THE STEREOSCOPE : ITS THEORY. 197 



THE STEREOSCOPE: ITS THEOKY. 



By W. LE CONTE STEVENS. 



II. 



ALL of the forms under which the stereoscope has come into gen- 

 eral 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 uncon- 

 scious 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 circum- 

 ference, 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- 



