van Monckhoven, Désiré van. A Popular Treatise on Photography. Translated By W.H. Thornthwaite. London, 1863.

Chapter XIV.
The Stereoscope

THIS instrument shows in relief, as one picture, two dissimilar views of the same object, mounted together either on plate or paper. The following is an explanation of this singular phenomenon. If, for example, a square-based pyramid be examined with the left eye, the eye being placed in the axis of this pyramid, it is very evident that it will present the appearance of a square intersected by its two diagonals, and a drawing of it would, in fact, be represented by such a figure. But if the left eye be now closed, and the right eye opened, it is equally clear that one side of the pyramid will be more visible than the other, and that in proportion to its height.

Now, if this same pyramid be viewed with both eyes at the same time, the two different images combine upon the retina in such a way as to produce the appearance of a single solid pyramid, in perfect relief, or, in other words, the effect is stereoscopic.

If the pyramid, instead of being in relief, were hollow, and the left. eye, as before. placed on a line with its axis, a square figure intersected by its two diagonals will be seen; but immediately upon opening the right eye, the effect will be exactly the reverse of what occurred in the case of the figure in relief. In the former case, it was the side of the pyramid to the right which impinged mostly on the right eye, but in the case of the hollow

figure, it would be, on the contrary, the left eye which receives the larger impression, and in consequence thereof, the effect upon the two eyes would be different, and instead of producing the sensation of solidity or relief, the combined images produce the appearance of hollowness, or in other words, are pseudoscopic.

It is upon this principle that the stereoscope is founded, the two views employed together are each taken by a camera, as near as possible under the same conditions as naturally presented to each eye. The prismatic glasses of the instrument superpose the two views in such away that but one image is perceived by the two eyes, and that in relief.

To Professor Wheatstone is due the credit for the elaboration of this theory and the application of photography thereto; also the invention of one modification of the instrument. Mirrors were employed in its construction, and it is known as the reflecting stereoscope. Sir David Brewster replaced the mirrors by prisms, and it is this improvement which has rendered it so popular and portable.

The relief of near objects is easily explained, by the large angle formed by the visual rays brought to each eye; but as this angle becomes more and more diminished in proportion to the distance of the objects, it becomes at last very difficult to appreciate relief, and inconsequence, the situation of large masses can only be judged of by comparison with those which are near. The original stereo scope was composed of small half lenses, which turned in brass tubes in such a way as completely to superpose the two views; but now the small lenses are replaced by large square prisms, let into the upper portion of the stereoscope, and this not only affords a larger field of view, but also makes the instrument more easy to manage.

Fig. 75. Prismatic Stereoscope.

Fig. 75 represents a prismatic stereoscope. The slides to be viewed are introduced through a groove at the bottom; it is furnished with a centre partition, which assists in the superposition of the two images. The upper portion is shown detached, or open, the better to exhibit its general construction.

For the best instruments the two prisms are achromatic, or else replaced by two achromatic lenses, by which their definition is rendered as perfect as possible. A very convenient form of stereoscope for showing a number of elides is now constructed, whereby as many as fifty pictures can be viewed consecutively, by merely turning a small button. This arrangement consists of a stereoscope fixed at the top of a deep box containing the views, which are attached to a kind of endless chain, on moving which the pictures are presented as required.

There are many other forms of stereoscope, the whole of which, however, with the exception, perhaps, of the reflecting stereoscope for viewing large pictures, being constructed on the general principle and arrangements of the foregoing, any further description would be unnecessary.

Fig. 76. Twin-lens Camera with Double Back.

Fig. 77.

Stereoscopic views are always of small size, except those before mentioned for the reflecting stereoscope, and are taken on glass and paper; those on glass are viewed as transparencies, but the paper slides are examined by reflected light, for which purpose there is usually attached to the stereoscope some such contrivance as is represented at Fig. 75, like a small door in front.

The most simple method of obtaining stereoscopic pictures certainly consists in taking the two proofs at the same time, with the apparatus as represented in Fig. 76. It is composed, 1st, of two double achromatic combinations of exactly the same focal length; 2nd, of an ordinary expanding camera, furnished, however, with a back, having two shutters (Fig. 77).

