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 372 which disappears immediately upon opening the other eye; plainly proving that judgment and experience, without any optical sense of relief, were guiding your first efforts. A similar uncertainty will be experienced on endeavouring to approach and snuff a candle with one eye shut. But to return to our argument.

When once it became clearly understood that in natural vision it was the two pictures carried to the brain from the retinæ, which were by their coalescence the cause of the perception of relief, an inference was soon made that if it were by any means possible to produce on paper a faithful copy of either image as seen by each eye separately, these pictures might on being properly viewed be naturally expected to coincide as their originals do, and give a similar sensation of relief. On experiment this actually proved to be the case. There is no need now to enter upon a description of the first approximately correct drawings which were made to test the truth of the theory; they were produced at a period just prior to the rapid modern development of photography, and it is not therefore surprising that this art was soon brought to bear upon them. Without photographic aid, indeed, the stereoscope could never be more than a piece of philosophical machinery, useless out of the study or lecture-room, inasmuch as nothing but the camera could possibly produce pictures of the requisite accuracy. To give one illustration, however, of the means by which the draughtsman’s art may be made to prove the truth of the binocular theory, we take the following simple figure:

Here A A represent the eyes; B, a square based cone cut off at the top, which will be seen with either eye, as shown by the lines of sight.

Proceeding now to draw the two figures mechanically as they appear at both A and A, and doubling the size of B, for the sake of greater distinctness, we get the following result:

two pictures which, when viewed in the manner immediately to be described, will form a single image, having all the appearance of perfect solidity.

It being now clearly understood that the common stereoscopic slide is nothing more nor less than two exact representations of any given view or object as we naturally see it with our two eyes, and that these pictures need only to be looked at, one with the right and one with the left eye, to ensure all the conditions necessary for the production of a sense of distance and relief, I proceed to describe the means by which any person having eyes of tolerably equal value as regards sight, may be enabled to make the images combine and get the stereoscopic effect.

On looking casually at a slide, or any other object, the natural tendency is of course to concentrate the axes of both eyes upon particular points of it successively; so rapidly is this done, that the whole surface of the picture is traversed instantaneously, and the general appearance gathered in a moment. This disposition to concentration of the optic axes is so strong, that some little practice will be required to overcome it, and make each eye act independently of the other with readiness, notwithstanding the assistance which will be given by the following plan. Procure a stereoscopic slide, consisting of a group of statuary, or any subject having a single, strongly-marked, and prominent feature, with considerable natural relief; bore two holes three-eighths of an inch diameter, and two and a quarter inches apart, through the centres of each photograph, as in the sketch below:

Then, sitting at one end of the room and in a good light, select some object, such as a vase or book, about twelve feet distant, hold the perforated slide close to the eyes, and look steadily at the thing determined upon through the holes, as through a pair of spectacles.

Now gradually remove the cardboard from the face, taking care to preserve its level, and keeping the eyes still fixed upon the distant object. As the picture is withdrawn, the two perforations will be found to coalesce, and form a central hole between two others, through which the vase or book selected will be seen; the appearance then presented by the slide will be as in the diagram:

for it follows that, having succeeded in bringing the holes drilled through the centres of the photographs into coincidence, the pictures must coincide as well and form a central image, over which, if the eyes are allowed to wander without changing the inclination of their axes, a perfectly solid stereoscopic view will be the result. This, however, will not be obtained all at once. On the first few trials, it is probable that so soon as the eyes are released from their allegiance to the distant point of convergence, and brought to dwell upon the slide, the old natural instinct of concentration upon the object viewed will instantly return, the central picture vanish, and the experiment become a failure. When this happens, repeat the operation described, always beginning with the slide used spectacle-wise, close to the face, and then gradually removed: in the course of half-a-dozen efforts the relieved image