Trans. N. Y. Ac. Sci. 11<^) Feb. 13, 



Journal of Scie?tce. (Mr. Stevens here read an extract from the 

 original paper, published by Professor HiMES, in \.\\& American Journal 

 ,0/ P/wlO£-rap^j ior September I, 1862.) Shortly before the appear- 

 ance of this paper, two Germans, Rollet and Becker,'' published 

 a method of securing binocular fusion of similar images with optic 

 divergence, the possibility of such fusion having been already mentioned 

 by BURCKHARDT. Professor Himes' experiments were doubtless con- 

 temporaneous with those of the Germans just named, and were wholly 

 independent of them. 



" My own discovery of the possibility of stereoscopic vision by optic 

 divergence was likewise independent. I subsequently learned of the 

 previous experiments on this subject by ROLLET, Becker, and Helm- 

 HOLTZ, and duly credited them in my paper before this Academy on 

 the 6th of last June. Professor HiMES' papers were sent me after mine 

 had been published in the American Journal of Science for last Novem- 

 ber. He deserves the credit of being the first in the country to secure 

 stereoscopic vision with conscious optic divergence. My own claim is 

 based rather upon the analysis than the discovery of this mode of 

 stereoscopy. For years past, oculists have subjected their patients to 

 the use of prisms for the purpose of testing the strength of the external 

 rectus muscles of the eyeballs, in diverging the visual lines while the 

 eyes receive, from an object in front, rays of light that are refracted by 

 transmission through the glasses. In view of this fact, it is remarkable 

 that Brewster's geometric theory of stereoscopic vision should still 

 hold its place in our text-books of physics. His theory of color has 

 been abandoned, and his theory of binocular perspective is awaiting the 

 same fate. 



" In regard to the phenomenon of the apparent enlargement of the 

 moon when seen at the horizon, Professor Hlmes' stereograph gives 

 an excellent illustration, superior to that of Brewster, ^ because the 

 two parts, which are to be contrasted in the external projection of the 

 binocular image, are more nearly aligned, and no motion of the stereo- 

 graph is necessary. While the angular diameter of the moon remains 

 nearly constant, being slightly greater when near the zenith, because the 

 observer's true distance is diminished by nearly the earth's semi-diame- 

 ter, its apparent magnitude varies with its estimated distance, and it 

 appears much smaller when near the zenith, instead of larger. The 

 cause of this illusion of distance, producing an illusion in regard to size, 

 which is well illustrated by the present stereograph, has been the sub- 

 ject of much discussion. In so late a book as Lockyer's Astronomy,^ 



2. Helmholtz, Optique Physiologique, pp. 827, 828. 



3. The Stereoscope. London, 1856, p. 201 et seg. 



, Elements of Astronomy, Appleton & Co., N. Y., 1873, p. 116. 



