STEREOSCOPE. (Gr. erEpEoc solid, crow*, I see) This is an instrument for exhibiting two plane perspective views of an object, taken from different stations, as one having the appearance of solidity. It is the invention of Professor Wheatstone, and was first made public and the theory of it explained by him in the year 1839. At that time the pictures were exhibited by means of reflectors, but the inventor suggested that lenses might be used instead. Some years after this, an instrument was brought out by Sir David Brewster, in which small photographic pictures are placed in a dark box, and viewed through half-lenses mounted in tubes ; and, soon after the introduction of that very imperfect instrument, Messrs. Knight, of Foster Lane, patented an improved form of stereoscope, in which large semi-lenses are used, mounted in the front of the instrument, without tubes ; and this, in the course of time, became so generally preferred to the other, as to supersede it to a great extent. But it was soon found that the eemi-lenses, whether large or small, pro duced an unbearable amount of distortion in the solid picture, making straight lines look as if they were concave to the spectator ; so, in order to obviate this evil, whole lenses were tried, and these were found to answer in certain cases. But as the theory of the lenticular stereoscope is even now but little understood by opti cians or the public at large, it was not perceived that, in order to render that form of instrument perfect, it was necessary not only to use whole lenses to get rid of the distortion, but to take the pictures in a camera suitably constructed, and mount them pro perly. The popular form of the stereoscope as manufactured by Mr. Knight, and called by him the Cosmorama Stereoscope, is however, notwithstanding its defects of principle, a very amusing instrument, and it is not till the spectator has become familiar with the true principles of stereoscopic representation that the defects are perceived. We shall endeavour, then, in the present article to explain the theory of the stereoscope in a clear and intelligible manner. But the reader must first consult and study the article on " Binocular Vision," for it is here assumed that the principles of binocular vision are clearly understood.
Theory of the Stereoscope.
The theory of the stereoscope consists in showing—first, how the instrument may be employed to represent things truthfully as we see them in nature ; secondly, how it may be employed to repre sent them falsely as we should see them if our eyes were wider apart ; and, thirdly, to explain how it happens that distortion is introduced by using an instrument of improper construction— straight lines being represented by curves, and so on.
Let L, R be the eyes of a spectator; A, C, B lamp posts of different heights having lights, or luminous points, A, B, C, at the top. Draw the visual rays L A, L C, L B, R A, R C, R B. Cut them by a vertical plane P Q parallel to the line which joins L and R. Then the points a,c,b, a,c,b, where the visual rays pass through this plane will be the images of A, B, C, as seen from the stations L and R, P Q, being supposed to be the plane of a perspective picture. (See " Perspective.") If the plane P Q be placed as shown in the figure, so that the perspective view as seen from L may be completely exterior to that seen from R, the pictures will be as represented on a card beneath L and R. (See the Lower Part of the Fig,ure.) Now if the perpendicular distance between L and the plane P Q be such that the points a,b,c, can be distinctly seen by an eye at L, —that is to say if this perpendicular distance be not too short for distinct vision, and if we place the card P Q before the eyes, as in the fig-ure, the right picture being cut off from the left eye and the left picture from the right eye by a partition, as shewn in the figure, then instead of seeing two pictures abc, abc, only one image will be seen, and that one image will appear to be the lights A,B,C in their na tural position, and at their true distance. For when the left eye is directed to a along the line La, the right eye is directed to a alono• the line Ra, and the optic axes L a, R a, being produced meet at A°, so that the spectator instead of seeing two images a,a, upon a plane P Q sees one image A at the true distance and in the true position of the light A. Similarly with respect to the other images b,c,b,c, which combine and produce single images at B and C.