EYE PIECES
The eyepiece of a telescope is merely an instrument for magnifying
the image produced by the objective or mirror. If one looks through a
telescope without its eyepiece, drawing the eye back from the focus to
its ordinary distance of distinct vision, the image is clearly seen as
if suspended in air, or it can be received on a bit of ground glass.
It appears larger or smaller than the object seen by the naked eye, in
proportion as the focal length of the objective is larger or smaller
than the distance to which the eye has to drop back to see the image
clearly.
This real image, the quality of which depends on the exactness of
correction of the objective or mirror, is then to be magnified so much
as may be desirable, by the eyepiece of the instrument. In broad terms,
then, the eyepiece is a simple microscope applied to the image of an
object instead of the object itself.
And looking at the matter in the simplest way the magnifying power of
any simple lens depends on the focal length of that lens compared with
the ordinary seeing distance of the eye. If this be taken at 10 inches
as it often conventionally is, then a lens of 1 inch focus brings clear
vision down to an inch from the object, increases the apparent angle
covered by the object 10 times and hence gives a magnifying power of 10.
But if the objective has a focal length of 100 inches the image, as we
have just seen, is already magnified 10 times as the naked eye sees it,
hence with an objective of 100 inches focus and a 1 inch eyepiece the
total magnification is 100 diameters. And this expresses the general
law, for if we took the normal seeing distance of the naked eye at some
other value than 10 inches, say 12½ inches then we should have to
reckon the image as magnified by 8 times so far as the objective inches
is concerned, but 12½ times due to the 1 inch eyepiece, and so
forth. Thus the magnifying power of any eyepiece is F/f where F is the
focal length of the objective or mirror and f that of the eyepiece.
The focal distance of the eye quite drops out of the reckoning.
All these facts appear very quickly if one explores the image from an
objective with a slip of ground glass and a pocket lens. An ordinary
camera tells the same story. A distant object which covers 1° will
cover on the ground glass 1° reckoned on a radius equal to the focal
length of the lens. If this is equal to the ordinary distance of clear
vision, an eye at the same distance will see the image (or the distant
object) covering the same 1°.
The geometry of the situation is as follows: Let _o_ Fig. 5, Chap.
1, be the objective. This lens, as in an ordinary camera, forms an
inverted image of an object A B at its focus, as at _a b_, and for
any point, as _a_, of the image there is a corresponding point of the
object lying on the straight line from A to that point through the
center, _c_, of the objective.
A pair of rays 1, 2, diverging from the object point A pass through
rim and center of _o_ respectively and meet in A. After crossing at
this point they fall on the eye lens _e_, and if _a_ is nearly in the
principal focus of _e_, the rays 1 and 2 will emerge substantially
parallel so that the eye will unite them to form a clear image.
Now if F is the focal length of _o_, and f that of _a_, the object
forming the image subtends at the center of the objective, o, an angle
_A c B_, and for a distant object this will be sensibly the angle under
which the eye sees the same object.
If L is the half linear dimension of the image, the eye sees half the
object covering the angle whose tangent is L/F. Similarly half the
image _ab_ is seen through the eye lens _e_ as covering a half angle
whose tangent is L/f. Since the magnifying power of the combination,
m, is directly as the ratio of increase in this tangent of the visual
angle, which measures the image dimension
m = F/f, as before
Further, as all the light which comes in parallel through the whole
opening of the objective forms a single conical beam concentrating into
a focus and then diverging to enter the eye lens, the diameter of the
cone coming through the eye lens must bear the same relation to the
diameter of _o_, that f does to F.
Any less diameter of _e_ will cut off part of the emerging light; any
more will show an emergent beam smaller than the eye lens, which is
generally the case. Hence if we call p the diameter of the bright
pencil of light which we see coming through the eye lens then for that
particular eye lens,
m = _o_/p
That is, f = pF/_o_ which is quite the easiest way of measuring the
focal length of an eyepiece.
Point the telescope toward the clear sky, focusing for a distant object
so that the emergent pencil is sharply defined at the ocular, and then
measure its diameter by the help of a fine scale and a pocket lens,
taking care that scale and emergent pencil are simultaneously in sharp
focus and show no parallax as the eye is shifted a bit. This bright
circle of the emerging beam is actually the projection by the eye lens
of the focal image of the objective aperture.
