SECTION V.. _on Binocular Vision and the Stereoscope._

On Binocular Vision and the Stereoscope.

An object is said to be "stereoscopic" (στρεοϛ solid, and σκοπεω, I see) when it stands out in relief, and gives to the eye the impression of solidity.

This subject was first explained by Professor Wheatstone in a memoir on binocular vision, published in the 'Philosophical Transactions' for 1838; in which he shows that solid bodies project different perspective figures upon each retina, and that the illusion of solidity may be artificially produced by means of the "Stereoscope."

The phenomena of binocular vision may be simply sketched as follows:--If a cube, or a small box of an oblong form, be placed at a short distance in front of the observer, and viewed attentively with the right and left eye separately and in succession, it will be found that the figure perceived in the two cases is different; that each eye sees more of one side of the box, and less of the other; and that in neither instance is the effect exactly the same as that given by the two eyes employed conjointly.

A silver pencil-case, or a pen-holder, may be used to illustrate the same fact. It should be held at about six or eight inches distant from the root of the nose, and quite at right angles to the face, so that the length of the pencil is concealed by the point. Then, whilst it remains fixed in this position, the left and right eye are to be alternately closed: in each case a portion of the opposite side of the pencil will be rendered visible.

The preceding diagrams exhibit the appearance of a bust as seen by each eye successively.

Observe that the second figure, which represents the impression received by the right eye, is more of a full face than fig. 1, which, being viewed from a point removed a little to the left, partakes of the character of a profile.

The human eyes are placed about 2-1/2 inches, or from that to 2-5/8 inches, asunder; hence it follows that, the points of sight being separated, a dissimilar image of a solid object is formed by each eye. We do not however see two images, but a single one, which is stereoscopic.

In looking at a picture painted on a flat surface the case is different: the eyes, as before, form two images, but these images are in every respect similar; consequently the impression of solidity is wanting. A single picture, therefore, cannot be made to appear stereoscopic. To convey the illusion two pictures must be employed, the one being a right and the other a left perspective projection of the object. The pictures must also be so arranged, that each is presented to its own eye, and that the two appear to proceed from the same spot.

The reflecting stereoscope, employed to effect this, forms luminous images of the binocular pictures, and throws these images together, so that, on looking into the instrument, only a single image is seen, in a central position. It should, however, be understood, that no optical arrangement of any kind is indispensably required, since it is quite possible, with a little effort, to combine the two images by the unaided organs of vision. The following diagram will make this obvious:--

The circles A and B represent two wafers, which are stuck on paper at a distance of about three inches from each other. They are then viewed by squinting strongly, or turning the eyes inwards towards the nose, until the right eye looks at the left wafer, and the left eye at the right wafer. Each wafer will then appear to become double, four images being seen, the two central of which will gradually approach each other until they coalesce. Stereoscopic pictures, properly arranged, may be examined in the same manner; and it will be found that the resultant solid image is formed midway, at a point where two lines, drawn across from the eyes to the pictures, cut one another. The experiment here mentioned is sometimes a painful one, and cannot easily be made if the eyes are not of equal strength; but it will serve to show that the essential principle resides in the binocular representation of the object, and not in the instrument employed to view it.

In Mr. Wheatstone's reflecting Stereoscope mirrors are used. The principle of the instrument is as follows:-- objects placed in front of a mirror have their reflected images apparently behind the mirror. By arranging two mirrors at a certain inclination to each other, the images of the double picture may be made to approach until they coalesce, and the eye perceives a single one only. The following diagram will explain this.

The rays proceeding from the star on either side pass in the direction of the arrows, being thrown off from the mirror (represented by the thick black line) and entering the eyes at R and L. The reflected images appear behind the mirror, uniting at the point A.

The reflecting Stereoscope is adapted principally for viewing large pictures. It is a very perfect instrument, and admits of a variety of adjustments, by which the apparent size and distance of the Stereoscopic image may be varied almost at pleasure.

The "lenticular" Stereoscope of Sir David Brewster is a more portable form of apparatus. A sectional view is given in the diagram.

