is demonstrable, that _a tube is not necessary_ in the construction of
a reflecting telescope--at least on the principle now stated--whether
it be used by day or by night for terrestrial or celestial objects;
for I have frequently used these telescopes in the open air in the day
time, without any inconvenience from extraneous light. Therefore, were
a reflecting telescope of 50 or 60 feet in length to be constructed, it
might be fitted up at a comparatively small expence, after the expense
of the metallic substances, and of casting, grinding, and polishing
the speculum is defrayed. The largest instrument of this description
which has hitherto been constructed is the 40 feet reflector of Sir
W. Herschel. This complicated and most unwieldy instrument had a tube
of rolled or sheet iron 39 feet 4 inches in length, about 15 feet in
circumference, and weighed about 8000 pounds. Now, I conceive that
such enormous tubes, in instruments of such dimensions, are altogether
unnecessary. Nothing more is requisite than a short tube for holding
the speculum. Connected with one side of this tube (or with both sides
were it found necessary), two strong bars of wood, projecting a few
feet beyond the speculum end, and extending in front as far as the
focal length of the mirror, and connected by cross bars of wood, iron
or brass--would be quite sufficient for a support to the eye-piece, and
for directing the motion of the instrument. A telescope of 40 or 50
feet in length, constructed on this plan, would not require one fifth
of the expense, nor one fourth of the apparatus and mechanical power
for moving it to any required position, which were found necessary
in the construction of Sir W. Herschel’s large reflecting telescope.
The idea here suggested will perhaps be more readily appreciated by
an inspection of fig. 71, where A is the short tube, BC and DE the
two large bars or arms, connected with cross bars, for the purpose of
securing strength and steadiness. At I and K, behind the speculum,
weights might be applied, if necessary, for counterbalancing the lever
power of the long arm. F represents the position of the eye-piece,
and GH the joint and part of the pedestal on which the instrument is
placed. With regard to telescopes of smaller dimensions, as from 5
to 15 feet in focal length--with the exception of the expense of the
specula and eye-pieces--they might be fitted up for a sum not greater
than from 3 to 10 or 15 guineas.
[Illustration: _figure 71._]
Were any person to attempt the construction of those telescopes, it
is possible he might not succeed in his first attempts without more
minute directions than I have yet given. The following directions
may perhaps tend to guide the experimenter in adjusting the eye-tube
to the speculum, which is a point that requires to be particularly
attended to, and on which depends the accurate performance of the
instrument. After having fixed the eye-piece nearly in the position
it should occupy, and directed the instrument to a particular object,
look along the arm of the telescope, from K (fig. 69.) to the extremity
of the eye-piece at H, and observe, whether it nearly coincides with
the object. If the object appear lower than this line of vision, the
eye-piece must be lowered, and if higher, it must be raised, by means
of the nuts and screws at _gd_ and _fe_, till the object and the line
of vision now stated nearly coincide. The eye-piece should be directed
as nearly perpendicular to the front of the speculum as possible, but
so that the reflected image of one’s head from the mirror shall not
interfere to obstruct the rays from the object. An object may be seen
with an approximate degree of distinctness, but not accurately, unless
this adjustment be pretty accurately made. The astronomical eye-pieces
used for these telescopes are fitted with a brass cap which slides on
the end next the eye, and is capable of being brought nearer to or
farther from the first eye-glass. In the centre of this cap, next the
eye, is a small hole, about the 1/40th or 1/50th of an inch diameter,
or about as wide as to admit the point of a pin or a moderate-sized
needle. The distance of this hole from the lens next the eye must be
adjusted by trial, till the whole field of view appear distinct. A
common astronomical eye-piece, without this addition, does not answer
well. I find by experience, that terrestrial eye-pieces, such as those
used in good achromatic telescopes, are, on the whole, best adapted to
this construction of a reflecting telescope.
I have sometimes used these instruments for the purpose of viewing
perspective prints, which they exhibit in a beautiful and interesting
manner. If a coloured perspective be placed at one end of a large
room or gallery, and strongly illuminated either by the sun or by two
candles, and one of the reflectors furnished with a _small magnifying
power_, placed at the opposite end of the room--the representation of
a street or a landscape will be seen in its true perspective, and will
appear even more pleasant and interesting than when viewed through
the common _optical diagonal machine_. If an inverting eye-piece be
used--which is most eligible in this experiment--the print, of course,
must be placed in an inverted position.
That reflecting telescopes of the descriptions now stated are original
in their construction, appears from the uniform language of optical
writers, some of whom have pronounced such attempts to be altogether
impracticable. Sir David Brewster, one of the latest and most
respectable writers on this subject, in the ‘Edinburgh Encyclopedia’
art _optics_, and in the last edition of his _appendix_ to ‘Ferguson’s
Lectures,’ has the following remarks:--‘If we could dispense with
the use of the small specula in telescopes of moderate length, by
inclining the great speculum, and using an oblique, and consequently
a _distorted_ reflection, as proposed first by La Maire, we should
consider the Newtonian telescope as perfect; and on a large scale, or
when the instrument exceeds 20 feet, it has undoubtedly this character,
as nothing can be more simple than to magnify, by a single eye-glass,
the image formed by a single speculum. As the _front view is quite
impracticable_, and indeed _has never been attempted_ in instruments
of a small size, it becomes of great practicable consequence to remove
as much as possible, the evils which arise from the use of a small
speculum,’ &c.
The instruments now described have effectuated, in some degree, the
desirable object alluded to by this distinguished philosopher, and
the mode of construction is neither that of Sir W. Herschel’s front
view, nor does it coincide with that proposed by La Maire, which
appears to have been a mere hint that was never realized in the
construction of reflecting telescopes of a small size. The simplicity
of the construction of these instruments, and the excellence of their
performance, have been much admired by several scientific gentlemen and
others to whom they have been exhibited. Prior to the description of
them in the Edin. Philos. Journal, they were exhibited in the Calton
Hill Observatory, Edinburgh, in the presence of Professor Wallace,
and another gentleman, who compared their performance with that of an
excellent Gregorian. As this instrument is distinguished from every
other telescope, in being used without a tube, it has been denominated
‘_The aerial reflector_.’
SECT. 4.--EARL OF ROSSE’S REFLECTING TELESCOPES.
This nobleman, unlike many of his compeers, has, for a considerable
number of years past, devoted his attention to the pursuits of science,
and particularly to the improvement of reflecting telescopes. He is
evidently possessed of high mathematical attainments, combined with
an uncommon degree of mechanical ingenuity. About 14 or 15 years ago,
he engaged in various experiments with the view of counteracting the
effects of the spherical aberration of the specula of reflecting
telescopes--which imperfection, if it could be completely remedied,
would render the reflecting telescope almost a perfect instrument,
as it is not affected by the different refrangibility of the rays of
light. His method, we believe, consisted in forming a large speculum of
two or three separate pieces of metal, which were afterwards accurately
combined into one--a central part which was surrounded by one or two
rings ground on the same tool. When the images formed by the separate
pieces, were made exactly to coincide, the image of the object towards
which the whole speculum was directed, was then found to be as distinct
as either image had been when separate. But at the period referred to,
a sufficient number of experiments had not been made to determine that
his lordship had completely accomplished the object he intended.
