THE STEREOSCOPE : ITS THEORY. 



199 



A- 



_s- 



-B 



Fig. 13. Theory of Visual 

 Tri angulation . 



R C and L C, as the attention is successively directed to different 

 points in the field of view. If accepted at all, it must be accepted 

 fully. If we suppose the semi-lenses removed, and that R and L 

 together represent a binocular camera, the diagram shows the exact 

 relation between this and an object to be 

 pictured, and the admirable mathematical dis- 

 cussion which Helmholtz gives subsequently 

 in full is strictly applicable. But, if the ob- 

 server's eyes be too near together, or the stereo- 

 graphic interval be too great, the relation 

 between the visual lines ceases to be the same 

 as that between the camera axes, and we no 

 longer have the conditions under which the 

 geometric discussion can be applied. It is but 

 due to Helmholtz to add that he closes with 

 the following remark : " These conditions are 

 not generally fulfilled for the photographic 

 proofs and the stereoscopes of commerce." 

 The same credit can not be given to the 

 writers of the ordinary text-books. This quali- 

 fication is of the last importance, for without it the theory is absurd, 

 the apparent position of the image determined by intersection of 

 visual lines being behind the observers head when optic divergence 

 is induced, and at an infinite distance when they are parallel. But, 

 even when camera axes and visual lines bear the same relation among 

 themselves, the abnormal muscular condition necessitated in stereo- 

 scopic vision introduces a disturbing element. The theory is hence 

 not applicable at all to the stereoscope, but must be limited to the dis- 

 cussion of the binocular camera. 



With a view to enabling persons with untrained eyes easily to per- 

 form many of the experiments through which variation in appearance 

 of the binocular image is produced by varying the conditions un- 

 der which the same stereograph is viewed, the writer has devised an 

 adjustable stereoscope (Fig. 14), which presents the additional very 

 important advantage of rendering vision as nearly painless as it can 

 be with the ideal stereograph, even although the stereographic inter- 

 val on the one employed be so great as to produce only confusion, or 

 strain of the eyes, when the common form of stereoscope is used. 

 Instead of being fixed in position, the semi-lenses are lightly rested in 

 a pair of boxes, with openings in front and rear so as to transmit the 

 light. Attached to the partition between them are a pair of springs 

 against which the thin edges of the semi-lenses are pressed by ad- 

 justing-screws in contact with their thick bases. By turning these so 

 that the glasses are pressed as close as possible together, the light 

 which enters the eyes passes through the thicker part of each glass, 

 where the planes^ that may be supposed to touch the opposite curved 



