
concave, although he approved of the parabolic form, but found himself
unable to accomplish it. These telescopes were of a construction
somewhat different from what Gregory had suggested, and though only 6
inches long, were considered as equal to a 6 feet common refracting
telescope. It is not a little singular, however, that we hear no more
about the construction of reflectors till more than half a century
afterwards. It was not till the year 1723, that any reflectors were
known to have been made, adapted to celestial observations. In that
year, Mr. Hadley, the inventor of the reflecting quadrant, which goes
by his name, published in No. 376 of the Philosophical Transactions, an
account of a large reflector on Newton’s plan, which he had just then
constructed, the performance of which left no room to doubt that this
invention would remain any longer in obscurity. The large speculum of
this instrument was 62-5/8 inches focal distance and 5 inches diameter,
was furnished with magnifying powers of from 190 to 230 times, and
equalled in performance the famous aerial telescope of Huygens of 123
feet in length.[25] Since this period, the reflecting telescope has
been in general use among astronomers in most countries of Europe,
and has received numerous improvements, under the direction of Short,
Mudge, Edwards and Herschel--the last of whom constructed reflectors
of 7, 10, 20, and even 40 feet in focal length, which far surpassed,
in brightness and magnifying power, all the instruments of this
description, which had previously been attempted.
I shall now proceed to give a brief sketch of the nature of a
reflecting telescope, and the different forms in which they have been
proposed to be constructed.
Fig. 62 represents the reflecting telescope as originally proposed
by Gregory. ABEF represents a tube open at AF towards the object; at
the other end is placed a concave speculum BE, with a hole CD in its
centre, the focus of which is at _e_. A little beyond this focus,
towards the object end of the telescope AF, is placed another small
concave mirror G, having its polished face turned towards the great
speculum, and is supported by an arm GH fastened to a slider connected
with the tube. At the end of the great tube BE is screwed in a small
tube CDKI, containing a small plano-convex lens IK. Such are the
essential parts of this instrument and their relative positions. It
will be recollected in our description of the properties of concave
mirrors (see page 92), that, when rays proceed from a distant object,
and fall upon a concave-speculum, they paint an image or representation
of the object in its focus before the speculum. Now suppose two
parallel rays _ab_ falling on the speculum BE, in _cd_; they are
reflected to its focus _e_ where an inverted image of the object is
formed. This image is formed at a little more than the focal distance
of the small speculum from its surface, and serves as it were for an
object on which the small mirror may act. By the action of this mirror
this first image is reflected to a point about _f_, where a second
image is formed very large and erect. This image is magnified in the
proportion of _f_G to _e_G, the rays from which are transmitted to the
eye glass IK, through which the eye perceives the object clear and
distinct, after the proper adjustments have been made.
[Illustration: _fig. 62._]
[Illustration: _fig. 63._]
[Illustration: _fig. 64._]
[Illustration: _fig. 65._]
[Illustration: _figure 66._]
Suppose the focal distance of the great mirror was 9 inches, and the
focal distance of the small mirror 1-1/2 inch--were we to remove the
eye piece of this telescope, and look through the hole of the great
mirror, we should see the image of the object depicted upon the face of
the small speculum, and magnified, in the proportion of 9 to 1-1/2, or,
6 times, on the same principle as a common convex object glass 9 inches
focal length, with an eye glass whose focus is 1-1/2 inch magnifies 6
times. This may be regarded as the first part of the magnifying power.
If now, we suppose the small speculum placed a little more than 1-1/2
inch from the image formed by the great speculum, a second image is
formed about _f_, as much exceeding the first in its dimensions as it
exceeds it in distance from the small speculum, on the principle on
which the object glass of a compound microscope forms a large image
near the eye glass. Suppose this distance to be 9 times greater, then
the whole magnifying power will be compounded of 6 multiplied by 9, or
54 times. As a telescope it magnifies 6 times, and in the microscope
part 9 times.--Such is a _general_ idea of the Gregorian telescope, the
minute particulars and structure of which can only be clearly perceived
by a direct inspection of the instrument.
_The Newtonian Reflector._--This instrument is somewhat different
both in its form and in its mode of operation from that of Gregory.
It is represented in fig. 63, where BAEF is the tube, and BE, the
object concave mirror, which reflects the parallel rays _ab_ to a
_plane_ speculum G, placed 45°, or half a right angle to the axis of
the concave speculum. This small plane reflector must be of an oval
form, the length of the oval should be to the breadth as 7 to 5, on
account of the obliquity of its position. It is supported on an arm
fixed to the side of the tube; an eye-glass is placed in a small tube,
moveable in the larger tube, so as to be perpendicular to the axis of
the large reflector, the perpendicular line passing through the centre
of the small mirror. The small mirror is situated between the large
mirror and its focus, that its distance from this focal point may be
equal to the distance from the centre of the mirror to the focus of the
eye-glass. When the rays _ab_ from a distant object fall upon the large
speculum at _cd_, they are reflected towards a focus at _h_; but being
intercepted by the plane mirror G, they are reflected perpendicularly
to the eye-glass at I, in the side of the tube, and the image formed
near that position at _e_ is viewed through a small plano-convex lens.
The magnifying power of this telescope is in the proportion of the
focal distance of the speculum to that of the eye-glass. Thus, if the
focal distance of the speculum be 36 inches, and that of the eye-glass
1/3 of an inch, the magnifying power will be 108 times. It was this
form of the reflecting telescope, that Newton invented, which Sir. W.
Herschel adopted, and with which he made most of his observations and
discoveries.
_The Cassegrainian Reflector._--This mode of the reflecting telescope,
suggested by M. Cassegrain, a Frenchman, is represented in fig. 64. It
is constructed in the same way as the Gregorian, with the exception of
a small _convex_ speculum G being substituted in the room of the small
concave in Gregory’s construction. As the focus of a convex mirror is
negative, it is placed at a distance from the large speculum equal to
the difference of their foci, that is, if the focal length of the large
speculum be 18 inches, and that of the small convex 2 inches, they are
placed at 16 inches distant from each other, on a principle similar
to that of the Galilean telescope, in which the concave eye-glass is
placed within the focus of the object-glass by a space equal to the
focal length of the eye-glass. In this telescope, likewise, instead
of two there is only _one image_ formed, namely that in the focus
of the eye-glass; and, on this account some are of opinion that the
distinctness is considerably greater than in the Gregorian. Mr. Ramsden
was of opinion that this construction is preferable to either of the
former reflectors, because the aberrations of the two metals have a
tendency to correct each other, whereas in the Gregorian both the
metals being concave, any error in the specula will be doubled. It is
his opinion that the aberrations in the Cassegrainian construction
to that of the Gregorian is as 3 to 5. The length of this telescope
is shorter than that of a Gregorian of equal focal length, by twice
the focal length of the small mirror, and it shows every thing in
an _inverted_ position, and consequently is not adapted for viewing
terrestrial objects.
