DESCRIPTION OF DIFFERENT STEREOSCOPES.
Although the lenticular stereoscope has every advantage that such
an instrument can possess, whether it is wanted for experiments on
binocular vision—for assisting the artist by the reproduction of
objects in relief, or for the purposes of amusement and instruction,
yet there are other forms of it which have particular properties,
and which may be constructed without the aid of the optician, and of
materials within the reach of the humblest inquirers. The first of
these is—
1. _The Tubular Reflecting Stereoscope._
In this form of the instrument, shewn in Fig. 28, the pictures are seen
by reflexion from two specula or prisms placed at an angle of 90°, as
in Mr. Wheatstone’s instrument. In other respects the two instruments
are essentially different.
In Mr. Wheatstone’s stereoscope he employs two mirrors, each _four
inches_ square—that is, he employs _thirty-two_ square inches of
reflecting surface, and is therefore under the necessity of employing
glass mirrors, and making a clumsy, unmanageable, and unscientific
instrument, with all the imperfections which we have pointed out in a
preceding chapter. It is not easy to understand why mirrors of such a
size should have been adopted. The reason of their being made of common
looking-glass is, that metallic or prismatic reflectors of such a size
would have been extremely expensive.
[Illustration: FIG. 28.]
It is obvious, however, from the slightest consideration, that
reflectors of such a size are wholly unnecessary, and that _one
square_ inch of reflecting surface, in place of _thirty-two_, is quite
sufficient for uniting the binocular pictures. We can, therefore, at
a price as low as that of the 4-inch glass reflectors, use mirrors of
speculum metal, steel, or even silver, or rectangular glass prisms,
in which the images are obtained by total reflexion. In this way the
stereoscope becomes a real optical instrument, in which the reflexion
is made from surfaces single and perfectly flat, as in the second
reflexion of the Newtonian telescope and the microscope of Amici, in
which pieces of looking-glass were never used. By thus diminishing the
reflectors, we obtain a portable tubular instrument occupying nearly
as little room as the lenticular stereoscope, as will be seen from
Fig. 28, where ABCD is a tube whose diameter is equal to the largest
size of one of the binocular pictures which we propose to use, the
left-eye picture being placed at CD, and the right-eye one at AB. If
they are transparent, they will be illuminated through paper or ground
glass, and if opaque, through openings in the tube. The image of AB,
reflected to the left eye L from the small mirror _mn_, and that of
CD to the right eye R from the mirror _op_, will be united exactly
as in Mr. Wheatstone’s instrument already described. The distance of
the two ends, _n_, _p_, of the mirrors should be a little greater than
the smallest distance between the two eyes. If we wish to magnify the
picture, we may use two lenses, or substitute for the reflectors a
totally reflecting glass prism, in which one or two of its surfaces are
made convex.[40]
[40] We may use also the lens prism, which I proposed many years ago in
the _Edinburgh Philosophical Journal_.
2. _The Single Reflecting Stereoscope._
This very simple instrument, which, however, answers only for
symmetrical figures, such as those shewn at A and B, which must be
either two right-eye or two left-eye pictures, is shewn in Fig. 29.
A single reflector, MN, which may be either a piece of glass, or a
piece of mirror-glass, or a small metallic speculum, or a rectangular
prism, is placed at MN. If we look into it with the left eye L, we see,
by reflexion from its surface at C, a reverted image, or a right-eye
picture of the left-eye picture B, which, when seen in the direction
LCA, and combined with the figure A, seen directly with the right eye
R, produces a _raised_ cone; but if we turn the reflector L round, so
that the right eye may look into it, and combine a reverted image of
A, with the figure B seen directly with the left eye L, we shall see
a _hollow_ cone. As BC + CL is greater than RA, the reflected image
will be slightly less in size than the image seen directly, but the
difference is not such as to produce any perceptible effect upon the
appearance of the hollow or the raised cone. By bringing the picture
viewed by reflexion a little nearer the reflector MN, the two pictures
may be made to have the same apparent magnitude.
[Illustration: FIG. 29.]
If we substitute for the single reflector MN, two reflectors such as
are shewn at M, N, Fig. 30, or a prism P, which gives two internal
reflexions, we shall have a general stereoscope, which answers for
landscapes and portraits.
[Illustration: FIG. 30.]
The reflectors M, N or P may be fitted up in a conical tube, which has
an elliptical section to accommodate two figures at its farther end,
the major axis of the ellipse being parallel to the line joining the
two eyes.