This method of measuring power is easy and rather accurate. But it
leads to trouble if the measured diameter of the objective is in
fact contracted by a stop anywhere along the path of the beam, as
occasionally happens. Examine the telescope carefully with reference to
this point before thus testing the power.[17]
[17] A more precise method, depending on an actual measurement of the
angle subtended by the diameter of the eyepiece diaphragm as seen
through the eye end of the ocular and its comparison with the same
angular diameter reckoned from the objective, is given by Schaeberle.
M. N. =43=, 297.
The eye lens of Fig. 5 is a simple double convex one, such as was used
by Christopher Scheiner and his contemporaries. With a first class
objective or mirror the simple eye lens such as is shown in Fig. 98a
is by no means to be despised even now. Sir William Herschel always
preferred it for high powers, and speaks with evident contempt of
observers who sacrificed its advantages to gain a bigger field of view.
Let us try to fathom the reason for his vigorously expressed opinion,
strongly backed up by experienced observers like the late T. W. Webb
and Mr. W. F. Denning.
First of all a single lens saves about 10% of the light. Each surface
of glass through which light passes transmits 95 to 96% of that light,
so that a single lens transmits approximately 90%, two lenses 81%
and so on. This loss may be enough to determine the visibility of an
object. Sir Wm. Herschel found that faint objects invisible with the
ordinary two lens eyepiece came to view with the single lens.
Probably the actual loss is less serious than its effect on seeing
conditions. The loss is due substantially to reflection at the
surfaces, and the light thus reflected is scattered close to, or
into, the eye and produces stray light in the field which injures the
contrast by which faint objects become visible.
In some eyepieces the form of the surfaces is such that reflected light
is strongly concentrated where the eye sees it, forming a “ghost”
quite bright enough greatly to interfere with the vision of delicate
contrasts.
The single lens has a very small sharp field, hardly 10° in angular
extent, the image falling off rapidly in quality as it departs from the
axis. If plano-convex, as is the eye lens of common two-lens oculars,
it works best with the curved side to the eye, i.e., reversed from
its usual position, the spherical aberration being much less in this
position.
[Illustration: FIG. 98.—Simple Oculars.]
Herschel’s report of better definition with a single lens than with an
ordinary two lens ocular speaks ill for the quality of the latter then
available. Of course the single lens gives some chromatic aberration,
generally of small account with the narrow pencils of light used in
high powers.
A somewhat better form of eye lens occasionally used is the so-called
Coddington lens, really devised by Sir David Brewster. This, Fig. 98b,
is derived from a glass sphere with a thick equatorial belt removed
and a groove cut down centrally leaving a diameter of less than half
the radius of the sphere. The focus is, for ordinary crown glass, 3/2
the radius of the sphere, and the field is a little improved over the
simple lens, but it falls off rather rapidly, with considerable color
toward the edge.
The obvious step toward fuller correction of the aberrations while
retaining the advantages of the simple lens is to make the ocular
achromatic, like a minute objective, thus correcting at once the
chromatic and spherical aberrations over a reasonably large field. As
the components are cemented the loss of light at their common surface
is negligible. Figure 98c shows such a lens. If correctly designed it
gives an admirably sharp field of 15° to 20°, colorless and with very
little distortion, and is well adapted for high powers.
[Illustration: _a_ _b_
FIG. 99.—Triple Cemented Oculars.]
Still better results in field and orthoscopy can be attained by
going to a triple cemented lens, similar to the objective of Fig.
57. Triplets thus constituted are made abroad by Zeiss, Steinheil
and others, while in this country an admirable triplet designed by
Professor Hastings is made by Bausch & Lomb.
[Illustration: FIG. 100.—Path of Rays Through Huygenian Ocular.]
Such lenses give a beautifully flat and sharp field over an angle of
20° to 30°, quite colorless and orthoscopic. Fig. 99_a_, a form used
by Steinheil, is an excellent example of the construction and a most
useful ocular. The late R. B. Tolles made such triplets, even down to
⅛ inch focus, which gave admirable results.
A highly specialized form of triplet is the so-called monocentric of
Steinheil Fig. 99_b_. Its peculiarity is less in the fact that all the
curves are struck from the same center than in the great thickness
of the front flint and the crown, which, as in some photographic
lenses, give added facilities for flattening the field and eliminating
distortion.
The monocentric eyepiece has a high reputation for keen definition
and is admirably achromatic and orthoscopic. The sharp field is about
32°, rather the largest given by any of the cemented combinations.