_Dr. Hook’s Reflector._--Before the reflecting telescope was much
known, Dr. Hook contrived one, the form of which is represented, fig.
65, which differs in little or nothing from the Gregorian, except that
the eye-glass I is placed in the hole of the great speculum BE.
_Martin’s Reflector._--Mr. Bengamin Martin, a distinguished writer on
optical and philosophical science, about a century ago, described a
new form of the reflecting telescope, approximating to the Newtonian
structure, which he contrived for his own use. It is represented in
fig. 66. ABEF is the tube, in which there is an opening or aperture OP,
in the upper part. Against this hole within the tube is placed a large
plane speculum GH, at half a right angle with the axis or sides of the
tubes, with a hole CD perforated through its middle. The parallel rays
_a b_ falling on the inclined plane GH are reflected perpendicularly
and parallel on the great speculum BE in the bottom of the tube. From
thence they are reflected converging to a focus _e_ through the hole of
the plane mirror CD, which being also the focus of the eye-glass IK,
the eye will perceive the object magnified and distinct.
In the figures referred to in the above descriptions, only one
eye-glass is represented to avoid complexity; but in most reflecting
telescopes, the eye-piece consists of a combination of two plano-convex
glasses, as in fig. 67, which produces a more correct and a larger
field of view than a single lens. This combination is generally known
by the name of the _Huygenian eye-piece_ which shall be described in
the section on the _eye-pieces_ of telescopes.
The following rule has been given for finding the magnifying power of
the Gregorian telescope:--Multiply the focal distance of the great
mirror by the distance of the small mirror from the image next the
eye; and multiply the focal distance of the small mirror by the focal
distance of the eye-glass; then divide the product of the former
multiplication by the product of the latter, and the quotient will
express the magnifying power. The following are the dimensions of one
of the reflecting telescopes constructed by Mr. Short--who was long
distinguished as the most eminent maker of such instruments, on a large
scale, and whose large reflectors are still to be found in various
observatories throughout Europe.
The focal distance of the great mirror 9.6 inches; or P _m_, fig. 67,
its breadth FD 2.3; the focal distance of the small mirror L _n_
1.5--or 1-1/2 inch--its breadth _g h_ 0.6--or 6/10 of an inch; the
breadth of the hole in the great mirror UV, 0.5--or half an inch--the
distance between the small mirror and the next eye-glass LR, 14.2; the
distance between the two eye-glasses SR, 2.4; the focal distance of the
eye-glass next the metal, 3.8.; and the focal distance of the eye-glass
next the eye, S _a_ 1.1, or one inch and one tenth. The magnifying
power of this telescope was about 60 times. Taking this telescope as a
standard, the following table of the dimensions and magnifying powers
of Gregorian reflecting telescopes, as constructed by Mr. Short, has
been computed.
[Illustration: _figure 67._]
A: Focal distance of the great mirror.
B: Breadth of the great mirror.
C: Focus of the small speculum.
D: Breadth of the hole in the great speculum.
E: Distance between the small speculum and the first eye-glass.
F: Focal distance of the glass next the metals.
G: Focal distance of the glass next the eye.
H: Distance between the plain sides of the two glasses.
I: Magnifying power.
J: Distance between the second glass and the small eye-hole.
--------+--------+--------+--------+--------
| | | |
A. | B. | C. | D. | E.
--------+--------+--------+--------+--------
| | | |
P _m_ | D F | L _n_ | U V | L R
| | | |
In. Dec.|In. Dec.|In. Dec.|In. Dec.|In. Dec.
| | | |
5. 65 | 1. 54 | 1. 10 | 0. 31 | 8. 54
9. 60 | 2. 30 | 1. 50 | 0. 39 |14. 61
15. 50 | 3. 30 | 2. 14 | 0. 50 |23. 81
36. 00 | 6. 26 | 3. 43 | 0. 65 |41. 16
60. 00 | 9. 21 | 5. 00 | 0. 85 |68. 17
--------+--------+---------+-----+------
| | | |
F. | G. | H. | I. | J.
--------+--------+---------+-----+------
| | | |
R | S | R S | |
| | | |
In. Dec.|In. Dec.|In. Dec. | In. |
| | | |
2. 44 | 0. 81 | 1. 68 | 39 | 0. 41
3. 13 | 1. 04 | 2. 09 | 60 | 0. 52
3. 94 | 1. 31 | 2. 63 | 86 | 0. 66
5. 12 | 1. 71 | 3. 41 | 165 | 0. 85
6. 43 | 2. 14 | 4. 28 | 243 | 1. 07
Mr. Short--who was born in Edinburgh in 1710, and died near London,
1768--was considered as the most accurate constructor of reflecting
telescopes, during the period which intervened from 1732, to 1768. In
1743, he constructed a reflector for Lord Thomas Spencer, of 12 feet
focal length, for which he received 600 guineas. He made several other
telescopes of the same focal distance, with greater improvements and
higher magnifiers; and in 1752, finished one for the king of Spain, for
which, with its whole apparatus, he received £1200. This was considered
the noblest instrument of its kind that had then been constructed, and
perhaps it was never surpassed, till Herschel constructed his twenty
and forty feet reflectors. High as the prices of large telescopes now
are, Mr. Short charged for his instruments at a much higher rate than
opticians now do, although the price of labour, and every other article
required in the construction of a telescope, is now much dearer. But
he had then scarcely any competitor, and he spared neither trouble nor
expense to make his telescopes perfect, and put such a price upon them
as properly repaid him. The following table contains a statement of the
apertures, powers, and prices of Gregorian telescopes, as constructed
by Mr. James Short.[26]
A: Number.
B: Focal length in inches.
C: Diameter of aperture in inches.
D: Prices in guineas.