3. _The Double Reflecting Stereoscope._
This instrument differs from the preceding in having a single
reflector, MN, M′N′, for each eye, as shewn in Fig. 31, and the effect
of this is to exhibit, _at the same time, the raised and the hollow
cone_. The image of B, seen by reflexion from MN at the point C, is
combined with the picture of A, seen directly by the _right_ eye R, and
forms a _hollow_ cone; while the image of A, seen by reflexion from
M′N′ at the point C′, is combined with the picture of B, seen directly
by the left eye L, and forms a _raised_ cone.
[Illustration: FIG. 31.]
[Illustration: FIG. 32.]
Another form of the double reflecting stereoscope is shewn in Fig. 32,
which differs from that shewn in Fig. 31 in the position of the two
reflectors and of the figures to be united. The reflecting faces of
the mirrors are turned outwards, their distance being less than the
distance between the eyes, and the effect of this is to exhibit at the
same time the _raised_ and the _hollow_ cone, the hollow cone being now
on the right-hand side.
If in place of two right or two left eye pictures, as shewn in Figs.
29, 31, and 32, we use one right eye and one left eye picture, and
combine the reflected image of the one with the reflected image of the
other, we shall have a _raised_ cone with the stereoscope, shewn in
Fig. 31, and a _hollow_ cone with the one in Fig. 32.
The double reflecting stereoscope, in both its forms, is a general
instrument for portraits and landscapes, and thus possesses properties
peculiar to itself.
The reflectors may be glass or metallic specula, or total reflexion
prisms.
4. _The Total Reflexion-Stereoscope._
This form of the stereoscope is a very interesting one, and possesses
valuable properties. It requires only a small prism and _one_ diagram,
or picture of the solid, as seen by one eye; the other diagram, or
picture which is to be combined with it, being created by total
reflexion from the base of the prism. This instrument is shewn in Fig.
33, where D is the picture of a cone as seen by the left eye L, and ABC
a prism, whose base BC is so large, that when the eye is placed close
to it, it may see, by reflexion, the whole of the diagram D. The angles
ABC, ACB must be equal, but may be of any magnitude. Great accuracy in
the equality of the angles is not necessary; and a prism constructed,
by a lapidary, out of a fragment of thick plate-glass, the face BC
being one of the surfaces of the plate, will answer the purpose. When
the prism is placed at _a_, Fig. 34, at one end of a conical tube LD,
and the diagram D at the other end, in a cap, which can be turned round
so as to have the line _mn_, Fig. 33, which passes through the centre
of the base and summit of the cone parallel to the line joining the two
eyes, the instrument is ready for use. The observer places his left
eye at L, and views with it the picture D, as seen by total reflexion
from the base BC of the prism, Figs. 33 and 35, while with his right
eye R, Fig. 33, he views the real picture directly. The first of these
pictures being the reverse of the second D, like all pictures formed
by one reflexion, we thus combine two dissimilar pictures into a
_raised_ cone, as in the figure, or into a _hollow_ one, if the picture
at D is turned round 180°. If we place the images of two diagrams,
one like one of those at A, Fig. 31, and the other like the one at B,
vertically above one another, we shall then see, at the same time,
the _raised_ and the _hollow_ cone, as produced in the lenticular
stereoscope by the three diagrams, two like those in Fig. 31, and a
third like the one at A. When the prism is good, the dissimilar image,
produced by the two refractions at B and C, and the one reflexion at
E, is, of course, more accurate than if it had been drawn by the most
skilful artist; and therefore this form of the stereoscope has in this
respect an advantage over every other in which two dissimilar figures,
executed by art, are necessary. In consequence of the length of the
reflected pencil DB + BE + EC + CL being a little greater than the
direct pencil of rays DR, the two images combined have not exactly the
same apparent magnitude; but the difference is not perceptible to the
eye, and a remedy could easily be provided were it required.
[Illustration: FIG. 33.]
[Illustration: FIG. 34.]
If the conical tube LD is held in the left hand, the left eye must be
used, and if in the right hand the right eye must be used, so that
the hand may not obstruct the direct vision of the drawing by the eye
which does not look through the prism. The cone LD must be turned round
slightly in the hand till the line _mn_ joining the centre and apex of
the figure is parallel to the line joining the two eyes. The same line
must be parallel to the plane of reflexion from the prism; but this
parallelism is secured by fixing the prism and the drawing.