The following is the method of operating:--

Whenever the two lenses are not exactly to the same focus, it is necessary to begin by focussing a near object, such, for instance, as a plaster bust. When a perfectly sharp image is obtained upon the ground glass with each of the lenses, the rack and pinion adjustment must not be again touched, but the focussing of any subsequent figure must be accomplished by drawing in or out the expanding or inner body of the camera.

This, however, will not suffice, if the distance of the bust from the camera has not been so regulated as to be that which will be most suitable for a portrait. For this apparatus will serve only for portraits, and that,, too, for portraits taken at a very short distance; otherwise, the proofs will be wanting in relief.

As would be inferred from the shape of the back, a glass is chosen much longer than it is wide, upon which the two negatives are taken at the same time. The prints obtained from these negatives are reversed, that is to say, when each positive is cut for the purpose of being mounted upon card, it will be necessary to paste the left-hand picture on the right side of the card, and the right-hand print on the left of the card.

When, with this apparatus, it is desired to take views, a diaphragm or stop must be inserted between the two lenses, but the two proofs can, however, no longer be taken at the same time, for the distance between the two lenses is not sufficient to give the proper amount of stereoscopic relief for distant objects.

A board with moveable guides is therefore placed upon the camera-stand, as represented at Fig. 81, and two cross lines are drawn upon the ground glass of the camera. One marls the centre on one side; the second,

that on the other. The camera is then pointed in such a way that the same object in the landscape comes on the centre of the cross lines, for without this the proof would not be properly centred.

The two negatives then are taken separately, and require transposing when they are mounted on the card. But this reversing or transposition may be, however, accomplished in the instrument itself, so effectively, that when the negative is placed in the stereoscope, the objects appear in relief.

For this purpose, suppose the apparatus, Fig. 76, is in the position as represented in Fig. 81. Instead of taking with the right-hand lens the right-hand negative, and with the left-band lens the left-hand negative, let the operations be reversed. Take the left picture with the right lens, and vice versa--the right picture with the left lens.

When it is desired to take landscapes only, the camera represented in Fig. 78 is generally used. It is an ordinary quarter-plate camera, which is converted into a stereoscopic camera by the addition of three pieces of apparatus, two of which are represented in Figs. 79 and 80.

In Fig. 79 will be seen a grooved piece of wood, with two rebates and a catch in the middle of the upper one, and a ground glass sliding between them; this is stopped in the middle by the catch before mentioned. This piece of wood is screwed to an ordinary camera-back, the door of which is reversed.

Fig. 78. Camera for Stereoscopic Views.

Two diagonal lines are drawn upon the ground glass. When it is desired to take a stereoscopic view, the board with two moveable rebates, as represented in Fig. 81, is brought into use.

The following is the method of proceeding. The camera, Fig. 78, is first placed in one of its positions, and directed towards the view-then in the second position, and directed towards the same point. These two views will appear upon the ground glass exactly the same, but they are not so in reality.

Fig. 79. Frame with Ground Glass.

Fig. 80. Frame for Sensitive Plate.

The ground glass being withdrawn, the camera is placed on the right hand of the operator, who looks at the view which is before him. Replacing the focussing glass by the camera-back, which is introduced on the right side of the piece of wood (Fig. 79), it is obvious that it is that part of the glass which is on the left in the back which will be first uncovered, as is shown in Fig. 80. The elide being drawn, and the time of exposure having expired, the door is closed. Fig. 78 shows the position of the camera for the fast picture, that is to say, the right-hand one.

In the second position, that of the left, the camera-back is made to slide between the two rebates until it is checked by the brass catch; the second picture is then taken.

These directions seem complicated, but in practice they prove very simple. The principle being once understood, it is easy to follow the description.

There remains a third method of operating, which is still more difficult. When it is desired to take views of animated objects or groups, the necessary interval which elapses between the two positions of the camera, just described, renders it inapplicable for the purpose, as in all probability the objects must have moved, and the two views would not be alike. Under these circumstances two quarter-plate cameras are used, furnished with lenses whose focal lengths are exactly equal, and the operation is conducted as is shown in Fig. 81. Great care must be taken to open and close the two shutters at the same time, or at least very quickly one after the other, as it is obvious that if one plate be longer exposed than the other, the development. moat take place unequally, and the two proofs will be unequal also.