-----+---------+------+-----------------------------------+----
| | | |
A. | B. | C. | Magnifying powers. | D.
-----+---------+------+-----------------------------------+----
1 | 3 | 1.1 | 1 Power of 18 times | 3
2 | 4-1/2 | 1.3 | 1 " 25 " | 4
3 | 7 | 1.9 | 1 " 40 " | 6
4 | 9-1/2 | 2.5 | 2 Powers 40 and 60 " | 8
5} | 12 | 3.0 | 2 " 55 and 85 " | 10
6} | 12 | 3.0 | 4 " 35, 55, 85, and 110 " | 14
7 | 18 | 3.8 | 4 " 55, 95, 130, and 200 " | 20
8 | 24 | 4.5 | 4 " 90, 150, 230, and 300 " | 35
9 | 36 | 6.3 | 4 " 100, 200, 300, and 400 " | 75
10 | 48 | 7.6 | 4 " 120, 260, 380, and 500 " | 100
11 | 72 | 12.2 | 4 " 200, 400, 600, and 800 " | 300
12 | 144 | 18.0 | 4 " 300, 600, 900, and 1200 " | 800
From this table, it appears that Mr. Short charged 75 guineas for a 3
feet reflector, whereas such an instrument is now marked in the London
opticians’ catalogues at £23, when mounted on a common brass stand, and
£39. 18s., when accompanied with rack-work motions and other apparatus.
It is now generally understood that in the above table, Short always
greatly _overrated_ the higher powers of his telescopes. By experiment
they were generally found to magnify _much less_ than here expressed.
_General remarks on Gregorian Reflectors._--1. In regard to the hole
UV, of the great speculum--its diameter should be equal, or nearly
so, to that of the small speculum L, fig. 67. For if it be less, no
more parallel rays will be reflected than if it were equal to _g h_,
and it may do harm in contracting the visible area within too narrow
limits. Nor must it be larger than the mirror L, because some parallel
rays will then be lost, and those of most consequence as being nearest
the centre. 2. The small hole at _e_ to which the eye is applied,
must be nicely adjusted to the size of the cone of rays proceeding
from the nearest lens S. If it be larger, it will permit the foreign
light of the sky or other objects to enter the eye, so as to prevent
distinct vision; for the eye should receive no light, but what comes
from the surface of the small mirror L. If the hole be smaller than
the cylinder of rays at _e_ then some of the necessary light will be
excluded, and the object rendered more obscure. The diameter of this
hole may be found by dividing the aperture of the telescope in inches
by its magnifying power. Thus, if we divide the diameter of one of
Short’s telescopes, the diameter of whose large speculum is 2.30, by
60, the magnifying power, the quotient will be .0383, which is nearly
the 1/25 of an inch. Sometimes this hole is made so small as the 1/50
of an inch. When this hole is, by any derangement, shifted from its
proper position, it sometimes requires great nicety to adjust it, and,
before it is accurately adjusted, the telescope is unfit for accurate
observation. 3. It is usual to fix a plate with a hole in it, at _a b_,
the focus of the eye glass S, of such a diameter as will circumscribe
the image, so as to exhibit only that part of it which appears
distinct, and to exclude the superfluous rays. 4. There is an adjusting
screw on the outside of the great tube, connected with the small
speculum, by which that speculum may be pushed backwards or forwards to
adjust the instrument to distinct vision. The hand is applied for this
purpose at T.
_Newtonian Telescopes._--These telescopes are now more frequently
used for celestial observations than during the last century, when
Gregorian reflectors were generally preferred. Sir W. Herschel was
chiefly instrumental in introducing this form of the reflecting
telescope to the more particular attention of astronomers, by the
splendour and extent of the discoveries which it enabled him to make.
In this telescope there is no hole required in the middle of the great
speculum, as in the Gregorian construction, which circumstance secures
the use of all the rays which flow from the central parts of the mirror.
The following table contains a statement of the apertures and
magnifying powers of Newtonian Telescopes, and the focal distances of
their eye-glasses. The first column contains the focal length of the
great speculum in feet; the second, its linear aperture in inches;
the third, the focal distance of the single glass in decimals, or in
1000ths of an inch, and the fourth column, contains the magnifying
power. This portion of the table was constructed by using the
dimensions of Mr. Hadley’s Newtonian Telescope, formerly referred to,
as a standard--the focal distance of the great mirror being 62-1/2
inches, its medium aperture 5 inches, and power 208. The fifth, sixth,
and seventh columns contains the apertures of the concave speculum,
the focal lengths of the eye-glasses and the magnifying powers, as
calculated by Sir D. Brewster, from a telescope of Mr. Hauksbee, taken
as a standard; whose focal length was 3 feet 3 inches, its aperture
about 4 inches, and magnifying power 226 times.
A: Focal distance of concave metal.
B: Aperture of concave metal.
C: Focal distance of single eye-glass.
D: Magnifying power.
E: Aperture of the concave speculum.
F: Focal length of the eye-glass.
G: Magnifying power.
--------+----------+----------++----------------------------------
| | ||
| | || Sir D. Brewster’s Numbers.
A. | B. | C. ++----------------------------------
| | || | | |
| | || D. | E. | F. | G.