All these optically single lenses are quite free of ghosts, reduce
scattered light to a minimum, and leave little to be desired in precise
definition. The weak point of the whole tribe is the small field,
which, despite Herschel’s opinion, is a real disadvantage for certain
kinds of work and wastes the observer’s time unless his facilities for
close setting are more than usually good.
Hence the general use of oculars of the two lens types, all of
them giving relatively wide fields, some of them faultless also in
definition and orthoscopy. The earliest form, Fig. 100, is the very
useful and common one used by Huygens and bearing his name, though
perhaps independently devised by Campani of Rome. Probably four out of
five astronomical eyepieces belong to this class.
The Huygenian ocular accomplishes two useful results—first, it gives a
wider sharp field than any single lens, and second it compensates the
chromatic aberration, which otherwise must be removed by a composite
lens. It usually consists of a plano-convex lens, convex side toward
the objective, which is brought inside the objective focus and forms
an image in the plane of a rear diaphragm, and a similar eye lens of
shorter focus by which this image is examined.
Fig. 100 shows the course of the rays—_A_ being the field lens, _B_
the diaphragm and _C_ the eye lens. Let _1_, _2_, be rays which are
incident near the margin of _A_. Each, in passing through the lens, is
dispersed, the blue being more refracted than the red. Both rays come
to a general focus at _B_, and, crossing, diverge slightly towards _C_.
But, on reaching _C_, ray _1_, that was nearer the margin and the more
refracted because in a zone of greater pitch, now falls on _C_ the
nearer its center, and is less refracted than ray _2_ which strikes _C_
nearer the rim. If the curvatures of _A_ and _C_ are properly related
_1_ and _2_ emerge from _C_ parallel to each other and thus unite in
forming a distinct image.
Now follow through the two branches of _l_ marked _l_r_, and _l_v_, the
red and violet components. Ray _l_v_, the more refrangible, strikes
_C_ nearer the center, and is the less refracted, emerging from _C_
substantially parallel with its mate _l_r_, hence blending the red and
violet images, if, being of the same glass, _A_ and _C_ have suitably
related focal lengths and separation.
As a matter of fact the condition for this chromatic compensation is
d = (f + f′)/2
where d is the distance between the lenses and f, f′, their respective
focal lengths. If this condition of achromatism be combined with
that of equal refraction at _A_ and _C_, favorable to minimizing
the spherical aberration, we find f = 3f′ and d = 2f′. This is the
conventional Huygenian ocular with an eye lens ⅓ the focus of the
field lens, spaced at double the focus of the eye lens, with the
diaphragm midway.
In practice the ratio of foci varies from 1:3 to 1:2 or even 1:1.5, the
exact figure varying with the amount of overcorrection in the objective
and under-correction in the eye that has to be dealt with, while the
value of d should be adjusted by actual trial on the telescope to
obtain the best color correction practicable. One cannot use any chance
ocular and expect the finest results.
[Illustration: FIG. 101_a_.—Airy and Mittenzuey Oculars.]
The Huygenian eyepieces are often referred to as “negative” inasmuch
as they cannot be used directly as magnifiers, although dealing
effectively with an image rather than an object. The statement is
also often made that they cannot be used with cross wires. This is
incorrect, for while there is noticeable distortion toward the edge of
the wide field, to say nothing of astigmatism, in and near the center
of the field the situation is a good deal better.
Central cross wires in the plane of the diaphragm are entirely suitable
for alignment of the instrument, and over a moderate extent of field
the distortion is so small that a micrometer scale in the plane of
the diaphragm gives very good approximate measurements, and indeed is
widely used in microscopy.
It should be noted that the achromatism of this type of eyepiece is
compensatory rather than real. One cannot at the same time bring the
images of various colors to the same size, and also to the same plane.
As failure in the latter respect is comparatively unimportant, the
Huygenian eyepiece is adjusted so far to compensate the paths of the
various rays as to bring the colored images to the same size, and in
point of fact the result is very good.
The field of the conventional form of Huygenian ocular is fully 40°,
and the definition, particularly centrally, is very excellent. There
are no perceptible ghosts produced, and while some 10% of light is lost
by reflection in the extra lens it is diffused in the general field
and is damaging only as it injures the contrast of faint objects. The
theory of the Huygenian eyepiece was elaborately given by Littrow,
(Memoirs R. A. S. Vol. 4, p. 599), wherein the somewhat intricate
geometry of the situation is fully discussed.
Various modifications of the Huygenian type have been devised and used.