Great interest, however, has of late been excited by the improvements
which his lordship has made in the formation of specula. Sir W.
Herschel never made public the means by which he succeeded in giving
such gigantic developement to the reflecting telescope: and therefore
the construction of a _large_ reflector has been considered as a
perilous adventure. But, according to a report of Dr. Robinson of
Armagh, to the Irish academy, the Earl of Rosse has overcome the
difficulties which have hitherto been met with, and carried to an
extent which even Herschel himself did not venture to contemplate,
the illuminating power of this telescope, along with a sharpness
of definition little inferior to that of the achromatic; and it is
scarcely possible, he observes, to preserve the necessary sobriety of
language in speaking of the moon’s appearance with this instrument,
which Dr. Robinson believes to be the most powerful ever constructed.
The difficulty of constructing large specula, and of imparting to them
the requisite degree of polish, has hitherto been considered so great,
that from 8 to 12 inches diameter has been in general their utmost
size. Indeed, except with the greatest reluctance, London opticians
would not accept of orders for specula of more than 9 inches in
diameter. It appears, however, that the Earl of Rosse has succeeded,
by a peculiar method of moulding, in casting object-mirrors of true
_speculum metal of three feet in diameter, and of a weight exceeding
17 cwt_. He is about to construct a telescope, the speculum of which
is _six feet_ in diameter, _fifty feet focal distance_, and of the
weight of _four tons_; and from what he has already accomplished, it
is not doubted that he possesses the power to carry his design into
effect. These great masses of metal, which, in the hands of all other
makers of specula would have been as untractable as so much unannealed
flint-glass, the Earl of Rosse has further succeeded in bringing to
the highest degree of polish, and the utmost perfection of curvature
by means of machinery. The process is conducted under water, by which
means those variations of temperature, so fatal to the finest specula
hitherto attempted, are effectually guarded against. To convince Dr.
Robinson of the efficacy of this machinery, the earl took the three
feet speculum out of its telescope, destroyed its polished surface,
and placed it under the mechanical polisher. In six hours it was
taken out with a perfect new surface as bright as the original. Under
the old system of hand-polishing, it might have required months,
and even years, to effect this restoration. Even before achieving
these extraordinary triumphs on the solid substance, his lordship
had constructed a six feet reflector by covering a curved surface of
brass with squares of the true _speculum metal_, which gave an immense
quantity of light, though subject to some irregularities, arising
from the number of joinings necessary in such a mosaic work. Of the
performance of his lordship’s great telescope, mounted with this
reflector, those who have seen it speak in terms of high admiration;
but in reference to the smaller and more perfect instrument, furnished
with the solid three feet speculum, the language of the Armagh
astronomer assumes a tone of enthusiasm and even of sublimity. By means
of this exquisite instrument, Dr. Robinson and Sir J. South, in the
intervals of a rather unfavourable night, saw several new stars, and
corrected numerous errors of other observers. For example, the planet
Uranus, supposed to possess a ring similar to that of Saturn, was found
not to have any such appendage; and those nebulæ, hitherto regarded,
from their apparently circular outline, as ‘coalescing systems,’
appeared, when tested by the three feet speculum, to be very far
indeed from presenting a globular appearance; numerous off-shoots and
appendages, invisible by other telescopes, appearing in all directions
radiating from their edges. Such discoveries, which reflect great
honour on the Earl of Rosse, will doubtless have great effect on the
interests of astronomical science.[27]
SECT. 5.--REFLECTING TELESCOPES WITH GLASS SPECULA.
After making a variety of experiments with aerial telescopes
constructed of metallic specula of different focal lengths, I
constructed a telescope on the same plan, with a concave glass mirror.
Having obtained a fragment of a very large convex mirror which happened
accidentally to have been broken, I caused the convex side to be
foliated, or silverised, and found its focal length to be about 27
inches. This mirror, which was about 5 inches diameter, I placed in one
of the aerial reflectors, instead of the metallic speculum, and tried
its effects with different terrestrial eye-pieces. With a power of
about 35 or 40 times, it gave a beautiful and splendid view of distant
terrestrial objects--the quantity of light reflected from them, being
considerably greater than when a metallic speculum was used, and they
appeared on the whole well-defined. The only imperfection--as I had
foreseen--consisted in a double image being formed of objects which
were remarkably bright and white, such as a light-house whitened on
the outside, and strongly illuminated by the sun. One of the images
was bright and the other faint. This was obviously owing to the two
reflections from the two surfaces of the mirror--one from the convex
silverised side, and the other from the concave side next the eye,
which produced the faint image--which circumstance has been generally
considered as a sufficient reason for rejecting the use of glass
specula in telescopes. But although very bright objects exhibited
a double image, almost all the other objects in the terrestrial
landscape appeared quite distinct and without any secondary image, so
that a common observer could scarcely have noticed any imperfection.
When the instrument, however, was directed to celestial objects, the
secondary image was somewhat vivid, so that every object appeared
double. Jupiter appeared with two bodies, at a little distance from
each other, and his four satellites appeared increased to eight.
The moon likewise appeared as a double orb, but the principal image
was distinct and well-defined. Such a telescope, therefore, was not
well-adapted for celestial observations, but might answer well enough
for viewing terrestrial objects.
Considering that the injurious effects of the secondary image arose
from the images reflected from the two surfaces being formed near the
same point, and at nearly the same focal distance, I formed a plan for
destroying the secondary image, or at least counteracting its effects,
by forming the concavity of the mirror next the eye of a portion of a
sphere _different_ from that of the convex side which was silverised,
and from which the principal image is formed. But, for a long time, I
could find no opticians possessed of tools of a sufficient length of
radii for accomplishing my design. At length a London working optician
undertook to finish a glass speculum, according to my directions,
which were, that the convex surface of the mirror should be ground on
a tool which would produce a focal distance by reflection of about 4
feet; and that the concave surface should have its focal distance at
about 3 feet 3 inches, so that the secondary image might be formed
at about 9 inches, within the focal distance of the silverised side,
and not interfere to disturb the principal image. But, either from
ignorance or inattention, the artist mistook the radius for the half
radius of concavity, and the speculum turned out to be only 23 inches
focal distance by reflection. This mirror was fitted up as a telescope,
on the aerial plan, and I found, as I expected, the secondary image
completely destroyed. It produced a very beautiful and brilliant view
of land objects, and even the brightest objects exhibited no double
image. The mirror was nearly 5 inches in diameter, but the image was
most accurately defined when the aperture was contracted to about 3
inches. It was fitted with a terrestrial eye-piece which produced a
magnifying power of about 25 times. When directed to the moon, it
gave a very distinct and luminous view of that orb, without the least
appearance of a secondary image. But as the focal distance of the
speculum was scarcely half the length I had prescribed, I did not
apply to it any high astronomical powers; as I find, that these can
only be applied with effect, in this construction, to a speculum of a
considerable focal length. Happening to have at hand a convex lens 10
feet focal length, and 4 inches in diameter--the one side of which had
been ground to a certain degree of concavity--I caused the convex side
to be foliated, which produced a focus by reflection, at 13-1/2 inches
distant. To this mirror I applied terrestrial powers of 15 and 24, with
considerable distinctness. The power of 15 produced a very brilliant
and distinct view of land objects. Had the mirror been at least 3 times
the focal length, it would have formed an excellent telescope, with the
same aperture.