--------+----------+----------++-----+----------+------------+----
| | || | | |
Feet. | In. Dec. | In. Dec. || | In. Dec. | In. Dec. |
0-1/2 | 0. 86 | 0. 167 || 36 | 1. 34 | 0. 107 | 56
1 | 1. 44 | 0. 199 || 60 | 2. 23 | 0. 129 | 93
2 | 2. 45 | 0. 236 || 102 | 3. 79 | 0. 152 | 158
3 | 3. 31 | 0. 261 || 138 | 5. 14 | 0. 168 | 214
4 | 4. 10 | 0. 281 || 171 | 6. 36 | 0. 181 | 265
5 | 4. 85 | 0. 297 || 202 | 7. 51 | 0. 192 | 313
6 | 5. 57 | 0. 311 || 232 | 8. 64 | 0. 200=1/5| 360
7 | 6. 24 | 0. 323 || 260 | 9. 67 | 0. 209 | 403
8 | 6. 89 | 0. 334 || 287 | 10. 44 | 0. 218 | 445
9 | 7. 54 | 0. 344 || 314 | 11. 69 | 0. 222 | 487
10 | 8. 16 | 0. 353 || 340 | 12. 65 | 0. 228 | 527
11 | 8. 76 | 0. 362 || 365 | 13. 58 | 0. 233 | 566
12 | 9. 36 | 0. 367 || 390 | 14. 50 | 0. 238 | 604
13 | 9. 94 | 0. 377 || 414 | 15. 41 | 0. 243 | 642
14 | 10. 49 | 0. 384 || 437 | 16. 25 | 0. 248 | 677
15 | 11. 04 | 0. 391 || 460 | 17. 11 | 0. 252 | 713
16 | 11. 59 | 0. 397 || 483 | 17. 98 | 0. 256 | 749
17 | 12. 14 | 0. 403 || 506 | 18. 82 | 0. 260 | 784
18 | 12. 67 | 0. 409 || 528 | 19. 63 | 0. 264 | 818
19 | 13. 20 | 0. 414 || 550 | 20. 45 | 0. 268 | 852
20 | 13. 71 | 0. 420 || 571 | 21. 24 | 0. 271 | 885
One great advantage of reflecting telescopes above common refractors,
is, that they will admit of eye glasses of a much shorter focal
distance, and consequently, will magnify so much the more, for the
rays are not coloured by reflection from a concave mirror, if it be
ground to a true figure, as they are by passing through a convex
glass though figured and polished with the utmost exactness. It will
be perceived from the above table, that the focal length of the eye
glasses is very small, the lowest there stated being only about 1/10
of an inch, and the highest little more than 1/4 of an inch focal
distance. Sir W. Herschel obtained the high powers which he sometimes
put upon his telescopes, by using small double convex lenses for eye
glasses, some of which did not exceed the _one fiftieth of an inch_ in
focal length. When the focal length of the concave speculum, and that
of the eye glass are given, the magnifying power is found by dividing
the former by the latter, after having reduced the focal length of the
concave speculum to inches. Thus the 6 feet speculum, multiplied by 12,
produces 72 inches, which, divided by Brewster’s number for the focus
of the eye glass = 200, or 1/5 of an inch, produces a quotient of 360
as the magnifying power. It has been calculated that, if the metals
of a Newtonian telescope be worked as exquisitely as those in Sir W.
Herschel’s 7 feet reflector, the highest power that such a telescope
should bear with perfect distinctness, will be found by multiplying the
diameter of the great speculum in inches, by 74, and the focal distance
of the single eye glass may be found by dividing the focal distance of
the great mirror by the magnifying power. Thus 6.25--the aperture in
inches of Herschel’s 7 feet Newtonian--multiplied by 74 is 462-1/2, the
magnifying power; and 7 multiplied by 12, and divided by 462.5 is 0.182
of an inch, the focal distance of the single eye glass required. But
it is seldom that more than one half of this power can be applied with
effect to any of the planetary bodies. For general purposes the power
produced by multiplying the diameter of the speculum by 30, or 40, will
be found most satisfactory.
The following are the general prices of reflecting telescopes as made
by the London opticians.
£ _s._
A four feet, seven inch aperture, Gregorian Reflector; with
the vertical motions upon a new invented principle, as well as
apparatus to render the tube more steady in observation;
according to the additional apparatus of small speculums,
eye-pieces, micrometers, &c. from 80 to 120 0
Three feet long, mounted on a plain brass stand 23 2
Ditto, with rack-work motions, improved mounting, and
metals 39 18
Two feet long without rack-work, and with 4 magnifying
powers, improved 15 15
Ditto with rack-work motion 22 1
Eighteen inch on a plain stand 9 9
Twelve inch Ditto 6 6
The above are the prices stated in Messrs. W. and S. Joneses catalogue.
The following list of prices of the various kinds of reflecting
telescopes is from Messrs. Tulley’s (of Islington) catalogue.
£ _s._
1 foot _Gregorian_ reflector, on pillar and claw stand, metal
2-1/2 inches diameter, packed in a mahogany box 6 6
1-1/2 foot ditto, on pillar and claw stand, metal 3 inches
diameter, packed in mahogany box 11 11
2 feet ditto, metal 4 inches diameter 16 16
Ditto, ditto, with rack-work motions 25 4
3 feet ditto, metal 5 inches diameter, with rack-work
motions 42 0
Ditto, metal 6 inches diameter, on a tripod stand, with centre
of gravity motion 68 5
4 feet ditto, metal 7 inches diameter, as above 105 0
6 feet ditto, metal 9 inches diameter, on an improved iron
stand 210 0
7 feet _Newtonian_ reflectors, 6 inches aperture, mounted on
a new and improved stand 105 0
Ditto, ditto, metal 7 inches diameter 126 0
9 feet ditto, metal 9 inches diameter 210 0
10 feet ditto, metal 10 inches diameter 315 0
12 feet ditto, metal 12 inches diameter 525 0
_Comparative brightness of achromatic and reflecting telescopes._ The
late astronomer royal, Dr. Maskelyne, from a comparison of a variety of
telescopes, was led to the following conclusion,--‘that the aperture
of a common reflecting telescope, in order to show objects as bright
as the achromatic must be to that of an achromatic telescope as 8 to
5,’--in other words, an achromatic whose object glass is 5 inches
diameter, will show objects with as great a degree of brightness as a
reflector whose large speculum is 8 inches in diameter. This result,
if correct, must be owing to the small number of rays reflected from
a speculum compared with the number transmitted through an achromatic
object glass.
SECT. 2.--THE HERSCHELIAN TELESCOPE.
Soon after Sir William Herschel commenced his astronomical career, he
introduced a new era in the history of reflecting telescopes. After he
had cast and polished an immense variety of specula for telescopes of
different sizes-he, at length, in the year 1782, finished a 20 feet
reflector with a large aperture. Being sensible of the vast quantity
of light which is lost by a second reflection from the small speculum,
he determined to throw it aside altogether, and mounted this 20 feet
reflector on a stand that admitted of being used without a small
speculum in making _front observations_--that is, in sitting with his
back to the object, and looking directly towards the surface of the
speculum. Many of his discoveries and measurements of double stars were
made with this instrument, till, at length, in the year 1785 he put
the finishing hand to that gigantic speculum, which soon became the
object of universal astonishment, and which was intended for his _forty
feet_ reflecting telescope; he had succeeded so well in constructing
reflecting telescopes of comparatively small aperture, that they would
bear higher magnifying powers than had ever previously been applied;
but he found that a deficiency of light could only be remedied by an
increased diameter of the large speculum, which therefore was his main
object, when he undertook to accomplish a work which to a man less
enterprising, would have appeared impracticable. The difficulties he
had to overcome were numerous; particularly in the operative department
of preparing, melting, annealing, grinding, and polishing a mass of
metal that was too unwieldly to be moved without the aid of mechanical
powers. At length, however, all difficulties having been overcome,
this magnificent instrument was completed with all its complicated
apparatus, and erected for observation, on the 28th of August, 1789,
and on the same day the sixth satellite of Saturn was detected, as a
prelude of still farther discoveries which were afterwards made by this
instrument, in the celestial regions.