Fig. 81. Position of the two Cameras for Instantaneous Views.

In order that the two negatives should be exactly alike, the two plates should be collodionised at the same time, and immersed one after the other in the same nitrate-bath, from which they should be simultaneously withdrawn.

In reference to the development, a dish is need with a glass bottom and wooden sides, or of solid glass, and the two plates are placed therein, one by the side of the other, in order that both may be developed with the same pyrogallic acid. This is the only way to obtain proofs of nearly equal intensity, for it is, indeed, quite a chance that they should be exactly alike; a little difference will not, however, materially affect the result in printing.

Positive proof's should be reversed: the one which was taken on the right should be on the left in the card, and in order that this may always be done correctly, it is advisable to mark each of the negatives when taken separately.

The apparatus for cartes de visite, at page 39, is, in fact, nothing more than the apparatus, Fig. 76, with a back like Fig. 80. It consists of a camera to which four lenses of equal focal length have been adapted, furnished with a sliding back. From this arrangement it follows that when the lenses, are arranged as shown in Fig. 76, there will be eight negatives taken on each plate, and that each pair will be stereoscopic. The cost of the apparatus may, however, be materially lessened by using a whole-plate camera fitted with a pair of twin-lenses, as in Fig. 76, and arranged so as to slide together on its rising front; in this way four carte de visite pictures may be obtained on one plate. A number of negatives are taken on one plate, because the printing of an equal number of proofs separately would be much more costly and troublesome.

It remains to consider the proper angular distance which should separate the two cameras represented in Fig. 81, so as to produce the desired result.

If several square-based pyramids, of various heights, be placed successively on the same spot, and examined with one eye from a fixed point over and slightly to the side of the pyramids, it will be perceived that each time a pyramid is changed, that the relative position of the apex alters, and that this displacement is greater the taller the pyramid viewed, or in other words, the relief is more visible. If these pyramids be now viewed again with one eye, but this time not on one side, but directly on a line with their axes, they will, although all differing in height, nevertheless present the same appearance of a square cut with its diagonals.

Thus, each time that the height of the pyramid varies, the visual angle becomes greater, and the relief is consequently more striking.

But suppose for a moment that it is desired to reproduce the pyramid, what angle should be employed? should it be the angle of vision, or a larger one ?

In view of what has been said above, it is evident that if a larger angle be used, the pyramid will appear in the stereoscope higher than it really is, and as the photographic apparatus does not furnish any standard of comparison by means of an object of known height placed side by side, a false idea will be conveyed of the true size of the pyramid.

When the angle is too great, the object is distorted. This defect is, so to speak, very general in all reproductions of statues, portraits, &c. The relief in these cases is, with very few exceptions, greatly exaggerated. And thus it happens that when a statue is reproduced in this way, the bead often seems so much in advance, that it becomes really monstrous.

It should always be borne in mind when objects are to be reproduced which are of known size and proportions, such as statues, portraits, animals, &c., that. if a large angle be employed the image will be distorted.

For views and landscapes an excessive relief may be obtained without detriment to the picture, on the contrary, with a decided improvement in effect, at the same time attention must be had to the character of the view. If, for example, the foreground of the picture be very near the operator, a small angle (about 2°) must be used, otherwise the two pictures will not become superposed in the stereoscope. If, on the contrary, the view offers but little difference between the various planes, advantage may be taken of this, and a larger angle, 4° for example, may be employed.

The stereoscopic angle is reckoned from the nearest point of the view to be taken; this point forming the apex of the triangle, and the distance between the two cameras being the base, or rather the arc, of the angle.

When the operator has chosen his point of sight, the eye should estimate the difference between the cameras and the nearest point in the view to be taken, which will determine whether an angle of 2° or 4° be preferable. The following table will, to a certain extent, serve as a guide for beginners to judge the value of an angle, that of 2° being selected as an example:-