| THE STEREOSCOPE: ITS THEORY. |
By W. LE CONTE STEVENS.
II.
ALL of the forms under which the stereoscope has come into general use have been devised with a view to creating to the utmost the illusion of natural binocular perspective by reproducing as nearly as possible the conditions of natural vision. That this end is not successfully attained is painfully felt by those who linger too long over an attractive collection of stereographs. To secure comfortable vision the muscles of the eyes must suffer no unusual strain. It is not easy to explain briefly how such strain is necessarily implied in the use of this instrument. Suffice it to say that, in looking at a point a few inches distant, the ciliary muscle which surrounds the crystalline lens in each eye is strongly contracted, and so is the muscle on the inner side of each eyeball. These contractions usually accompany each other, and to dissociate them is always more or less painful. The stereograph is but a few inches distant, but, because there are two pictures, the convergence of the visual lines is much less than normal; indeed, optic divergence is not unfrequently necessary. The unconscious interpretation which is put upon the retinal sensation is due partly to imagination; but also largely to the temporary condition of the muscles of the eyes. This includes not only the ciliary but also the rectus muscles, external and internal, by which the eyeballs are controlled, as the angle between the visual lines is varied. The effect of varying this angle is best studied with a modification of Wheatstone's stereoscope, which the writer has constructed for this purpose. A pair of conjugate pictures are chosen, which present as little as possible of mathematical perspective. A stereograph of the moon, divided at the middle, is one of the best for this purpose. The twin photographs are placed upon cross-bars (Fig. 12) which rest on graduated arms that are pivoted at the proper point in the base of a cubical block to which the mirrors are cemented. These arms move in contact with part of a circle, marked off in degrees at the circumference, the center of this being in the pivot. If the two arms make a straight line, and the pictures are properly adjusted, the visual lines must be parallel, for the eyes to receive the reflected rays. If pulled forward toward the observer, the visual lines must converge in order to retain single vision, and the angle of convergence is at once obtained from the circle. If pushed slightly back, as represented in the figure, single vision can be retained only by optic divergence. Most eyes that are healthy will be found capable of enduring a few degrees of such divergence. The real distance of the object is thus kept unchanged, and the card appears always directly across the visual line. The varia-