It is scarcely necessary to state that this stereoscope is applicable
only to those diagrams and forms where the one image is the reflected
picture of the other.
[Illustration: FIG. 35.]
If we wish to make a microscopic stereoscope of this form, or to
magnify the drawings, we have only to cement plano-convex lenses,
of the requisite focal length, upon the faces AB, AC of the prism,
or, what is simpler still, to use a section of a deeply convex lens
ABC, Fig. 35, and apply the other half of the lens to the right eye,
the face BC having been previously ground flat and polished for the
prismatic lens. By using a lens of larger focus for the right eye, we
may correct, if required, the imperfection arising from the difference
of paths in the reflected and direct pencils. This difference, though
trivial, might be corrected, if thought necessary, by applying to the
right eye the central portion of the same lens whose margin is used for
the prism.
[Illustration: FIG. 36.]
If we take the drawing of a six-sided pyramid as seen by the right eye,
as shewn in Fig. 36, and place it in the total-reflexion stereoscope at
D, Fig. 33, so that the line MN coincides with _mn_, and is parallel to
the line joining the eyes of the observer, we shall perceive a perfect
raised pyramid of a given height, the reflected image of CD, Fig. 36,
being combined with AF, seen directly. If we now turn the figure round
30°, CD will come into the position AB, and unite with AB, and we shall
still perceive a raised pyramid, with less height and less symmetry. If
we turn it round 30° more, CD will be combined with BC, and we shall
still perceive a raised pyramid with still less height and still less
symmetry. When the figure is turned round other 30°, or 90° degrees
from its first position, CD will coincide with CD seen directly,
and the combined figures will be perfectly flat. If we continue the
rotation through other 30°, CD will coincide with DE, and a slightly
hollow, but not very symmetrical figure, will be seen. A rotation of
other 30° will bring CD into coalesence with EF, and we shall see a
still more hollow and more symmetrical pyramid. A further rotation of
other 30°, making 180° from the commencement, will bring CD into union
with AF; and we shall have a perfectly symmetrical hollow pyramid of
still greater depth, and the exact counterpart of the raised pyramid
which was seen before the rotation of the figure commenced. If the
pyramid had been square, the _raised_ would have passed into the
_hollow_ pyramid by rotations of 45° each. If it had been rectangular,
the change would have been effected by rotations of 90°. If the space
between the two circular sections of the cone in Fig. 31 had been
uniformly shaded, or if lines had been drawn from every degree of the
one circle to every corresponding degree in the other, in place of
from every 90th degree, as in the Figure, the raised cone would have
gradually diminished in height, by the rotation of the figure, till it
became flat, after a rotation of 90°; and by continuing the rotation it
would have become hollow, and gradually reached its maximum depth after
a revolution of 180°.
5. _The Single-Prism Stereoscope._
Although the idea of uniting the binocular pictures by a single
prism applied to one eye, and refracting one of the pictures so as
to place it upon the other seen directly by the other eye, or by a
prism applied to each eye, could hardly have escaped the notice of
any person studying the subject, yet the experiment was, so far as I
know, first made and published by myself. I found two prisms quite
unnecessary, and therefore abandoned the use of them, for reasons which
will be readily appreciated. This simple instrument is shewn in Fig.
37, where A, B are the dissimilar pictures, and P a prism with such
a refracting angle as is sufficient to lay the image of A upon B, as
seen by the right eye. If we place a _second_ prism before the eye
R, we require it only to have half the refracting angle of the prism
P, because each prism now refracts the picture opposite to it only
half way between A and B, where they are united. This, at first sight,
appears to be an advantage, for as there must always be a certain
degree of colour produced by a single prism, the use of two prisms,
with half the refracting angle, might be supposed to reduce the colour
one-half. But while the colour produced by each prism is thus reduced,
the colour over the whole picture is the same. Each luminous edge with
two prisms has both red and blue tints, whereas with one prism each
luminous edge has only one colour, either red or blue. If the picture
is very luminous these colours will be seen, but in many of the finest
opaque pictures it is hardly visible. In order, however, to diminish
it, the prism should be made of glass with the lowest dispersive power,
or with rock crystal. A single plane surface, ground and polished by
a lapidary, upon the edge of a piece of plate-glass, a little larger
than the pupil of the eye, will give a prism sufficient for every
ordinary purpose. Any person may make one in a few minutes for himself,
by placing a little bit of good window glass upon another piece
inclined to it at the proper angle, and inserting in the angle a drop
of fluid. Such a prism will scarcely produce any perceptible colour.