SECT. 6.--A REFLECTING TELESCOPE, WITH A SINGLE MIRROR AND NO EYE-PIECE.
[Illustration: _figure 72._]
On the same principle as that by which a refracting telescope may be
constructed by means of a single lens--as represented fig. 51, (page
234) we may form a telescope by reflection with a single mirror, and
without an eye-piece. Let AB, fig. 72, represent a large concave
speculum, and C its focus--if an eye be placed at D, about 8 or 10
inches within the focal point C, all the objects in the direction of C,
or behind the spectator, will be seen magnified by reflection on the
face of the mirror, and strongly illuminated. The magnifying power, in
this case, will be nearly in the proportion of the focal length of the
mirror to the focal length of the eye for near objects. If for example,
the focal distance of the mirror be 8 feet, and the distance from the
eye at which we see near objects most distinctly, be 8 inches--the
magnifying power will be in the ratio of 8 to 96, or 12 times. I have
a glass mirror of this description, whose focal length is 4 feet 8
inches, and diameter 6 inches, which magnifies distant objects about 7
times, takes in a large field of view, and exhibits objects with great
brilliancy. It presents a very distinct picture of the moon, showing
the different streaks of light and shade upon her surface; and, in some
cases, shows the larger spots which traverse the solar disc. This mode
of viewing objects is extremely easy and pleasant, especially when the
mirror is of a large diameter; and the observer is at first struck and
gratified with the novel aspect in which the objects appear.
Were a concave mirror of this description--whether of glass or of
speculum metal--to be formed to a very long focus, the magnifying
power would be considerable. One of 50 feet focal length, and of a
corresponding diameter, might produce a magnifying power, to certain
eyes, of about 75 times; and, from the quantity of light with which the
object would be seen, its effect would be much greater than the same
power applied to a common telescope. Sir W. Herschel states, that, on
one occasion, by looking with his naked eye on the speculum of his 40
feet Reflector, without the interposition of any lens or mirror, he
perceived distinctly one of the satellites of Saturn, which requires
the application of a considerable power to be seen by an ordinary
telescope. Such an instrument is one of the most simple forms of a
telescope, and would exhibit a brilliant and interesting view of the
moon, or of terrestrial objects.
PRICES OF REFLECTING TELESCOPES.
1. Prices as stated by Messrs. W. and S. Jones, Holborn, London.
£ s.
A 4 feet, 7 inch aperture Gregorian reflector, with the
vertical motions upon a new invented principle, as well as
apparatus to render the tube more steady for observation,
according to the additional apparatus of small speculums,
eye-pieces, micrometers, &c. from 80_l._ to 120 0
Three feet long, mounted on a plain brass stand 23 2
Ditto with rack-work motions, improved mountings and
metals 39 18
Two feet long without rack-work, and with 4 magnifying
powers, improved 15 15
Ditto improved, with rack-work motions 22 1
Eighteen inch, on a plain stand 9 9
Twelve inch ditto 6 6
2. Prices as stated by Messrs. Tulley, Islington.
£ 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 a mahogany box 11 11
2 feet ditto, metal 4 inches diameter 16 16
Ditto with rack-work motions 25 4
3 feet ditto, metal 5 inches diameter, rack-work motions 42 0
4 feet ditto, metal 7 inches diameter, on a tripod stand
with centre of gravity motion 105 0
6 feet ditto, metal 9 inches diameter 210 0
7 feet _Newtonian_, 6 inches aperture 105 0
12 feet ditto, metal 12 inches diameter 525 0
3. Prices stated by Mr. G. Dollond, St. Paul’s Church Yard.
£ s.
Reflecting telescopes 14 inches long, in a mahogany box 9 9
Ditto, 18 inches 12 12
Ditto 2 feet 18 18
Ditto with 4 different powers, and rack-work stand
supporting the telescope in the centre of gravity 36 15
Ditto 3 feet, with ditto 50 0
4. Prices of single speculums and reflecting telescopes, as made by Mr.
Grub, Charlemont Bridge works, Dublin.
-----------------------------------------+-+----------------------------------------
NEWTONIAN TELESCOPES. | | GREGORIAN REFLECTORS.
--------+---------+----------+-----------+ +--------+---------+----------+----------
Diameter| Focal | Price of | Price of | |Diameter| Focal | Price of | Price of
in | length | Mirrors | telescope | |in | length | Mirrors | telescope
inches. | in feet.| alone. | complete | |inches. | in feet.| alone. | complete
| | | without | | | | | without
| | | stand. | | | | | stand.
--------+---------+----------+-----------+ +--------+---------+----------+----------
| | £ s. | £ s. | | | | £ s. | £ s.
| | | | | 6 | 3 | 17 10 | 25 0
| | | | | | | |
7 | 7 | 17 10 | 27 10 | | 7 | 3 | 25 0 | 34 0
| | | | | | | |
9 | 10 | 25 0 | 40 0 | | 9 | 4-1/2 | 35 0 | 50 0
| | | | | | | |
12 | 12 | 60 0 | 90 0 | | 12 | 7 | 70 0 | 100 0
| | | | | | | |
15 | 15 | 120 0 | 170 0 | | 15 | 9 | 150 0 | 200 0
| | | | | | | |
18 | 18 | 200 0 | 260 0 | | 18 | 12 | 240 0 | 300 0
ON THE EYE-PIECES OF TELESCOPES.
Although the performance of telescopes chiefly depends on the goodness
of the object-glass, or the object-speculum of the instrument, yet it
is of considerable importance, in order to distinct vision, and to
obtain a large and uniformly distinct field of view, that the eye-piece
be properly constructed. The different kinds of eye-pieces may be
arranged into two general divisions--_Astronomical_ and _terrestrial_.