The brass tubes to which the eyes of the observer are applied contain each a semi-lens, formed by dividing a common lens through the centre and cutting each half into a circular form (fig. 1 in the following page). The half-lens viewed in section (fig. 2) is therefore of a prismatic shape, and when placed with its sharp edge as in the diagram above, alters the direction of the rays of light proceeding from the picture, bending them outwards or away from the centre, so that in accordance with well-known optical laws they appear to come in the direction of the dotted lines in the diagram (in the last page), and the two images coalesce at their point of junction. In the instrument as it is often sold, one of the lenses is made movable, and by turning it round with the finger and thumb it will be seen that the positions of the images may be shifted at pleasure.

Fig. 1.

Fig. 2.

Rules for taking Binocular Photographs.--In viewing very distant objects with the eyes, the images formed on the retinæ are not sufficiently dissimilar to produce a very Stereoscopic effect; hence it is often required, in taking binocular pictures, to separate the Cameras more widely than the two eyes are separated, in order to give a sufficient appearance of relief. Mr. Wheatstone's original directions were, to allow about one foot of separation for each twenty-five feet of distance, but considerable latitude may be permitted.

If the Cameras be not placed far enough apart, the dimensions of the stereoscopic image from before backwards will be too small,--statues looking like bas-reliefs, and the circular trunks of trees appearing oval, with the long diameter transverse. On the other hand, when the separation is too wide, the reverse obtains,--objects for instance which are square, assuming an oblong shape pointing towards the observer.

To understand the cause of this, the following law in optics should be studied:--"The distance of objects is estimated by the extent to which the axes of the eyes must be converged to view them." If we have to turn our eyes strongly inwards, we judge the object to be near; but if the eyes remain nearly parallel, we suppose it to be distant.

The above figures represent six-sided truncated pyramids, each with its apex towards the observer, the centres of the two smaller interior hexagons being more widely separated than those of the larger exterior ones. By converging the eyes upon them so as to unite the central images in the manner represented in page 68 a greater amount of convergence will be required to bring together the two summits than the bases, and hence the summits will appear the nearest to the eye; that is to say, the resultant central figure will acquire the additional dimension of height, and appear as a solid cone, standing perpendicularly upon its base: further, the more widely the summits are separated in relation to the bases, the taller will the cone be, although a greater effort will be required to coalesce the figures.

Binocular Photographs taken with too much separation of the Cameras, are distorted from a similar cause,--so strong a convergence being required to unite them that certain parts of the picture appear to approach near to the eye; and the depth of the solid image is increased.

This effect is most observable when the picture embraces a variety of objects, situated in different planes. In the case of views which are quite distant, no near objects being admitted, the Cameras may be placed with especial reference to them, even as far as twelve feet apart, without producing distortion.

It is sometimes observable, in looking at Stereoscopic pictures, that they convey an erroneous impression of the real size and distance of the object. For instance, in using the large reflecting Stereoscope, if, when the adjustments have been made and the images properly united, the two pictures be moved slowly forward, the eyes remaining fixed upon the mirrors, the Stereoscopic image will gradually change its character, the various objects it embraces appearing to become diminished in size, and approaching near to the observer: whilst if the pictures be pushed backwards, the image will enlarge and recede to a distance. So, again, if an ordinary slide for the lenticular Stereoscope be divided in the centre, and, looking into the instrument until the images coalesce, the two halves be slowly separated from each other, the solid picture will seem to become larger and to recede from the eye.

It is easy to understand the cause of this. When the pictures in the reflecting Stereoscope are moved forwards, the convergence of the optic axes is increased: the image therefore appears nearer, in accordance with the last-mentioned law. But to convey the impression of nearness is equivalent to an apparent diminution in size, for we judge of the dimensions of a body very much in relation to its supposed distance. Of two figures, for instance, appearing of the same height, one known to be a hundred yards off might be considered colossal, whilst the other, obviously near at hand, would be viewed as a statuette.

These facts, with others not mentioned, are of great interest and importance, but their further consideration does not fall within the bounds originally prescribed to us. The practical details of Stereoscopic Photography have been arranged in a distinct Section, and will be found included in the Second Part of the Work.

For a more full and detailed explanation of the Stereoscopic phenomena, see an abstract of Professor Tyndall's lectures in the third volume of the 'Photographic Journal.'