[Illustration: FIG. 37.]
If a single-prism reflector is to be made perfect, we have only to
make it achromatic, which could be done _extempore_, by correcting
the colour of the fluid prism by another fluid prism of different
refractive and dispersive power.
With a good achromatic prism the single-prism stereoscope is a very
fine instrument; and no advantage of any value could be gained by
using _two achromatic prisms_. In the article on New Stereoscopes,
published in the Transactions of the Royal Society of Arts for 1849,
and in the Philosophical Magazine for 1852, I have stated in a note
that _I believed_ that Mr. Wheatstone had used _two achromatic prisms_.
This, however, was a mistake, as already explained,[41] for such
an instrument was never made, and has never been named in any work
previous to 1849, when it was mentioned by myself in the note above
referred to.
[41] See Chap. i. pp. 33-36.
If we make a double prism, or join two, as shewn at P, P′ in Fig.
38, and apply it to two dissimilar figures A, B, one of which is the
reflected image of the other, so that with the left eye L and the prism
P we place the refracted image of A upon B, as seen by the right eye
R, we shall see a _raised_ cone, and if with the prism P′ we place
the image of B upon A we shall see a _hollow_ cone. If we place the
left eye L at O, behind the common base of the prism, we shall see with
one-half of the pupil the _hollow_ cone and with the other half the
_raised_ cone.
[Illustration: FIG. 38.]
6. _The Opera-Glass Stereoscope._
As the eyes themselves form a stereoscope to those who have the power
of quickly converging their axes to points nearer than the object which
they contemplate, it might have been expected that the first attempt
to make a stereoscope for those who do not possess such a power, would
have been to supply them with auxiliary eyeballs capable of combining
binocular pictures of different sizes at different distances from the
eye. This, however, has not been the case, and the stereoscope for this
purpose, which we are about to describe, is one of the latest of its
forms.
[Illustration: FIG. 39.]
[Illustration: FIG. 40.]
In Fig. 39, MN is a small inverting telescope, consisting of two
convex lenses M, N, placed at the sum of their focal distances, and
OP another of the same kind. When the two eyes, R, L, look through the
two telescopes directly at the dissimilar pictures A, B, they will see
them with perfect distinctness; but, by the slightest inclination of
the axes of the telescopes, the two images can be combined, and the
stereoscopic effect immediately produced. With the dissimilar pictures
in the diagram a _hollow_ cone is produced; but if we look at B with
the telescope M′N′, as in Fig. 40, and at A′ with O′P′, a _raised_ cone
will be seen. With the usual binocular slides containing portraits or
landscapes, the pictures are seen in relief by combining the right-eye
one with the left-eye one.
The instrument now described is nothing more than a double opera-glass,
which itself forms a good stereoscope. Owing, however, to the use of
a concave eye-glass, the field of view is very small, and therefore a
convex glass, which gives a larger field, is greatly to be preferred.
The little telescopes, MN, OP, may be made one and a half or even
one inch long, and fitted up, either at a fixed or with a variable
inclination, in a pyramidal box, like the lenticular stereoscope, and
made equally portable. One of these instruments was made for me some
years ago by Messrs. Horne and Thornthwaite, and I have described it in
the _North British Review_[42] as having the properties of a _Binocular
Cameoscope_, and of what has been absurdly called a _Pseudoscope_,
seeing that every inverting eye-piece and every stereoscope is entitled
to the very same name.
The little telescope may be made of one piece of glass, _convex_
at each end, or _concave_ at the eye-end if a small field is not
objectionable,—the length of the piece of glass, in the _first_
case, being equal to the _sum_, and, in the _second_ case, to the
_difference_ of the focal lengths of the virtual lenses at each end.[43]
[42] For 1852, vol. xvii. p. 200.
[43] These solid telescopes may be made achromatic by cementing concave
lenses of flint glass upon each end, or of crown glass if they are made
of flint glass.
7. _The Eye-Glass Stereoscope._
As it is impossible to obtain, by the ocular stereoscope, pictures in
relief from the beautiful binocular slides which are made in every
part of the world for the lenticular stereoscope, it is very desirable
to have a portable stereoscope which can be carried safely in our
purse, for the purpose of examining stereoscopically all such binocular
pictures.