1. _Astronomical eye-pieces._--The most simple astronomical eye-piece
is that which consists of a single convex lens; and when the focal
distance of this lens, and that of the object-glass of the instrument
is accurately ascertained, the magnifying power may be nicely
determined, by dividing the focal length of the object-lens by that
of the eye-glass. But, as the pencil of white light transmitted by
the object-glass, will be divided by the eye-glass into its component
colours, the object will appear bordered with coloured fringes, and the
distinctness of vision consequently injured. Besides, the spherical
aberration, when a single lens is used, is much greater than when two
or more glasses are employed. Hence astronomical eye-pieces are now
formed by a combination of at least two lenses.
[Illustration: _figure 73._]
The combination of lenses now generally used for astronomical purposes,
is that which is usually denominated the _Huygenian eye-piece_, having
been first proposed by the celebrated Huygens, as a great improvement
on the single lens eye-piece. The following figure (73) represents a
section of this eye-piece. Let AB be a compounded pencil of white light
proceeding from the object-glass; BF a plano-convex field-glass, with
its plane side next the eye-glass E. The red rays of the pencil AB,
after refraction would cross the axis in R, and the violet rays in V,
but meeting the eye-glass E, the red rays will be refracted to O, and
the violet nearly in the same direction, when they will cross each
other about the point O, in the axis, and unite. The distance of the
two glasses FE, to produce this correction, when made of crown glass,
must be equal to half the sum of their focal distances nearly. For
example, suppose the focal distance of the largest, or field lens, to
be 3 inches, and the focal distance of the lens next the eye, 1 inch,
the two lenses should be placed exactly at the distance of 2 inches;
the sum of their focal length being 4, the half of which is 2. In other
words, the glass next the eye should be placed as much _within_ the
focus of the field-glass as is equal to its own focal distance. The
focal length of a single lens, that has the same magnifying power as
this compound eye-piece--is equal to twice the product of the focal
lengths of the two lenses, divided by the sum of the same numbers.
Or, it is equal to half the focal length of the field-glass. Thus, in
reference to the preceding example, twice the product of the focal
length of the two lenses--is equal to 6, and their sum is 4. The former
number divided by the latter, produces a quotient of 1-1/2, which
is the focal length of a single lens, which would produce the same
magnifying power as the eye-piece; and 1½ is just half the focal length
of the field-glass. The proportion of the focal lengths of the two
lenses to each other, according to Huygens, should be as 3 to 1; that
is, if the field-glass be 4-1/2 inches, the eye-glass should be 1-1/2;
and this is the proportion most generally adopted. But some opticians
have recommended that the proportions should be as 3 to 2. Boscovich
recommended two similar lenses; and in this case the distance between
them was equal to half the sum of their focal distances, as in the
Huygenian eye-piece.
The image is formed at IM, at the focal distance of the lens next the
eye, and at the same distance from the field-glass. When distinct
vision is the principal object of an achromatic telescope, the two
lenses are usually both plano-convex, and fixed with their curved
faces towards the object glass, as in the figure. Sometimes, however,
they consist of what is called _crossed_ lenses, that is lenses ground
on one side to a short focus, and on the other side to a pretty
long focus, the sides with the deepest curves being turned towards
the object glass. A diaphragm, or aperture of a proper diameter, is
placed at the focus of the eye lens, where the image formed by the
object-glass falls, for the purpose of cutting off the extreme rays of
the field lens, and rendering every part of the field of view equally
distinct. This is likewise the form of the eye-piece generally applied
to Gregorian reflectors. In short, when accurately constructed, it is
applicable to telescopes of every description. This eye-piece, having
the image viewed, by the eye behind the inner lens, is generally called
the _negative_ eye-piece, and is that which the optical-instrument
makers usually supply, of three or four different sizes, for so many
magnifying powers, to be applied to different celestial objects,
according to their nature or the state of the atmosphere in which they
are used.
_Ramsden’s eye-piece._--There is another modification of lenses,
known by the name of the _Positive_, or Ramsden’s eye-piece, which is
much used in Transit instruments, and telescopes which are furnished
with micrometers, and which affords equally good vision as the other
eye-piece. In this construction the lenses are plano-convex, and
nearly of the same focus, but are placed at a distance from each other
less than the focal distance of the glass next the eye, so that the
image of the object viewed is beyond both the lenses, when measuring
from the eye. The flat faces of the two lenses are turned into
contrary directions in this eye-piece--one facing the object-glass,
and the other the eye of the observer; and as the image formed at
the focus of the object-glass, lies parallel to the flat face of the
contiguous lens, every part of the field of view is distinct at the
same adjustment, or, as opticians say, there is a _flat field_, which,
without a diaphragm, prevents distortion of the object. This eye-piece
is represented in fig. 74, where AB and CD are two plano-convex
lenses, with their convex sides inwards. They have nearly the same
focal length, and are placed at a distance from each other, equal to
about two thirds of the focal length of either. The focal length of
an equivalent single lens is equal to three fourths the focal length
of either lens, supposing them to have equal focal distances. This
eye-piece is generally applied, when wires of spider’s lines are used
in the common focus; as the piece containing the lenses can be taken
out without disturbing the lines, and is adjustable for distinct
vision; and whatever may be the measure of any object given by the
wire micrometer, at the solar focus, it is not altered by a change of
the magnifying power, when a second eye-piece of this construction is
substituted.
[Illustration: _figure 74._]
[Illustration: _figure 75._]
[Illustration: _figure 76._]
_Aberration of lenses._--In connection with the above descriptions,
the following statements respecting the spherical aberration of lenses
may not be inappropriate. Mr. John Dollond, in a letter to Mr. Short,
remarks, that ‘the aberration in a single lens is as the cube of the
refracted angle; but if the refraction be caused by two lenses, the sum
of the cubes of each half will be 1/4 of the refracted angle, twice
the cube of 1 being 1/4 the cube of 2. So three times the cube of 1 is
only _one ninth_ of the cube of 3.’ &c. Hence the indistinctness of the
borders of the field of view of a telescope is diminished by increasing
the number of lenses in an eye piece. Sir J. Herschel has shown that if
two plano-convex lenses are put together as in fig. 75, the aberration
will be only 0.2481, or _one fourth_ of that of a single lens in its
best form. The focal length of the first of these lenses, must be to
that of the second as 1 to 2.3. If their focal lengths are equal, the
aberration will be 0.603, or nearly one half. The spherical aberration,
however, may be _entirely_ destroyed by combining a meniscus and double
convex lens, as shown in fig. 76, the convex sides being turned to the
eye when they are used as lenses, and to parallel rays, when they are
used as burning glasses. Sir J. Herschel has computed the following
curvatures for such lenses.