Figure 101_a_ is the Airy form devised as a result of a somewhat full
mathematical investigation by Sir George Airy, later Astronomer Royal.
Its peculiarity lies in the form of the lenses which preserve the usual
3:1 ratio of focal lengths. The field lens is a positive meniscus with
a noticeable amount of concavity in the rear face while the eye lens
is a “crossed” lens, the outer curvature being about ⅙ of the inner
curvature. The marginal field in this ocular is a little better than in
the conventional Huygenian.
[Illustration: FIG. 101_b_.—Airy and Mittenzwey Oculars.]
A commoner modification now-a-days is the Mittenzwey form, Fig. 101_b_.
This is usually made with 2:1 ratio of focal lengths, and the field
lens still a meniscus, but less conspicuously concave than in the
Airy form. The eye lens is the usual plano-convex. It is widely used,
especially abroad, and gives perhaps as large available field as any
ocular yet devised, approximately 50°, with pretty good definition out
to the margin.
Finally, we come to the solid eyepiece Fig. 102_a_, devised by the late
R. B. Tolies nearly three quarters of a century ago, and and often
made by him both for telescopes and microscopes. It is practically
a Huygenian eyepiece made out of a single cylinder of glass with a
curvature ratio of 1½:1 between the eye and the field lens. A groove
is cut around the long lens at about ⅓ its length from the vertex of
the field end. This serves as a stop, reducing the diameter of the
lens to about one-half its focal length.
It is in fact a Huygenian eyepiece free from the loss of light in the
usual construction. It gives a wide field, more extensive than in the
ordinary form, with exquisite definition. It is really a most admirable
form of eyepiece which should be used far more than is now the case.
The late Dr. Brashear is on record as believing that all negative
eyepieces less than ¾ inch focus should be made in this form.
[Illustration: _a_ _b_
FIG. 102.—Tolles’ Solid and Compensated Oculars.]
So far as the writer can ascertain the only reason that it is not
more used is that it is somewhat more difficult to construct than the
two lens form, for its curvatures and length must be very accurately
adjusted. It is consequently unpopular with the constructing optician
in spite of its conspicuous merits. It gives no ghosts, and the faint
reflection at the eye end is widely spread so that if the exterior of
the cylinder is well blackened, as it should be, it gives exceptional
freedom from stray light. Still another variety of the Huygenian
ocular sometimes useful is analogous to the compensating eyepiece used
in microscopy. If, as commonly is the case, a telescope objective is
over-corrected for color to correct for the chromatism of the eye in
low powers, the high powers show strong over correction, the blue focus
being longer than the red, and the blue image therefore the larger.
If now the field lens of the ocular be made of heavy flint glass and
the separation of the lenses suitably adjusted, the stronger refraction
of the field lens for the blue pulls up the blue focus and brings its
image to substantially the dimensions of the red, so that the eye lens
performs as if there were no overcorrection of the objective.
The writer has experimented with an ocular of this sort as shown
in Fig. 102_b_ and finds that the color correction is, as might be
expected, greatly improved over a Mittenzwey ocular of the same focus
(⅕ inch). There would be material advantage in thus varying the
ocular color correction to suit the power.
In the Huyghenian eyepiece the equivalent focal length F is given by,
F = 2ff′/(f + f′)
where f and f′ are the focal lengths of the field and eye lenses
respectively. This assumes the normal spacing, d, of half the sum of
the focal lengths, not always adhered to by constructors. The perfectly
general case, as for any two combined lenses is,
F = ff_{1}/(f + f_{1}-d)
[Illustration: FIG. 103.—Path of Rays Through Ramsden Ocular.]
To obtain a flatter field, and particularly one free from distortion
the construction devised by Ramsden is commonly used. This consists,
Fig. 103, of two plano convex lenses of equal focal length, placed with
their plane faces outward, at a distance equal to, or somewhat less
than, their common focal length. The former spacing is the one which
gives the best achromatic compensation since as before the condition
for achromatism is
d = ½(f + f′)
When thus spaced the plane surface of the field lens is exactly in the
focus of the eye lens, the combined focus F is the same as that of
either lens, since as just shown in any additive combination of two
lenses
F = ff′/(f + f′-d)
and while the field is flat and colorless, every speck of dust on the
field lens is offensively in view.
