The stereoscope

by David Brewster

As an auxiliary in the investigation of questions of difficulty and importance, both in physics and metaphysics, the stereoscope is peculiarly valuable. It enables us to place in its true light the celebrated theory of vision on which Bishop Berkeley reared the ideal philosophy, of which he was the founder, and it gives us powerful aid in explaining many physical phenomena which have long baffled the ingenuity of philosophers. It would be out of place to give any account of these in a work like this, but there is one so remarkable, and at the same time so instructive, as to merit special notice. In order to exhibit, by means of three diagrams, a solid in relief and hollow at the same time, which had not been previously done, I executed three drawings of the frustum of a cone, resembling those in [Fig. 31], so that the left-hand one and the middle one gave the hollow cone, while the middle one and the right-hand one gave the raised cone. Having their summits truncated, as in the figure, the cones exhibit, in the one case, a circle at the bottom of the hollow cone, and in the other, a circle on the summit of the raised cone. When these three diagrams[68] are placed in an open lenticular stereoscope, or are united by the convergency of the optical axes, so that we can not only see the hollow and the raised cones, but the flat drawing on each side of them, we are enabled to give an ocular and experimental proof of the cause of the large size of the horizontal moon, of her small size when in the meridian or at a great altitude, and of her intermediate apparent magnitude at intermediate altitudes,—phenomena which had long perplexed astronomers, and which Dr. Berkeley, rejecting previous and well-founded explanations, ascribed to the different degrees of brightness of the moon in these different positions.

As the circular summit of the raised cone appears to be nearest the eye of the observer, the summit of the hollow cone farthest off, and the similar central circle in the flat drawing on each side, at an intermediate distance, the apparent distances from the eye of different and equal circles will represent the apparent distance of the moon in the zenith, or very high in the elliptical celestial vault,—the same distance when she is in the horizon, and the same when at an intermediate altitude. Being in reality of exactly the same size, and at the same distance from the eye, these circular summits, or sections of the cone, are precisely in the same circumstances as the moon in the three positions already mentioned. If we now contemplate them in the lenticular stereoscope, we shall see the circular summit of the hollow cone the largest, like the horizontal moon, because it seems to be at the greatest distance from the eye,—the circular summit of the raised cone the smallest, because it appears at the least distance, like the zenith or culminating moon,—and the circular summits of the flat cones on each side, of an intermediate size, like the moon at an intermediate altitude, because their distance from the eye is intermediate. The same effect will be equally well seen by placing three small wafers of the same size and colour on the square summits of the drawings of the quadrangular pyramids, or more simply, by observing the larger size of the square summit of the hollow pyramid.

This explanation of the cause of the increased size of the horizontal moon is rigorously correct. If any person should suspect that the circles which represent the moon are unequal in size, or are at different distances from the eye, they have only to cut the diagram into three parts, and make each drawing of the frustum of the cone occupy a different place in the binocular slide, and they will obtain the very same results. Hence we place beyond a doubt the incorrectness of Dr. Berkeley’s theory of the size of the horizontal moon,—a theory to which the stereoscope enables us to apply another test, for if we make one or more of these circles less bright than the rest, no change whatever will be produced in their apparent magnitude.

CHAPTER XIV.
APPLICATION OF THE STEREOSCOPE
TO PURPOSES OF AMUSEMENT.

Every experiment in science, and every instrument depending on scientific principles, when employed for the purpose of amusement, must necessarily be instructive. “Philosophy in sport” never fails to become “Science in earnest.” The toy which amuses the child will instruct the sage, and many an eminent discoverer and inventor can trace the pursuits which immortalize them to some experiment or instrument which amused them at school. The soap bubble, the kite, the balloon, the water wheel, the sun-dial, the burning-glass, the magnet, &c., have all been valuable incentives to the study of the sciences.

In a list of about 150 binocular pictures issued by the London Stereoscopic Company, under the title of “Miscellaneous Subjects of the ‘Wilkie’ character,” there are many of an amusing kind, in which scenes in common life are admirably represented. Following out the same idea, the most interesting scenes in our best comedies and tragedies might be represented with the same distinctness and relief as if the actors were on the stage. Events and scenes in ancient and modern history might be similarly exhibited, and in our day, binocular pictures of trials, congresses, political, legislative, and religious assemblies, in which the leading actors were represented, might be provided for the stereoscope.