_Focal length of the convex lens_ +10.000
Radius of its first surface +5.833
Radius of its second surface -35.000
_Focal length of the meniscus_ +17.829
Radius of its first surface +3.688
Radius of its second surface +6.294
Focal length of the compound lens +6.407
On the general principles above stated, a good astronomical eye-piece
may be easily constructed with two proper lenses, either according to
the plan of Huygens or that of Ramsden; and, from what has been now
stated it is demonstrably certain, that, in all cases where two glasses
are properly combined, such an eye-piece is superior to a single lens,
both in point of distinctness, and of the enlargement of the field of
view. I lately fitted up an eye-piece, on Ramsden’s principle, with
two lenses, each about 3 inches focal length, and 1-3/8 inch diameter,
placed at half an inch distant, with their convex surfaces facing
each other as in fig. 74, which forms an excellent eye-piece for an
achromatic telescope, 6 feet 8 inches focal distance, and 4 inches
aperture, particularly for viewing clusters of stars, the Milky Way,
and the large nebulæ. The field of view is large, the magnifying power
is only between 50 and 60 times, and the quantity of light being so
great, every celestial object appears with great brilliancy, and it is
in general much preferable, when applied to the stars than any of the
higher powers. When applied to _Presepe_ in Cancer, it exhibits that
group at one view, as consisting of nearly a 100 stars which exhibit a
beautiful and most striking appearance.
It may appear a curious circumstance that any eye-piece which is good
with a short telescope, is also good with a long one, but that the
reverse is not true; for it is found to be more difficult to make
a good eye-piece for a short than for a long focal distance of the
object-glass.
Celestial eye-pieces are sometimes constructed so as to produce
_variable powers_. This is effected by giving a motion to the lens next
the eye, so as to remove it nearer to or farther from the field lens;
for at every different distance at which it is placed from the other
lens, the magnifying power will either be increased or diminished. The
greatest power is when the two lenses are nearly in contact, and the
power diminishes in proportion to the distance at which the glass next
the eye is removed from the other. The scale of distance, however,
between the two lenses, cannot be greater than the focal distance of
the field, or inner glass; for if it were, the lenses would no longer
form an eye-piece, but would be changed into an inverting opera-glass.
For effecting the purpose now stated, the eye-glass is fixed in a
tube which slides upon an interior tube on which is marked a scale of
distances, corresponding to certain magnifying powers; and, in this way
an eye-piece may be made to magnify about double the number of times,
when the lenses are in one position than when they are in another--as,
for example, all the powers from 36 to 72 times may be thus applied,
merely by regulating the distance between the two lenses. When the
glasses are varied in this manner the eye-piece becomes sometimes a
_positive_ eye-piece, like Ramsden’s, and sometimes a negative one like
that of Huygens.
_Diagonal eye-pieces._ The eye-pieces to which we have now adverted,
when adapted to refracting telescopes, both reverse and invert the
object, and therefore are not calculated for showing terrestrial
objects in their natural position. But as the heavenly bodies are of a
spherical form, this circumstance detracts nothing from their utility.
When the celestial object, however, is at a high altitude, the observer
is obliged to place his head in a very inconvenient position, and to
direct his eye nearly upwards; in which position he cannot remain long
at ease, or observe with a steady eye. To remedy this inconvenience,
the diagonal eye-piece has been invented, which admits of the eye
being applied at the side--or at the upper part of the eye-piece,
instead of the end; and when such an eye-piece is used, it is of no
importance in what direction the telescope is elevated, as the observer
can then either sit or stand erect, and look down upon the object with
the utmost ease. This object is effected by placing a flat piece of
polished speculum-metal at an angle of 45 degrees in respect to the two
lenses of the eye-piece, which alters the direction of the converging
rays, and forms an image which becomes erect with respect to altitude,
but is reversed with respect to azimuth;--that is, in other words,
when we look down upon the objects in the field of view, they appear
erect; but that part of an object which is in reality on our right hand
appears on our left; and if it be in motion, its _apparent_ is opposite
to its _real_ motion; if it be moving towards the west, it will seem to
move towards the east.
There are three situations in which the diagonal reflector in this
eye-piece may be placed. It may be placed either 1. before the
eye-piece,--or 2. behind it,--or 3. between the two lenses of which
the eye-piece consists. The most common position of the reflector is
between the lenses; and this may be done both in the negative and the
positive eye-pieces; but as the distance between the two lenses is
necessarily considerable, to make room for the diagonal position of the
reflector, the magnifying power cannot be great; otherwise, a diagonal
eye-piece of this construction remains always in adjustment, and is
useful in all cases where a high power is not required. The following
is a description and representation of a diagonal eye-piece of this
kind in my possession.
[Illustration: _figure 77._]
In fig. 77, AB represents the plano-convex lens next the object, which
is about 2 inches in focal length, and 3/4 inch in diameter; CD, a
plain metallic speculum of an oval form, well polished, and placed at
half a right angle to the axis of the tube; and EF another plano-convex
lens, about 1-1/2 inch focal distance. The centre of the speculum is
about 1-1/4 inch from the lens AB, and about 1/2 or 1/3 inch from EF;
so that this eye-piece is a _positive_ one, on the principle proposed
by Ramsden. The rays proceeding from the lens AB, and falling upon the
speculum, are reflected in a perpendicular direction to the lens EF,
where they enter the eye at G, which looks down upon the object through
the side of the tube. The real size of this eye-piece is much about the
same as that represented in the figure. When applied to an achromatic
telescope of 44-1/2 inches focal distance it produces a magnifying
power of 36 times, and exhibits a very beautiful view of the whole
of the full moon. It likewise presents a very pleasing prospect of
terrestrial objects, which appear as if situated immediately below us.
[Illustration: _figure 78._]
Another plan of the diagonal eye-piece is represented in fig. 78, where
the speculum is fixed _within_ the sliding tube which receives the
eye-piece, or immediately below it. The part of the tube at AB slides
into the tube of the telescope, CD is the speculum placed at half a
right angle to the axis of the tube, and EF, the tube containing the
lenses, which stands at right angles to the position of the telescope,
and slides into an exterior tube, and the eye is applied at G. This
construction of the diagonal eye-piece may be used with any eye-piece
whatever, whether the Huygenian or that of Ramsden. It will admit of
any magnifying power, and if several different eye-pieces be fitted
to the sliding tube, they may be changed at pleasure. This form of
the diagonal eye-piece, I therefore consider as the best and the
most convenient construction, although it is not commonly adopted by
opticians.
When any of these eye-pieces are applied to a telescope, with the lens
E on the upper part of it, we look down upon the object, if it be a
terrestrial one, as if it were under our feet. If we turn the eye-piece
round in its socket a quarter of a circle towards the left, an object
directly before us in the south, will appear as if it were in the
_west_ and turned upside down. If, from this position, it is turned
round a semicircle towards the right, and the eye applied, the same
object will appear as if it were situated in the east, and inverted;
and if it be turned round another quadrant, till it be directly
opposite to its first position, and the eye applied from below, the
object or landscape will appear as if suspended in the atmosphere above
us. This eye-piece, therefore, is capable of exhibiting objects in a
great variety of aspects, and the use of it is both pleasant and easy
for the observer. But there is a considerable loss of light, occasioned
by the reflection from the speculum, which is sensibly felt when very
high powers are applied; and therefore when very small stars are to be
observed, such as some of those connected with double or triple stars,
the observer should not study his own ease so much as the quantity of
light he can retain with a high power, which object is best attained
with an ordinary eye-piece and a telescope of large aperture.