If placed together with their plane sides in contact, a plano-convex
lens, AB, and a plano-concave one, CD, of the same glass and the same
focal length, will resemble a thick watch-glass, and on looking through
them, we shall see objects of their natural size and in their proper
place; but if we slip the concave lens, CD, to a side, as shewn in
Fig. 41, we merely displace the image of the object which we view, and
the displacement increases till the centre of the concave lens comes
to the margin of the convex one. We thus obtain a variable prism, by
means of which we can, with the left eye, displace one of the binocular
pictures, and lay it upon the other, as seen by the right eye. We may
use semi-lenses or quarters of lenses, and we may make them achromatic
or nearly so if we desire it. Double convex and double concave lenses
may also be used, and the motion of the concave one regulated by a
screw. In one which I constantly use, the concave lens slides in a
groove over a convex quarter-lens.
[Illustration: FIG. 41.]
By employing two of these variable prisms, we have an _Universal
Stereoscope_ for uniting pictures of various sizes and at various
distances from each other, and the prisms may be placed in a pyramidal
box, like the lenticular stereoscope.
8. _The Reading-Glass Stereoscope._
If we take a reading-glass whose diameter is not less than two inches
and three quarters, and look through it with both eyes at a binocular
picture in which the right-eye view is on the left hand, and the
left-eye view on the right hand, as in the ocular stereoscope, we shall
see each picture doubled, and the degree of separation is proportional
to the distance of the picture from the eye. If the distance of the
binocular pictures from each other is small, the two middle images of
the four will be united when their distance from the lens is not very
much greater than its focal length. With a reading-glass 4½ inches in
diameter, with a focal length of two feet, binocular pictures, in which
the distance of similar parts is _nine_ inches, are united without
any exertion of the eyes at the distance of eight feet. With the same
reading-glass, binocular pictures, at the usual distance of 2½ inches,
will be united at the distance of 2¼ or even 2½ feet. If we advance the
reading-glass when the distance is 2 or 3 feet, the picture in relief
will be magnified, but, though the observer may not notice it, the
separated images are now kept united by a slight convergency of the
optic axes. Although the pictures are placed so far beyond the anterior
focus of the lens, they are exceedingly distinct. The distinctness of
vision is sufficient, at least to long-sighted eyes, when the pictures
are placed within 16 or 18 inches of the observer, that is, 6 or 8
inches nearer the eye than the anterior focus of the lens. In this
case we can maintain the union of the pictures only when we begin to
view them at a distance of 2½ or 3 feet, and then gradually advance
the lens within 16 or 18 inches of the pictures. At considerable
distances, the pictures are most magnified by advancing the lens while
the head of the observer is stationary.
9. _The Camera Stereoscope._
The object of this instrument is to unite the transient pictures of
groups of persons or landscapes, as delineated in two dissimilar
pictures, on the ground-glass of a binocular camera. If we attach
to the back of the camera a lenticular stereoscope, so that the two
pictures on the ground-glass occupy the same place as its usual
binocular slides, we shall see the group of figures in relief under
every change of attitude, position, and expression. The two pictures
may be formed in the air, or, more curiously still, upon a wreath of
smoke. As the figures are necessarily inverted in the camera, they
will remain inverted by the lenticular and every other instrument but
the opera-glass stereoscope, which inverts the object. By applying
it therefore to the camera, we obtain an instrument by which the
photographic artist can make experiments, and try the effect which will
be produced by his pictures before he takes them. He can thus select
the best forms of groups of persons and of landscapes, and thus produce
works of great interest and value.
10. _The Chromatic Stereoscope._
The chromatic stereoscope is a form of the instrument in which relief
or apparent solidity is given to a single figure with different colours
delineated upon a plane surface.
If we look with both eyes through a lens LL, Fig. 42, about 2½ inches
in diameter or upwards, at any object having colours of different
degrees of refrangibility, such as the coloured boundary lines on
a map, a red rose among green leaves and on a blue background, or
any scarlet object whatever upon a violet ground, or in general any
two simple colours not of the same degree of refrangibility, _the
differently coloured parts of the object will appear at different
distances from the observer_.
[Illustration: FIG. 42.]
