T

HE only ground for expectation of a persistent and general popular interest in the stereoscope must be found in its practical value, and a unique practical value, as an optical aid. It must be realized, as its raison d'être, that something can be done with it that cannot be done without it, at least by the average individual, and that it can be done easily and comfortably, and that this something is of high value, and continual occurrence, not simply curious and of passing interest. Only after the instrument has experimentally demonstrated its practical value, the optical and physiological conditions. involved and psychological speculations in regard to it, acquire an interest for most persons. For this reason, in a previous article,* experiments, interesting in themselves and readily performed by any one, were suggested and illustrated, by which the fundamental facts were emphasized upon which the value of the stereoscope for

representation, education, record, and investigation depends. With these facts well in mind, not only will intelligent use of the instrument be promoted, but even the tyro will be apt to find himself experimenting with new subjects for its application. The first of these facts, verified by a simple experiment, is that the two eyes give us unmistakable impressions of relief solidity, position, that one eye cannot give. The other fact, also verifiable by a suggested

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Hugo Tollens.

experiment, and illustrated by camera pictures of a characteristic subject, is, that there is a difference, slight but very appreciable, between the pictures of the same object given by the separate eyes, so that we can speak of right-eye and left-eye pictures of an object. And, further, it appears that it is by reason of this very dissimilarity of the pictures that the two eyes give us infallible perception of solidity. The invention of the stereoscope consisted in the recognition of these two facts, and the additional thought, as it occurred to Wheatstone, that if a picture on the flat of an object as seen by the right eye could be presented to the right eye alone, and at the same time a picture of the same object as seen by the left eye could be presented to the left eye alone, the same impression of solidity should be produced as when the object itself was looked at with both eyes at once, since the impressions on the retinas of the two eyes would be the same in both cases. He made diagramatic drawings of a solid, as it would appear to the right and left eyes respectively, and he devised an instrument by aid of which the right eye was permitted to look at the right eye picture alone, and the left eye at the left eye picture at the same time. The result verified his expectations in the highest degree. He called the instrument a stereoscope. But the thought that preceded it was more than the instrument, for the latter, with practice, can be entirely dispensed with. The presentation of the discovery to the British Association for the Advancement of Science in 1838 created a decided sensation. Sir David Brewster saw in it a complete explanation of vision in three dimensions, and Sir John Herschel characterized it as "one of the most curious and beautiful for its simplicity in the entire range of experimental optics." But the difficulty, in many cases impossibility, of making suitable drawings gave it at the time an almost purely scientific value. The advent of photography, capable of furnishing pictures meeting fully all requirements, and of a great variety of subjects, soon popularized it. It was soon found everywhere. It was natural that to many it should have been simply a curious optical instrument without permanent interest. It is true that at present it is not utilized to its fullest extent, and is not found in many places where it could be expected. But these who speak deploringly of the decadence of the stereoscope, and the desirability of its revival are hardly aware of the activity of the stereoscopic trade, and the very

large output of stereograplis, and that in spite of the fact that the instrument retains the form of half a century ago. It is safe to say that at no period has the commercial production of stereographs approached that of to-day. The mass of the amateurs of to-day, it is true, have not as yet taken kindly to it. For this there may be many reasons, but prominent among them is a magnified impression of innate difficulties involved in taking and mounting stereographs, and in many cases also a want of knowledge of its peculiar possibilities. As to the difficulties they will become trifling to the intelligent amateur, with his wonderful capacity to take trouble, as soon as he fully realizes, not simply assents to, the fact that no single picture, on the flat, can in any way be made to give the same effect, the simulation of solidity, afforded by the two dissimilar pictures of the stereograph. This dissimilarity seems so slight that dealers not only sometimes are tempted to mount identical pictures from one negative as stereographs, and in some cases even of paintings, but they are encouraged to do so by the ready sale they often command, even to purchasers aware of their character. A few illustrations may serve to make this fundamental fact clear. To take a very simple case in Fig. 1, either

H

mote.

FIG. 1

H

diagram may be regarded as a geometrical drawing of a solid, with the face H more reBut on continued looking at it, accompanied by an exercise of the will and imagination, in some way, not to be described or determined, it passes into another solid, with the face H in front, and thus it can be made to represent in turn either solid. This experiment credited by Sir David Brewster to Professor Neckar, and elaborately discussed by him, is one that every tyro in solid geometry has discovered, to his annoyance, for himself. The simple fact is, that it is a drawing that would be made by one eye of either of the solids. It seems a sufficent explanation of the appearance of one, at first, rather than the other, that it is in the most natural or usual position. With the plane