It is therefore usual to make this ocular in the form suggested by
Airy, in which something of the achromatic correction is sacrificed to
obviate this difficulty, and to obtain a better balance of the residual
aberrations. The path of the rays is shown in Fig. 103. The lenses _A_
and _B_ are of the same focal length but are now spaced at ⅔ of this
length apart.
The two neighboring rays _1_, _2_, coming through the objective from
the distant object meet at the objective focus in a point, _a_, of the
image plane _a b_. Thence, diverging, they are so refracted by _A_
and _B_ as to leave the latter substantially parallel so that both
appear to proceed from the point c, of the image plane _c_, _d_, in the
principal focus of _B_.
From the ordinary equation for the combination, F = ¾ f. The
combination focusses ¼ f back of the principal focus of the
objective, and the position of the eye is ¼ F back of the eye lens,
which is another reason for shortening the lens spacing. At longer
spacing the eye distance is inconveniently reduced.
Thus constituted, the Ramsden ocular, known as “positive” from its
capability for use as a magnifier of actual objects, gives a good flat
field free from distortion over a field of nearly 35° and at some loss
of definition a little more. It is the form most commonly used for
micrometer work.
In all optical instruments the aberrations increase as one departs from
the axis, so that angular field is rather a loose term depending on the
maximum aberrations that can be tolerated.[18]
[18] The angular field a is defined by
tan ½a = γ/F
where γ is, numerically, the radius of the field sharp enough for the
purpose in hand, and F the effective focal length of the ocular.
Of the Ramsden ocular there are many modifications. Sometimes f and f′
are made unequal or there is departure from the simple plano-convex
form. More often the lenses are made achromatic, thus getting rid
of the very perceptible color in the simpler form and materially
helping the definition. Figure 104_a_ shows such an achromatic ocular
as made by Steinheil. The general arrangement is as in the ordinary
Ramsden, but the sharp field is slightly enlarged, a good 36°, and the
definition is improved quite noticeably.
A somewhat analogous form, but considerably modified in detail, is
the Kellner ocular, Fig. 104_b_. It was devised by an optician of that
name, of Wetzlar, who exploited it some three quarters of a century
since in a little brochure entitled “Das orthoskopische Okular,” as
notable a blast of “hot air” as ever came from a modern publicity agent.
As made today the Kellner ocular consists of a field lens which is
commonly plano-convex, plano side out, but sometimes crossed or even
equiconvex, combined with a considerably smaller eye lens which is
an over-corrected achromatic. The focal length of the field lens is
approximately 7/4 F, that of the eye lens 4/3 F, separated by about ¾
F.
This ocular has its front focal plane very near the field lens,
sometimes even within its substance, and a rather short eye distance,
but it gives admirable definition and a usable field of very great
extent, colorless and orthoscopic to the edge. The writer has one of
2⅝″ focus, with an achromatic triplet as eye lens, which gives an
admirable field of quite 50°.
[Illustration: FIG. 104.—Achromatic and Kellner Oculars.]
The Kellner is decidedly valuable as a wide field positive ocular, but
it has in common with the two just previously described a sometimes
unpleasant ghost of bright objects. This arises from light reflected
from the inner surface of the field lens, and back again by the front
surface to a focus. This focus commonly lies not far back of the field
lens and quite too near to the focus of the eye lens for comfort. It
should be watched for in going after faint objects with oculars of the
types noted.
A decidedly better form of positive ocular is the modern orthoscopic
as made by Steinheil and Zeiss, Fig. 105_a_. It consists of a triple
achromatic field lens, a dense flint between two crowns, with a
plano-convex eye lens of much shorter focus (⅓ to ½) almost in
contact on its convex side.
The field triplet is heavily over-corrected for color, the front focal
plane is nearly ½ F ahead of the front vertex of the field lens, and
the eye distance is notably greater than in the Kellner. The field
is above 40°, beautifully flat, sharp, and orthoscopic, free of
troublesome ghosts. On the whole the writer is inclined to rate it as
the best of two-lens oculars.
There should also here be mentioned a very useful long relief ocular,
often used for artillery sights, and shown in Fig. 105_b_. It consists
like Fig. 104_a_, of a pair of achromatic lenses, but they are placed
with the crowns almost in contact and are frequently used with a simple
plano convex field lens of much longer focus, to render the combination
more fully orthoscopic.
The field, especially with the field lens, is wide, quite 40° as
apparent angle for the whole instrument, and the eye distance is
roughly equal to the focal length. It is a form of ocular that might
be very advantageously used in finders, where one often has to assume
uncomfortable angles of view, and long relief is valuable.