We have said that a diagonal eye-piece may be constructed with a
reflector _before_ the eye-piece. In this case, the speculum is
sometimes made to slide before the eye at the requisite angle of
reclination, in which application each eye-piece must necessarily have
a groove to receive it, and the eye must be applied without a hole
to direct it, but it may be put on and taken off without disturbing
the adjustment for distinct vision, and is very simple in its
application. But, on the whole, the form represented in fig. 78, is
the most convenient, and should generally be preferred, as any common
astronomical eye-piece can be applied to it. I have used a diagonal
eye-piece of this kind, with good effect, when a power of 180 has been
applied to the sun and other celestial objects.
Instead of a metallic speculum, a _rectangular prism of glass_
is sometimes substituted; for the rays of light are then bent by
reflection from the second polished surface, which ought to be _dry_,
and undergo two refractions which achromatise them; and the same effect
is thus produced as by polished metal. Ramsden sometimes gave one of
the polished faces of a right angled prism a curve, which prism served
instead of a lens in an eye-piece, and also performed the office of a
reflector. A semi-globe, or what has been called a Bull’s eye, has also
been used as a diagonal eye-piece, and when the curve is well-formed,
and the glass good, it is achromatic, and is said to perform pretty
well, but it is not superior to the forms already described.
SECT. 2.--TERRESTRIAL EYE-PIECES.
When describing the common refracting telescope, (p. 228.) I have
noticed that three eye-glasses, placed at double their focal distances
from each other, formerly constituted the terrestrial eye-piece,
as represented in fig. 47. But this construction, especially for
achromatic instruments, has now become obsolete, and is never used,
except in small pocket spy-glasses formed with a single object lens. In
its place a four glassed eye-piece has been substituted, which is now
universally used in all good telescopes, and which, besides improving
the vision and producing an erect position of the images of objects,
presents a considerably larger field of view. During the progressive
stages of improvement made in the construction of erect eye-pieces by
Dollond and Ramsden, three, four, and five lenses were successively
introduced; and hence, in some of the old telescopes constructed by
these artists, we frequently find five lenses of different descriptions
composing the eye-piece. But four lenses, arranged in the manner I
am now about to describe, have ultimately obtained the preference.
In a telescope having a celestial eye-piece of the Huygenian form,
the image that is formed in the focus of the object glass, is that
which is seen magnified, and in an inverted position; but when a four
glassed eye-piece is used, which produces an erect view of the object,
the image is repeated, and the _second_ image, which is formed by
the inner pair of lenses AB on an enlarged scale, is that which the
pair of lenses CD at the eye-end render visible on a scale still more
enlarged. The modern terrestrial eye-piece, represented in fig. 79,
is, in fact, nothing else than a compound microscope, consisting of an
object lens, an amplifying lens, and an eye-piece composed of a pair
of lenses on the principle of the Huygenian eye-piece. Its properties
will be best understood by considering the first image of an object,
which is formed in the focus of the object glass, as a small luminous
object to be rendered visible, in a magnified state, by a compound
microscope. The object to be magnified may be considered as placed near
the point A, and the magnified image at _i_, which is viewed by the
lens D. Hence, if we look through such an eye-piece at a small object
placed very near the lens A, we shall find that it acts as a compound
microscope of a moderate magnifying power increasing, in some cases,
the diameter of the object about 10 times, and 100 times in surface.
[Illustration: _figure 79._]
In order to distinguish the different lenses in this eye-piece, we may
call the lens A, which is next to the first image, the _object-lens_,
the next to it B, the _amplifying-lens_, the third, or C, the
_field-lens_, and the one next the eye, D, the _eye-lens_. The first
image formed a little before A, may be denominated the _radiant_, or
the object from which the rays proceed. Now, it is well known as a
principle in optics, that if the radiant be brought nearer to the lens
than its principal focus, the emerging rays will _diverge_, and, on
the contrary, if the radiant be put farther from the lens than its
principal focal distance, the emerging rays will _converge_ to a point
at a distance beyond the lens, which will depend on the distance of the
radiant from the first face of the lens. In this place an image of the
radiant will be formed by the concurrence of the converging rays, but
in a contrary position; and the length of the image will exceed the
length of the radiant in the same proportion, as the distance of the
image from the radiant exceeds that of the radiant from the lens. This
secondary image of the radiant at _i_, is not well-defined, when only
one lens, as A, is used, owing to the great spherical aberrations, and
therefore the amplifying lens is placed at the distance of the shorter
conjugate focus, with an intervening diaphragm of a small diameter at
the place of the principal focus; the uses of which lens and diaphragm
are, first to cut off the coloured rays that are occasioned by the
dispersive property of the object lens,--and secondly, to bring the
rays to a shorter conjugate focus for the place of the image, than
would have taken place with a single lens having only one refraction.
As the secondary image is in this way much better defined and free from
colouration, the addition of this second lens is a great improvement to
vision. For this reason I am clearly of opinion, that the object glass
of a compound microscope, instead of consisting of a small single lens,
should be formed of two lenses on the principle now stated, which would
unquestionably add to the distinctness of vision.
With respect to _the proportions of the focal lengths of the lenses_
in this four glass eye-piece, Mr. Coddington states, that if the focal
lengths, reckoning from A to D, fig. 79, be as the numbers 3, 4, 4 and
3, and the distances between them on the same scale, 4, 6, and 5, 2,
the radii, reckoning from the outer surface of A, should be thus:--
A {First surface 27 }nearly plano-convex.
{Second surface 1 }
B {First surface 9 }a miniscus.
{Second surface 4 }
C {First surface 1 }nearly plano-convex.
{Second surface 21 }
D {First surface 1 }double convex.
{Second surface 24 }
Sir D. Brewster states, that a good achromatic eye-piece may be made of
4 lenses, if their focal lengths, reckoning from that next the object,
be as the numbers 14, 21, 27, 32; their distances 23, 44, 40; their
apertures 5.6; 3.4; 13.5; 2.6; and the aperture of the diaphragm placed
in the interior focus of the fourth eye-glass, 7. Another proportion
may be stated:--Suppose the lens next the object A, to be 1-7/8 inch
focal length, then B may be 2-1/2 inches, C 2 inches, and D 1-1/2; and
their distances AB 2-1/2; BC 3-5/8; and CD 2-3/8. In one of Ramsden’s
small telescopes, whose object glass was 8-1/2 inches in focal length,
and its magnifying power 15.4, the focal lengths of the eye glasses
were A 0.775 of an inch, B 1.025, C 1.01, D 0.79;--the distances AB
1.18, BC 1.83, and CD 1.105. In the excellent achromatic telescope
of Dollond’s construction which belonged to the Duc de Chaulnes, the
focal lengths of the eye glasses, beginning with that next the object,
were 14-1/4 lines, 19, 22-3/4, 14; their distances 22.48 lines, 46.17,
21.45, and their thickness at the centre, 1.23 lines, 1.25, 1.47. The
fourth lens was plano-convex, with the plane side to the eye, and the
rest were double convex lenses. This telescope was in focal length 3
feet 5-1/2 inches.