It would be too tedious to attempt a description of all the machinery
and apparatus connected with this noble instrument. The reader
who wishes to peruse a minute description of the stairs, ladders,
platform, rollers, and of every circumstance relating to joiner’s work,
carpenter’s work, smith’s work, and other particulars connected with
the formation and erection of this telescope, will find the details
recorded in the 85th volume of the Philosophical Transactions of the
Royal Society of London, for 1795, in which there are sixty-three pages
of letter press, and eighteen plates illustrative of the subject. I
shall content myself with giving a short outline of the essential parts
belonging to this instrument.
The _tube_ of this telescope is made of rolled or sheet iron, joined
together without rivets; the thickness of the sheets is somewhat less
than 1/36 part of an inch, or 14 pounds weight for a square foot;
great care was taken that the cylindrical form should be secured, and
the whole was coated over three or four times with paint, inside and
outside, to secure it against the damp. This tube was removed from
the place in which it was formed by twenty-four men, divided into six
sets; so that two men on each side, with a pole of 5 feet long in
their hands, to which was affixed a piece of course cloth, 7 feet long
going under the tube, and joined to a pole 5 feet long, in the hands
of two other men, assisted in carrying the tube. The _length_ of this
tube is 39 feet 4 inches, the diameter 4 feet 10 inches; and, on a
moderate computation, it was ascertained, that a wooden tube of proper
dimensions would have exceeded an iron one in weight by at least 3000
pounds. Reckoning the circumference of the tube 15 feet, its length
39-1/3 feet, and 14 lib. for the weight of a square foot, it must have
contained 590 square feet, and weighed 8,260 pounds. Various hoops were
fixed within the tube, and longitudinal bars of iron connecting some of
them are attached to the two ends of the tube, by way of bracing the
sheets, and preserving the shape perfect, when the pulleys are applied
to give the necessary elevation at the upper end, and that the speculum
may be kept secure at the lower end. The lower end of the tube is
firmly supported on rollers that are capable of being moved forwards or
backwards by a double rack, connected with a set of wheels and pinions.
By an adjustment at the lower extremity of the tube, the speculum is
turned to a small inclination, so that the line of collimation may not
be coincident with the longitudinal axis of the tube, but may cross the
tube diagonally, and meet the eye in the air at about two inches from
the edge of the tube, which is the peculiarity of the construction,
that supersedes the necessity of applying a second reflector. Hence no
part of the head of the observer intercepts the incident rays, and the
observation is taken with the face looking at the speculum, the back
being turned to the object to be observed.
The large speculum is enclosed in a strong iron ring, braced across
with bars of iron, and an enclosure of iron and ten sheets makes a case
for it. It is lifted by three handles of iron attached to the sides of
the ring, and is put into and taken out of its proper place in the tube
by the help of a moveable crane, running on a carriage, which operation
requires great care. The speculum is made of a metallic composition,
and is 49-1/2 inches in diameter; but the concave polished surface is
only 48 inches, or 4 feet in diameter. Its thickness is 31 inches; and
when it came from the cast its weight was 2118 pounds. The metals for
its formation were procured at a warehouse in Thames Street, London,
where they kept ingots of two kinds ready made, one of white, and the
other of bell-metal; and it was composed of two ingots of bell-metal
for one of white. It was not to be expected that a speculum of such
large dimensions, could have a perfect figure imparted to its surface,
nor that the curve, whatever it might be, would remain identically the
same in changes of temperature; therefore we are not surprised when we
are told, that the magnifying powers used with this telescope seldom
exceeded 200; the quantity of light collected by so large a surface
being the principal aim of the maker. The raising of the balcony, on
which the observer stands, and the sliding of the lower end of the
tube, in which the speculum rests, are effected by separate tackles,
and require only occasional motions; but the elevation of the telescope
requires the main tackle to be employed, and the motion usually given
in altitude at once was two degrees; the breadth of the zone in which
the observations were made, as the motion of the sphere in right
ascension brought the objects into view. A star, however, could be
followed for about a quarter of an hour. Three persons were employed
in using this telescope, one to work the tackle, another to observe,
and a third to mark down the observations. The elevation was pointed
out by a small quadrant fixed to the main tube, near the lower end, but
the polar distance was indicated by a piece of machinery, worked by a
string, which continually indicated the degree and minute on a dial in
the small house adjoining, while the time was shown by a clock in the
same place, Miss Herschel performing the office of Registrar.
At the upper end the tube is open, and directed to the part of the
heavens intended for observation, and the observer, standing on the
foot board, looks down the tube, and perceives the object by rays
reflected from the speculum, through the eye glass at the opening
of the tube. When the telescope is directed to any objects near the
zenith, the observer is necessarily at an elevation at least 40 feet
from the ground. Near the place of the eye glass is the end of a tin
pipe, into which a mouth-piece may be placed, so that, during an
observation, a person may direct his voice into this pipe, while his
eye is at the glass. This pipe, which is 1-1/2 inch in diameter runs
down to the bottom of the tube, where it goes into a turning joint,
thence into a drawing tube, and out of this into another turning joint,
from whence it proceeds, by a set of sliding tubes towards the front of
the foundation timber. Its use is to convey the voice of the observer
to his assistants, for at the last place, it divides itself into two
branches, one going into the observatory, the other into the workman’s
room, ascending in both places through the floor, and terminates in
the usual shape of speaking trumpets. Though the voice passes in this
manner through a tube, with many inflections, and through not less than
115 feet, it requires very little exertion to be well understood.
To direct so unwieldy a body to any part of the heavens at pleasure,
many mechanical contrivances were evidently necessary. The whole
apparatus rests upon rollers, and care was previously taken of the
foundation in the ground. This consists of concentrical brick walls,
the outermost 42 feet, the innermost 21 feet in diameter, 2 feet 6
inches deep under ground, 2 feet 3 inches broad at the bottom, and 1
foot 2 inches at the top, capped with paving stones 3 inches thick, and
12-3/4 inches broad.