For the purpose of amusement, the photographer might carry us even into the regions of the supernatural. His art, as I have elsewhere shewn, enables him to give a spiritual appearance to one or more of his figures, and to exhibit them as “thin air” amid the solid realities of the stereoscopic picture. While a party is engaged with their whist or their gossip, a female figure appears in the midst of them with all the attributes of the supernatural. Her form is transparent, every object or person beyond her being seen in shadowy but distinct outline. She may occupy more than one place in the scene, and different portions of the group might be made to gaze upon one or other of the visions before them. In order to produce such a scene, the parties which are to compose the group must have their portraits nearly finished in the binocular camera, in the attitude which they may be supposed to take, and with the expression which they may be supposed to assume, if the vision were real. When the party have nearly sat the proper length of time, the female figure, suitably attired, walks quickly into the place assigned her, and after standing a few seconds in the proper attitude, retires quickly, or takes as quickly, a second or even a third place in the picture if it is required, in each of which she remains a few seconds, so that her picture in these different positions may be taken with sufficient distinctness in the negative photograph. If this operation has been well performed, all the objects immediately behind the female figure, having been, previous to her introduction, impressed upon the negative surface, will be seen through her, and she will have the appearance of an aerial personage, unlike the other figures in the picture. This experiment may be varied in many ways. One body may be placed within another, a chicken, for example, within an egg, and singular effects produced by combining plane pictures with solid bodies in the arrangement of the persons and things placed before the binocular camera. Any individual in a group may appear more than once in the same picture, either in two or more characters, and no difficulty will be experienced by the ingenious photographer in giving to these double or triple portraits, when it is required, the same appearance as that of the other parties who have not changed their place. In groups of this kind curious effects might be produced by placing a second binocular slide between the principal slide and the eye, and giving it a motion within the stereoscope. The figures upon it must be delineated photographically upon a plate of glass, through which the figures on the principal slide are seen, and the secondary slide must be so close to the other that the figures on both may be distinctly visible, if distinct vision is required for those which are to move.

Another method of making solid figures transparent in a photograph has been referred to in the [preceding chapter], and may be employed in producing amusing combinations. The transparency is, in this case, produced by using a large lens, the margin of which receives the rays which issue from bodies, or parts of bodies, situated behind other bodies, or parts of bodies, whose images are given in the photograph. The body thus rendered transparent must be less in superficial extent than the lens, and the body seen through it must be so far behind it that rays emanating from it would fall upon some part of the lens, the luminosity of this body on the photograph being proportional to the part of the surface of the lens upon which the rays fall. This will be readily understood from [Figs. 48] and [49], and their description, and the ingenious photographer will have no difficulty in producing very curious effects from this property of large object-glasses.

One of the most interesting applications of the stereoscope is in combining binocular pictures, constructed like the plane picture, used in what has been called the cosmorama for exhibiting dissolving views. These plane pictures are so constructed, that when we view them by reflected light, as pictures are generally viewed, we see a particular scene, such as the Chamber of Deputies in its external aspect; but when we allow no light to fall upon it, but view it by transmitted light, we see the interior of the building brilliantly lighted up, and the deputies listening to the debate. In like manner, the one picture may represent two armies in battle array, while the other may represent them in action. A cathedral in all its architectural beauty may be combined with the same building in the act of being burned to the ground; or a winter scene covered with snow may be conjoined with a landscape glowing with the warmth and verdure of summer. In the cosmorama, the reflected light which falls upon the front of the one picture is obtained by opening a lid similar to that of the stereoscope, as shewn at CD, [Fig. 14], while another lid opening behind the picture stops any light which might pass through it, and prevents the second picture from being seen. If, when the first picture is visible, we gradually open the lid behind it, and close the lid CD before it, it gradually disappears, or dissolves, and the second picture gradually appears till the first vanishes and the second occupies its place. A great deal of ingenuity is displayed by the Parisian artists in the composition of these pictures, and the exhibition of them, either in small portable instruments held in the hand, or placed on the table, or on a great scale, to an audience, by means of the oxygen and hydrogen light, never fails to excite admiration.

The pictures thus exhibited, though finely executed, have only that degree of relief which I have called monocular, and which depends on correct shading and perspective; but when the dissolving views are obtained from binocular pictures, and have all the high relief given them by their stereoscopic combination, the effect must be singularly fine.