The magnifying power of this eye-piece, as usually made, differs only
in a small degree from what would be produced by using the first or
the fourth glass alone, in which case the magnifying power would be
somewhat greater, but the vision less distinct, and were the lens next
the eye used alone without the field glass, the field of view would
be much contracted. Stops should be placed between the lenses A and B,
near to B, and a larger one between C and D, to prevent any false light
from passing through the lenses to the eye. The more stops that are
introduced into a telescope--which should all be blackened--provided
they do not hinder the pencils of light proceeding from the object, the
better will the instrument perform.
For the information of amateur constructors of telescopes, I shall
here state the dimensions of two or three four glassed eye-pieces in
my possession, which perform with great distinctness, and present a
pretty large field of view. In one of these, adapted to a 44-1/2 inch
achromatic, the lens A, next the object, is 1-7/8 inch, focal length,
and about 1 inch diameter, with the plane side, next the object. The
focal length of the lens B 2-1/10 inches, diameter 7/10 inch, with its
plane side next A; distance of these lenses from each other 2-4/10
inches. Distance of the field lens C from the lens B 5-1/2 inches. The
small hole or diaphragm between A and B is at the focus of A, and is
about 1/6 inch diameter, and about 3/8 of an inch from the lens B. The
field lens C is 2 inches focal length, and 1-1/4 inch diameter, with
its plane side next the eye. The lens next the eye D is 1 inch focal
distance, 1/2 inch diameter, and is distant from the field glass 1-3/4
inch, with its plane side next the eye. The magnifying power of this
eye-piece is equivalent to that of a single lens whose focal length is
half an inch, and with the 44-1/2 inch object glass produces a power of
about 90 times. The lens next the eye can be changed for another 1-3/8
inch focal length, which produces a power of 65; and the two glasses
CD can be changed for another set, of a longer focal distance which
produces a power of 45 times. The whole length of this eye-piece is
11-1/2 inches.
In another eye-piece, adapted to a pocket achromatic, whose object
glass is 9 inches focal length, the lens A is 1 inch focal length,
and 1/2 inch diameter; the lens B 1-1/4 inch, and 1/2 inch diameter,
their distance 1-1/2 inch, the lens C 1-1/10 inch focal length, and
5/8 inch diameter; the eye-lens D 5/8 inch focal length, and 3/8 inch
diameter; distance between C and D 1-1/8 inch. The distance between B
and C 1-3/4 inch. The whole length of this eye-piece is 4-1/2 inches,
and its power is nearly equal to that of a single lens of 1/2 or 6/10
of an inch focal length, the magnifying power of the telescope being
about 16 times. Another eye-piece of much larger dimensions, has the
lens A of 2-1/2 inches focal length, and 3/4 inch diameter: the lens
B 2-3/4 inches focus and 5/8 inch diameter; and their distance 2-3/4
inches; the lens C 2-5/8 inches focus and 1-1/8 inch diameter; the lens
D 1-3/4 inch focus and 3/4 inch diameter; distance from each other
2-3/4 inches. The distance between the lenses B and C is 4 inches. The
magnifying power is equal to that of a single lens 1-1/8 inch focal
distance. When applied to an achromatic object glass 6 feet 7 inches
focal length, it produces a power of about 70 times. This eye-piece has
a moveable tube 9 inches in length in which the two lenses next the
eye are contained, by pulling out which, and consequently increasing
the distance between the lenses B and C, the magnifying power may be
increased to 100, 120 or 140, according to the distance to which this
moveable tube is drawn out. It has also a second and third set of
lenses, corresponding to C and D of a shorter focal distance, which
produce higher magnifying powers on a principle to be afterwards
explained.
_Description of an eye-piece, &c. of an old Dutch Achromatic Telescope._
About twenty or thirty years ago, I purchased, in an optician’s shop
in Edinburgh, a small achromatic telescope, made in Amsterdam, which
was supposed, by the optician, to have been constructed prior to the
invention of achromatic telescopes by Mr. Dollond. It is mounted wholly
of brass, and in all its parts is a piece of beautiful and exquisite
workmanship, and the utmost care seems to have been taken to have all
the glasses and diaphragms accurately adjusted. The object glass is a
double achromatic, 6-1/2 inches focal distance and 1 inch diameter,
but the clear aperture is only 7/8 inch diameter. It is perfectly
achromatic, and would bear a power of 50 times, if it had a sufficient
quantity of light. The following inscription is engraved on the tube
adjacent to the object glass:--“_Jan van Deyl en Zoon Invenit et
Fecit, Amsterdam, Ao. 1769_.” Although Dollond exhibited the principle
of an achromatic telescope, eight or ten years before the date here
specified, yet it is not improbable that the artist whose name is
here stated, may not have heard of Dollond’s invention; and that he
was really, as he assumes, one of the inventors of the achromatic
telescope. For, the invention of this telescope by Dollond was not very
generally known, except among philosophers and the London opticians,
till a number of years after the date above stated. Euler, in his
“Letters to a German Princess”--in which telescopes are particularly
described, makes no mention of, nor the least allusion to the invention
of Dollond, though this was a subject which particularly engaged his
attention. Now, these letters were written in 1762, but were not
published till 1770. When alluding to the defects in telescopes arising
from the different refrangibility of the rays of light, in Letter
43, and that they might possibly be rectified by means of different
transparent substances, he says, ‘But neither theory nor practice have
hitherto been carried to the degree of perfection necessary to the
execution of a structure which should remedy these defects.’ Mr. B.
Martin, in his ‘Gentleman and Lady’s Philosophy,’ published in 1781,
alludes to the achromatic telescope, but speaks of it as it were but
very little, if at all superior to the common refracting telescope. And
therefore, I think it highly probable that Jan van Deyl, was really an
inventor of an achromatic telescope, before he had any notice of what
Dollond and others had done in this way some short time before.
But my principal object in adverting to this telescope, is to describe
the structure of the eye-piece, which is a very fine one, and which
is somewhat different from the achromatic eye-piece above described.