[Illustration: _a_ _b_
FIG. 105.—Orthoscopic and Long Relief Oculars.]
Whatever the apparent angular field of an ocular may be, the real
angular field of view is obtained by dividing the apparent field by the
magnifying power. Thus the author’s big Kellner, just mentioned, gives
a power of 20 with the objective for which it was designed, hence a
real field of 2½°, while a second, power 65, gives a real field of
hardly 0°40′, the apparent field in this case being a trifle over 40°.
There is no escaping this relation, so that high power always implies
small field.
The limit of apparent field is due to increasing errors away from the
axis, strong curvature of the field, and particularly astigmatism in
the outer zones. The eye itself can take in only about 40° so that more
than this, while attainable, can only be utilized by peering around the
marginal field.
For low powers the usable field is helped out by the accommodation of
the eye, but in oculars of short focus the curvature of field is the
limiting factor. The radius of curvature of the image is, in a single
lens approximately 3/2 F, and in the common two lens forms about ¾ F.
In considering this matter Conrady has shown (M. N. _78_ 445) that for
a total field of 40° the sharpness of field fails at a focal length
of about 1 inch for normal power of accommodation. The best achromatic
combinations reduce this limit to about ½ inch.
At focal lengths below this the sharpest field is obtainable only with
reduced aperture. There is an interesting possibility of building an
anastigmatic ocular on the lines of the modern photographic lens, which
Conrady suggests, but the need of wide field in high powers is hardly
pressing enough to stimulate research.
[Illustration: FIG. 106.—Ordinary Terrestrial Ocular.]
Finally we may pass to the very simple adjunct of most small
telescopes, the terrestrial ocular which inverts the image and shows
the landscape right side up. Whatever its exact form it consists of
an inverting system which erects the inverted image produced by the
objective alone, and an eyepiece for viewing this erected image. In its
common form it is composed of four plano-convex lenses arranged as in
Fig. 106. Here A and B for the inverting pair and C and D a modified
Huygenian ocular. The image from the objective is formed in the front
focus of AB which is practically an inverted ocular, and the erected
image is formed in the usual way between C and D.
The apparent field is fairly good, about 35°, and while slightly
better corrections can be gained by using lenses of specially adjusted
curvatures, as Airy has shown, these are seldom applied. The chief
objection to this erecting system is its length, some ten times its
equivalent focus. Now and then to save light and gain field, the
erector is a single cemented combination and the ocular like Fig. 99_a_
or Fig. 102_a_. Fig. 107 shows a terrestrial eyepiece so arranged,
from an example by the late R. B. Tolles. When carefully designed an
apparent field of 40° or more can be secured, with great brilliancy,
and the length of the erecting system is moderate.
Very much akin in principle is the eyepiece microscope, such as is made
by Zeiss to give variable power and a convenient position of the eye
in connection with filar micrometers, Fig. 108. It is provided with
a focussing collar and its draw tube allows varying power just as in
case of an ordinary microscope. In fact eyepiece microscopes have long
been now and then used to advantage for high powers. They are easier on
the eye, and give greater eye distance than the exceedingly small eye
lenses of short focus oculars, and using a solid eyepiece and single
lens objective lose no more light than an ordinary Huygenian ocular.
The erect resultant image is occasionally a convenience in astronomical
use.
[Illustration: FIG. 107.—Tolles Triplet Inverting System.]
[Illustration: FIG. 108.—Microscope as Ocular.]
[Illustration: FIG. 109.—“Davon” Instrument.]
Quite analogous to the eyepiece microscope is the so-called “Davon”
micro-telescope. Originally developed as an attachment for the substage
of a microscope to give large images of objects at a little distance
it has grown also into a separate hand telescope, monocular or
binocular, for general purposes. The attachment thus developed is shown
complete in Fig. 109. D is merely a well corrected objective set in a
mount provided with ample stops. The image is viewed by an ordinary
microscope or special eyepiece microscope A, as the case may be,
furnished with rack focussing at A′ and assembled with the objective by
means of the carefully centered coupling C.
It furnishes a compact and powerful instrument, very suitable for
terrestrial or minor astronomical uses, like the Tolles’ short-focus
hand telescopes already mentioned. When properly designed telescopes
of this sort give nearly the field of prism glasses, weigh much less
and lose far less light for the same effective power and aperture. They
also have under fairly high powers rather the advantage in the matter
of definition, other things being equal.