Let us suppose the rays to be _red_ and _violet_, those which differ
most in refrangibility. If the red rays radiate from the anterior focus
R, or red rays of the lens LL, they will emerge parallel, and enter the
eye at _m_; but the violet rays radiating from the same focus, being
more refrangible, will emerge in a state of convergence, as shewn at
_mv_, _nv_, the red rays being _mr_, _nr_. The part of the object,
therefore, from which the red rays come, will appear nearer to the
observer than the parts from which the violet rays come, and if there
are other colours or rays of intermediate refrangibilities, they will
appear to come from intermediate distances.
If we place a small _red_ and _violet_ disc, like the smallest
wafer, beside one another, so that the line joining their centres is
perpendicular to the line joining the eyes, and suppose that rays from
both enter the eyes with their optical axes parallel, it is obvious
that the distance between the violet images on each retina will be
_less_ than the distance between the _red_ images, and consequently the
eyes will require to converge their axes to a _nearer_ point in order
to unite the red images, than in order to unite the violet images. The
red images will therefore appear at this nearer point of convergence,
just as, in the lenticular stereoscope, the more distant pair of points
in the dissimilar images appear when united nearer to the eye. By the
two eyes alone, therefore, we obtain a certain, though a small degree
of relief from colours. With the lens LL, however, the effect is
greatly increased, and we have the _sum_ of the _two_ effects.
From these observations, it is manifest that the reverse effect must
be produced by a concave lens, or by the common stereoscope, when
_two_ coloured objects are employed or united. The _blue_ part of the
object will be seen _nearer_ the observer, and the red part of it
more _remote_. It is, however, a curious fact, and one which appeared
difficult to explain, that in the stereoscope the colour-relief was not
brought out as might have been expected. Sometimes the red was nearest
the eye, and sometimes the blue, and sometimes the object appeared
without any relief. The cause of this is, that the colour-relief given
by the common stereoscope was the opposite of that given by the eye,
and it was only the _difference_ of these effects that ought to have
been observed; and though the influence of the eyes was an inferior
one, it often acted alone, and sometimes ceased to act at all, in
virtue of that property of vision by which we see only with one eye
when we are looking with two.
In the chromatic stereoscope, Fig. 42, the intermediate part _mn_ of
the lens is of no use, so that out of the margin of a lens upwards of
2½ inches in diameter, we may cut a dozen of portions capable of making
as many instruments. These portions, however, a little larger only than
the pupil of the eye, must be placed in the same position as in Fig. 42.
All the effects which we have described are greatly increased by using
lenses of highly-dispersing flint glass, oil of cassia, and other
fluids of a great dispersive power, and avoiding the use of compound
colours in the objects placed in the stereoscope.
It is an obvious result of these observations, that in painting, and in
coloured decorations of all kinds, the red or less refrangible colours
should be given to the prominent parts of the object to be represented,
and the _blue_ or more refrangible colours to the background and the
parts of the objects that are to retire from the eye.
11. _The Microscope Stereoscope._
The lenticular form of the stereoscope is admirably fitted for its
application to small and microscopic objects. The first instruments
of this kind were constructed by myself with quarter-inch lenses, and
were 3 inches long and only 1 and 1½ deep.[44] They may be carried in
the pocket, and exhibit all the properties of the instrument to the
greatest advantage. The mode of constructing and using the instrument
is precisely the same as in the common stereoscope; but in taking the
dissimilar pictures, we must use either a small binocular camera, which
will give considerably magnified representations of the objects, or
we must procure them from the compound microscope. The pictures may
be obtained with a small single camera, by first taking one picture,
and then shifting the object in the focus of the lens, through a space
corresponding with the binocular angle. To find this space, which we
may call _x_, make _d_ the distance of the object from the lens, _n_
the number of times it is to be magnified, or the distance of the image
behind the lens, and D the distance of the eyes; then we shall have
D
_nd_ : _d_ = D : _x_, and _x_ = ———.
_n_
that is, the space is equal to the distance between the eyes divided by
the magnifying power.
[44] _Phil. Mag._, Jan. 1852, vol. iii. p. 19.
With the binocular microscope of Professor Riddell,[45] and the same
instrument as improved by M. Nachet, binocular pictures are obtained
directly by having them drawn, as Professor Riddell suggests, by the
camera lucida, but it would be preferable to take them photographically.
Portraits for lockets or rings might be put into a very small
stereoscope, by folding the one lens back upon the other.
[45] _American Journal of Science_, 1852, vol. xv. p. 68.