In the centre is a large post of oak, framed together with braces under
ground, and walled fast to brick-work to make it steady. Round this
centre the whole frame is moved horizontally by means of 20 rollers,
12 upon the outer, and 8 upon the inner wall. The vertical motion is
given to the instrument by means of ropes and pullies, passing over the
main beam supported by the ladders. These ladders are 49 feet long,
and there is a moveable gallery with 24 rollers to ease its motion.
There is a stair-case intended for persons who wish to ascend into the
gallery, without being obliged to go up the ladder. The ease with which
the horizontal and vertical motions may be communicated to the tube
may be conceived, from a remark of Sir W. Herschel, that, in the year
1789, he several times observed Saturn, two or three hours before and
after its meridian passage with one single person to continue, at his
directions, the necessary horizontal and vertical motions.
By this telescope the sixth and seventh satellites of Saturn were
discovered, only one of which is within the reach of the 20 feet
reflector, or even of a 25 feet instrument. The discovery of the
satellites of the planet Uranus, however, was made by the 20 feet
reflector, but only after it had been converted from the Newtonian to
the Herschelian construction--which affords a proof of the superiority
of the latter construction over the former when the same speculum is
used. Never had the heavens before been observed with so extraordinary
an instrument as the forty feet reflector. The nebulosities which
are found among the fixed stars, in various regions of the heavens,
appeared almost all to resolve themselves into an innumerable
multitude of stars; others, hitherto imperceptible, seemed to have
acquired a distinct light. On the entrance of Sirius into the field
of the telescope, the eye was so violently affected, that stars of
less magnitude could not immediately after be perceived; and it was
necessary to wait for 20 minutes before these stars could be observed.
The ring of Saturn had always before ceased to be visible when its
plane was directed towards the earth; but the feeble light which it
reflects in that position was enough for Herschel’s instrument, and the
ring, even then, still remained visible to him.
It has been generally considered that this telescope was capable of
carrying a power of 6000 times; and perhaps for the purpose of an
experiment, and for trying its effect on certain objects, such a power
may have been applied,--in which case the eye-glass must have been
only 2/25 of an inch focal distance, or somewhat less than one twelfth
of an inch. But such a power could not be generally applied, with any
good effect, to the planetary bodies; and I question much whether any
power above 1000 times was ever generally used. For, it is the quantity
of light which the telescope collects, more than the magnifying power,
that enables us to penetrate, with effect, into the distant spaces of
the firmament: and hence, as above stated, the power seldom exceeded
200, which on account of the large diameter of the speculum, would
enable the instrument to penetrate into the distant celestial spaces
perhaps further than if a power of as many thousands of times had been
applied.
Sir John Herschel, who inherits all the science, skill, and industry of
his father, some time ago ground and polished a new speculum for the 20
feet tube, formerly noticed, which is connected with a stand, pulleys
and other appendages, similar to those above described, though of
smaller dimensions. This telescope shows the double stars exceedingly
well defined, and was one of the principal instruments used in forming
his catalogue of these objects which was presented to the Royal
Society, in conjunction with that of Sir James South, about the year
1828. I suppose, it is likewise the same telescope with which Sir John
lately made his Sidereal observations at the Cape of Good Hope.
SECT. 3.--RAMAGE’S LARGE REFLECTING TELESCOPE.
The largest _front view_ reflecting telescope in this country--next
to Herschel’s 40 feet instrument--is that which was erected at the
Royal Observatory at Greenwich, in the year 1820, by Mr. Ramage of
Aberdeen. The diameter of the concave reflector is 15 inches, and its
focal length 25 feet. It is erected on machinery which bears a certain
resemblance to that of Herschel’s, which we have now described; but the
mechanical arrangements are greatly simplified, so that the instrument
is manageable by an observer without an assistant. The tube is composed
of a twelve-sided prism of deal 5/8 inch thick. At the mouth is a
double cylinder of different diameters on the same axis; around this a
cord is wound by a winch, and passes up from the small cylinder, over
a pulley, and down through another pulley on to the large cylinder.
When the winch, therefore, is turned to raise the telescope, the
endless cord is unwound from the smaller cylinder, and wound on to the
larger, the difference of the size of the two cylinders will be double
the quantity raised, and a mechanical force to any extent may thus be
obtained, by duly proportioning the diameters of the two cylinders: by
this contrivance the necessity of an assistant is superseded. The view
through this instrument first astonished those observers who had not
been accustomed to examine a heavenly body with a telescope possessing
so much light; and its performance was deemed quite extraordinary. But
when the first impression had subsided, and different trials had been
made in different states of the atmosphere, it was discovered that the
central portion of the speculum was more perfectly figured than the
ring bordering on the extreme edges. When the aperture was limited to
ten or twelve inches, the performance as to the distinctness in its
defining power, was greatly improved, and the light was so brilliant,
that the Astronomer Royal was disposed to entertain an opinion, that it
might equal that of a good achromatic refractor of the same dimensions.
When, however, very small and obscure objects are to be observed,
the whole light of the entire aperture may be used with advantage on
favourable evenings.
The eye-pieces adapted to this telescope have powers which magnify
the object linearly from 100 to 1500 times, which are competent to
fulfil all the purposes of vision when cleared of aberration. When the
telescope is placed in the plane of the meridian and elevated together
with the gallery, into any required altitude, the _meridional sweeps_,
formerly practised by Sir W. Herschel, and continued by Sir John with
great success, in the examination of double stars and nebula, may be
managed with great ease.
Mr. Ramage had a telescope of about the same size, erected in an open
space in Aberdeen, which I had an opportunity of inspecting when I
paid a visit to that gentleman in 1833; but cloudy weather prevented
my obtaining a view of any celestial bodies through it. He showed me
at that time two or three large speculums, from 12 to 18 inches in
diameter, which he had finished some time before, and which appeared
most beautifully polished. He told me, too, that he had ground and
polished them simply with his hand, without the aid of any machinery
or mechanical power--a circumstance which, he said, astonished the
opticians of London, when it was stated, and which they considered as
almost incredible. His experience in casting and polishing metals of
various sizes, during a period of 15 or 16 years, qualified him to
prepare specula of great lustre, and with an unusually high polish. It
has been asserted that a fifty feet telescope by Ramage of 21 inches
aperture was intended to be substituted for the 25 feet instrument
erected at Greenwich, and the speculum it is understood, was prepared,
and ready for use, provided the Navy Board was disposed to defray the
expense of carrying the plan into execution. But, unfortunately, this
ingenious artist was unexpectedly cut off in the midst of his career,
about the year 1835.