It consists of four glasses, two combined next the eye, and two next
the object. Each of these combinations forms an astronomical eye-piece
nearly similar to the Huygenian. The lens A, next the object, fig. 80,
is 5/8 inch focal distance, and 4/10 inch diameter; the lens B 3/8 inch
focus, and 1/5 inch diameter, and the distance between them somewhat
less than 5/8 inch; the diameter of the aperture _e_ about 1/15 of an
inch. This combination forms an excellent astronomical eye-piece, with
a large flat field, and its magnifying power is equivalent to that of
a single lens 5/8 or 6/8 focal length. The lens C is 1/2 inch focal
length, and 4/10 inch diameter; the lens D 1/4 inch focus, and about
1/5 inch diameter; their distance about 1/2 inch, or a small fraction
more. The hole at _d_ is about 1/20 or 1/25 of an inch diameter, and
the distance between the lenses B and C about 1-1/2 inch. The whole
length of the eye-piece is 3-1/4 inches--exactly the same size as
represented in the engraving. Its magnifying power is equal to that of
a single lens 1/4 inch focal length; and consequently the telescope,
though only 9-1/2 inches long, magnifies 26 times, with great
distinctness, though there is a little deficiency of light when viewing
land objects, which are not well illuminated.
[Illustration: _figure 80._]
The glasses of this telescope are all plano-convex, with their
convex-sides towards the object--except the lens D, which is double
convex, but flattest on the side next the eye, and they are all very
accurately finished. The two lenses C and D form an astronomical
eye-piece nearly similar to that formed by the lenses A and B. The
focus of the telescope is adjusted by a screw, the threads of which are
formed upon the outside of a tube into which the eye-piece slides. The
eye-piece and apparatus connected with it, is screwed into the inside
of the main tube, when not in use, when the instrument forms a compact
brass cylinder 6 inches long, which is enclosed in a fish-skin case,
lined with silk velvet, which opens with hinges.
The lenses in the eye-pieces formerly described, though stated to be
plano-convexes, are for the most part _crossed glasses_, that is
ground on tools of a long focus on the one side, and to a short focus
on the other. The construction of the eye-piece of the Dutch telescope
above described, is one which might be adopted with a good effect
in most of our achromatic telescopes; and I am persuaded, from the
application I have made of it to various telescopes, that it is even
superior, in distinctness and accuracy, and in the _flatness of field_
which it produces to the eye-piece in common use. The two astronomical
eye-pieces of which it consists, when applied to large achromatic
telescopes, perform with great accuracy, and are excellently adapted
for celestial observations.
SECT. 3.--DESCRIPTION OF THE PANCRATIC EYE-TUBE.
From what we have stated, when describing the common terrestrial
eye-piece now applied to achromatic instruments, (p. 349, fig. 79.), it
appears obvious, that any variety of magnifying powers, within certain
limits, may be obtained by removing the set of lenses CD, fig. 79,
nearer to or farther from the tube which contains the lenses A and B,
on the same principle as the magnifying power of a compound microscope
is increased by removing the eye-glasses to a greater distance from
the object-lens. If then, the pair of eye-lenses CD be attached to an
inner tube that will draw out and increase their distance from the
inner pair of lenses, as the tube _a b c d_, the magnifying power may
be indefinitely increased or diminished, by pushing in or drawing out
the sliding tube, and a scale might be placed on this tube, which, if
divided into equal intervals, will be a scale of magnifying powers, by
which the power of the telescope will be seen at every division, when
the lowest power is once determined.
Sir David Brewster, in his ‘Treatise on New Philosophical instruments,’
Book i. chap. vii. page 59, published in 1813, has adverted to this
circumstance, in his description of an ‘Eye-piece wire micrometer,’ and
complains of Mr. Ezekiel Walker, having in the ‘Philosophical Magazine’
for August, 1811, described such an instrument as an invention of his
own. Dr. Kitchener some years afterwards, described what he called a
Pancratic or omnipotent eye-piece, and got one made by Dollond, with a
few modifications different from that suggested by Brewster and Walker,
which were little else than cutting the single tube into several parts,
and giving it the _appearance_ of a new invention. In fact, none of
these gentlemen had a right to claim it as his peculiar invention, as
the principle was known and recognised long before. I had increased the
magnifying powers of telescopes, on the same principle, several years
before any of these gentlemen communicated their views on the subject,
although I never formally constructed a scale of powers. Mr. B. Martin,
who died in 1782, proposed many years before, such a moveable interior
tube as that alluded to, for varying the magnifying power.
In order to give the reader a more specific idea of this contrivance,
I shall present him with a figure and description of one of Dr.
Kitchener’s Pancratic eye-pieces, copied from one lately in my
possession. The following are the exact dimensions of this instrument,
with the focal distances, &c. of the glasses, &c. of which it is
composed.
[Illustration: _fig. 81._]
In. Tenths.
Length of the whole eye-piece, consisting of
four tubes, when fully drawn out, or the distance
from A to B. fig. 81. 14 4
Length of the three tubes on which the
scale is engraved, from the commencement of
the divisions at B to their termination at C. 9 15
Each division into tens is equal to 3-10ths of an inch.
When the three inner tubes are shut up to
C, the length of the eye-piece is exactly 5 5
When these tubes are thus shut up, the magnifying
power for a 3-1/2 feet achromatic is 100
times, which is the smallest power. When the
inner tube is drawn out 1/3 of an inch, or to
the first division, the power is 110, &c.
Focal distance of the lens next the object 1 0
Breadth of Ditto. 0 65
The plane side of this glass is next the object.
Focal distance of the second glass from the
object 1 5
This glass is double and equally convex,
Breadth 0 5
Distance between these two glasses 1 7
Focal distance of the third or field lens,
which is plane on the side next the eye 1 1
Breadth of Ditto. 0 55
Focal distance of the lens next the eye 0 6
Breadth 0 43
This glass is plane on the side next the eye.
Distance between the third and fourth glasses. 1 1
From the figure and description, the reader will be at no loss to
perceive how the magnifying power is ascertained by this eye-piece.
If the lowest power for a 44 inch telescope be found to be 100, when
the three sliding tubes are shut into the larger one, then by drawing
out the tube next the eye 4 divisions, a power of 140 is produced; by
drawing out the tube next the eye its whole length, and the second
tube to the division marked 220, a power of 220 times is produced, and
drawing out all the tubes to their utmost extent, as represented in
the figure, a power of 400 is obtained. These powers are by far too
high for such a telescope, as the powers between 300 and 400 can seldom
or never be used. Were the scale to begin at 50, and terminate at
200, it would be much better adapted to a 3-1/2 feet telescope. Each
alteration of the magnifying power requires a new adjustment of the
eye-piece for distinct vision. As the magnifying power is increased,
the distance between the eye-glass and the object-glass must be
diminished. Dr. Kitchener says, that ‘the pancratic eye tube gives a
better defined image of a fixed star, and shows double stars decidedly
more distinct and perfectly separated than any other eye tube, and
that such tubes will probably enable us to determine the distances
of these objects from each other, in a more perfect manner than has
been possible heretofore.’ These tubes are made by Dollond, London,
and are sold for two guineas each. But I do not think they excel, in
distinctness, those which are occasionally made by Mr. Tulley and other
opticians.