195 



ducing on the ground glass the coalescence of the images of the two 

 sides of a slide placed before the camera. 



The slide itself being cut in two parts, the two images can also, 

 moving in a groove, be separated in a horizontal direction, until 

 they are sufficiently apart to be refracted on the ground glass by 

 the two lenses in the most oblique direction consistent with the pro- 

 duction of a well-defined image ; for it is to the increased degree of 

 obliquity of the refracted rays in falling on the ground glass that is 

 due the more effective extinction or evanescence of the image for the 

 eye whose axis consequently deviates in a greater degree from the 

 line of refraction. 



By the same principles which produce the phenomenon of relief 

 of the image formed on the ground glass of the camera obscura, the 

 right picture of the slide, being obliquely refracted on $e ground 

 glass by the right lens in a line coinciding with the axis of the left 

 eye, is visible only to that eye ; and the left picture, being refracted 

 obliquely by the left lens in an opposite direction coinciding with the 

 right eye, is only visible to that eye. Consequently each eye seeing 

 only one image, and that image having its own perspective, the optic 

 axes have to converge more or less according to the angular separa- 

 tion of the similar points of the two coincident images ; and by the 

 different degrees of convergence producing single vision of these 

 various similar points, we have the sensation of the comparative 

 distances of the objects represented on the ground glass. 



Before having constructed this new stereoscope and tried its effect,, 

 it would have been hasty on my part to pretend that its success was 

 certain,, and for this reason I took care in my former paper to propose 

 it as a mere speculative idea suggested by the phenomenon I had 

 discovered, without vouching for the result. Indeed it was not long 

 before I had to congratulate myself on my caution, when I found that, 

 the truth of my experiments being questioned and the deductions 

 from these experiments denied, my proposed stereoscope was declared 

 impossible, as being founded on principles completely at variance with 

 the laws of optics. 



However, these remarks did not shake my conviction, and after 

 the usual difficulties, I have now the satisfaction of being able to 

 prove that I was perfectly right, and that I had not been led astray 

 by any erroneous notion in my analytic and synthetic deductions. 