SECT. 4.--THE AERIAL REFLECTOR--CONSTRUCTED BY THE AUTHOR.
A particular description of this telescope was given in the ‘Edinburgh
New Philosophical Journal’ for April--July, 1826, conducted by
Professor Jameson, the greater part of which was copied in the ‘London
Encyclopedia,’ ] under the article _Telescope_. From this description I
shall endeavour to condense a brief account of this instrument with a
few additional remarks.
About the year 1822, an old speculum 27 inches in focal length, very
imperfectly polished happened accidentally to come into my possession;
and feeling no inclination to fit it up in the Gregorian form, I formed
the resolution of throwing aside the small speculum, and attempting
the _front view_ notwithstanding the uniform assertion of opticians,
that such an attempt in instruments of a small size is impracticable.
I had some ground for expecting success in this attempt, from several
experiments I had previously made, particularly from some modifications
I had made in the construction of astronomical eye-pieces, which have
a tendency to correct the aberration of the rays of light, when they
proceed somewhat obliquely from a lens or speculum. In the first
instance, I placed the speculum at the one end of a tube of the form
of a segment of a cone--the end next the eye being somewhat wider
than that at which the speculum was fixed, and its length about an
inch shorter than the focal distance of the mirror. A small tube for
receiving the different eye-pieces was fixed in the inside of the large
tube at the end next the eye, and connected with an apparatus by which
it could occasionally be moved either in a vertical or horizontal
direction. With the instrument fitted up in this manner, I obtained
some interesting views of the moon, and of terrestrial objects. But
finding that one side of the tube intercepted a considerable portion of
light from the object, I determined to throw aside the tube altogether,
and to fit up the instrument on a different plan.
A short mahogany tube, about 3 inches long, was prepared, to serve as
a socket for holding the speculum. To the side of this tube an arm was
attached, about the length of the focal distance of the mirror, at
the extremity of which a brass tube for receiving the eye-pieces, was
fixed, connected with screws and sockets, by which it might be raised
or depressed, and turned to the right hand or to the left, and with
adjusting apparatus by which it might be brought nearer to or farther
from the speculum. Fig. 69 exhibits a general representation of the
instrument in profile. AB is the short tube which holds the speculum;
CD the arm which carries the eye-tubes, which consists of two distinct
pieces of mahogany; the part D being capable of sliding along the under
side of C, through the brass sockets EF. To the under part of the
socket F is attached a brass nut with a female screw, in which the male
screw _ab_ acts by applying the hand to the knob _c_, which serves for
adjusting the instrument to distinct vision. G is the brass tube which
receives the eye-pieces. It is supported by a strong brass wire _de_,
which passes through a nut connected with another strong wire, which
passes through the arm D. By means of the nut _f_ this tube may be
elevated or depressed, and firmly fixed in its proper position; and by
the nut _d_ it may be brought nearer to or further from, the arm D.
[Illustration: _figure 69._]
By the same apparatus, it is also rendered capable of being moved
either in a vertical or horizontal direction: but when it is once
adjusted to its proper position, it must be firmly fixed, and requires
no further attention. The eye-piece represented in this figure is the
one used for terrestrial objects, which consists of the tubes belonging
to a pocket achromatic telescope. When an astronomical eye-piece is
used, the length of the instrument extends only to the point I. In
looking through this telescope, the right eye is applied at the point
H, and the observer’s head is understood to be uncovered, or, at
least, tightly covered with a thin cap. For those who use only the
left eye, the arm would require to be placed on the opposite side of
the tube, or the arm, along with the tube, be made to turn round 180
degrees.
[Illustration: _figure 70._]
Fig. 70 represents a front, or rather an oblique view of the
instrument, in which the position of the speculum may be seen. All the
specula which I fitted up in this form, having been originally intended
for Gregorian reflectors, have holes in their centres. The eye-piece
is therefore directed to a point nearly equi-distant from the hole to
the left hand edge of the speculum, that is, to the point _a_. In one
of these instruments fitted up with a four feet speculum, the line
of vision is directed to the point _b_ on the opposite side of the
speculum, but, in this case, the eye-tube is removed farther from the
arm, than in the former case. The hole in the centre of the speculum is
obviously a defect in this construction of a reflecting telescope, as
it prevents us from obtaining the full advantage of the rays which fall
near the centre of the mirror; yet the performance of the instruments,
even with this disadvantage, is superior to what we should previously
have been led to expect.
The principal nicety in the construction of this instrument, consists
in the adjustment and proper direction of the eye-tube. There is only
one position in which vision will be perfectly distinct. It must be
neither too high nor too low,--it must be fixed at a certain distance
from the arm,--and must be directed to a certain point of the speculum.
This position must be ultimately determined by experiment, when viewing
terrestrial objects. A person unacquainted with this construction
of the telescope, would, perhaps, find it difficult, in the first
instance, to make this adjustment; but were it at any time deranged,
through accident or otherwise, I can easily make the adjustment anew,
in the course of a minute or two.
In pointing this telescope to the object intended to be viewed, the
eye is applied at K, fig, 69, and looking along the arm, towards the
eye-piece, till it nearly coincide with the object, it will, in most
cases, be readily found. In this way I can easily point this instrument
to Jupiter or Saturn, or to any of the other planets, visible to the
naked eye, even when a power of 160 or 170 times is applied. When high
magnifying powers, however, are used, it may be expedient to fix,
on the upper part of the short tube in which the speculum rests, a
Finder, such as that which is used in Newtonian telescopes. When the
moon is the object intended to be viewed, she may be instantly found by
moving the instrument till her reflected image be seen from the eye-end
of the telescope on the face of the mirror.
I have fitted up several instruments of the above description with
specula of 16, 27, 35, and 49 inches focal distance. One of these
having a speculum of 27 inches focal length, and an astronomical
eye-piece, producing a magnifying power of about 90 times, serves
as a good astronomical telescope. By this instrument the belts
and satellites of Jupiter, the ring of Saturn, and the mountains
and cavities of the moon, may be contemplated with great ease and
distinctness. With a magnifying power of 35 or 40 times, terrestrial
objects appear remarkably bright and well-defined. When compared with
a Gregorian, the quantity of light upon the object appears nearly
doubled, and the image is equally distinct--although the speculum has
several blemishes, and its surface is but imperfectly polished. It
represents objects in their natural colours, without that dingy and
yellowish tinge which appears when looking through a Gregorian. Another
of these instruments is about four feet long. The speculum which
belongs to it is a very old one: when it came into my possession, it
was so completely tarnished, as scarcely to reflect a ray of light.
After it was cleaned, it appeared to be scarcely half polished, and
its surface is covered with yellowish stains which cannot be erased.
Were it fitted up upon the Gregorian plan, it would, I presume, be
of very little use, unless when a very small magnifying power was
applied. Yet, in its present form, it bears, with distinctness, a
magnifying power of 130 times, and is equal in its performance to a
3-1/2 feet achromatic. It exhibits distinct and interesting views of
the diversities of shade, and of the mountains, vales, cavities, and
other inequalities of the moon’s surface. With a power of about 50
times, and a terrestrial eye-piece, it forms an excellent telescope
for land objects, and exhibits them in a brilliant and novel aspect.
The smallest instrument I have attempted to construct on this plan,
is only 5-1/2 inches focal distance, and 1-3/4 inch diameter. With a
magnifying power of about 15 times, it shows terrestrial objects with
distinctness and brilliancy. But I should deem it inexpedient to fit
up any instrument of this description with specula of a shorter focal
distance than 20 or 24 inches. The longer the focal distance the more
distinctness may be expected, although the aperture of the speculum
should be comparatively small.
The following are some of the properties and advantages peculiar to
this construction of the reflecting telescope.
1. It is _extremely simple_, and may be fitted up at a comparatively
_small expense_. Instead of large and expensive brass tubes, such as
are used in the Gregorian and Newtonian construction, little more is
required than a short mahogany tube, two or three inches long, to
serve as a socket for the speculum, with an arm connected with it
about the focal length of the speculum. The expense of small specula,
either plain or concave, is saved, together with the numerous screws,
springs, &c., for centering the two specula, and placing the small
mirror parallel to the large one. The only adjustment requisite in this
construction, is that of the eye-tube to the speculum; and, by means of
the simple apparatus above described, it can be effected in the course
of a few minutes. Almost the whole expense of the instrument consists
in the price of the speculum and the eye-pieces. The expense of fitting
up the four feet speculum, alluded to above--_exclusive of speculum and
eye-piece_--but including mahogany tube and arm, brass sockets, screws,
eye-tube, brass joint, and a cast-iron stand painted and varnished,
did not amount to £1 : 8_s_. A Gregorian of the same size would have
required a brass tube at least 4-1/2 feet in length, which would cost 5
or 6 guineas, besides the apparatus connected with the small speculum,
and the additional expense connected with the fitting up of the joint
and stand requisite for supporting and steadying so unwieldy an
instrument. While the one instrument would require two persons to carry
it from one room to another, and would occupy a considerable space in
an ordinary apartment, the other can be moved, with the utmost ease,
with one hand, to any moderate distance, and the space it occupies is
extremely small.
2. _It is more convenient for viewing celestial objects at a high
altitude, than other telescopes._ When we look through a Gregorian
reflector or an achromatic telescope of 4 or 5 feet in length, to an
object elevated 50 or 60 degrees above the horizon, the body requires
to be placed in an uneasy and distorted position, and the eye is
somewhat strained, while the observation is continued. But when
viewing similar objects by the _Aerial Reflector_, we can either stand
perfectly erect, or sit on a chair, with the same ease as we sit at a
desk when reading a book or writing a letter. In this way, the surface
of the moon or any of the planets, may be contemplated for an hour or
two, without the least weariness or fatigue. A delineation of the
lunar surface may be taken with this instrument with more ease and
accuracy than with any other instrument, as the observer can sketch
the outline of the object by one eye on a tablet placed a little below
the eye-piece, while the other eye is looking at the object. For the
purpose of accommodating the instrument to a sitting or standing
posture a small table was constructed, capable of being elevated or
depressed at pleasure, on which the stand of the telescope is placed.
When the telescope is 4 or 5 feet long, and the object at a very high
elevation, the instrument may be placed on the floor of the apartment,
and the observer will stand in an erect position.
3. This instrument is considerably _shorter_ than a Gregorian telescope
whose mirror is of the same focal length. When an astronomical
eye-piece is used, the whole length of the instrument is nothing more
than the focal length of the speculum. But a Gregorian whose large
speculum is 4 feet focus, will be nearly 5 feet in length, including
the eye-piece.
4. The Aerial Reflector far excels the Gregorian in brightness. The
deficiency of light in the Gregorians is owing to the second reflection
from the small mirror; for it has been proved by experiment that nearly
the one half of the rays of light which fall upon a reflecting surface
is lost by a second reflection. The image of the object may also be
presumed to be more correct, as it is not liable to any distortion by
being reflected from another speculum.
5. There is _less tremor_ in these telescopes than in Gregorian
Reflectors. One cause, among others, of the tremors complained of in
Gregorians is, I presume, the formation of a second image at a great
distance from the first, besides that which arises from the elastic
tremor of the small speculum, when carried by an arm supported only
at one end. But as the image formed by the speculum in the aerial
telescope is viewed _directly_, without being exposed to any subsequent
reflection, it is not so liable to the tremors which are so frequently
experienced in other reflectors. Notwithstanding the length of the arm
of the 4 feet telescope above mentioned, a celestial object appears
remarkably steady, when passing across the field of view, especially
when it is at a moderate degree of altitude; and it is easily kept in
the field by a gentle motion applied to the arm of the instrument.
In prosecuting my experiments in relation to these instruments, I
wished to ascertain what effect might be produced by using a _part of
a speculum_ instead of the whole. For this purpose, I cut a speculum,
three feet in focal length, through the centre, so as to divide it
into two equal parts, and fitted up each part as a distinct telescope;
so that I obtained two telescopes from one speculum. In this case I
found that each half of the speculum performed nearly as well as the
whole speculum had done before, at least there appeared to be _no
very sensible_ diminution in the _brightness_ of the object, when
viewed with a moderate power, and the image was equally accurate and
distinct; so that if _economy_ were a particular object aimed at in
the construction of these instruments, two good telescopes might be
obtained from one speculum; or if a speculum happened to be broken
accidentally into large fragments, one or more of the fragments might
be fitted up on this principle to serve as a tolerably good telescope.