j882. 11^^^ Ti-ans. N. Y . Ac. Set. 



glasses or for binocular combination of images by direct vision, and to re- 

 duce the difficulty usually attendant upon stereoscopic vision by the 

 latter method. 



VI. To secure the means of producing stereoscopy from perfectly 

 similar pictures by making the retinal images of these dissimilar. 



The construction and manipulation of the instrument was illustrated 

 in full; but a description of this is not given in the present abstract, 

 because already furnished to the American Journal of Sczejice. A 

 printed description also accompanies each instrument, as issued by the 

 manufacturers, Messrs. E. & H. T. Anthony & Co., 591 Broadway. 



The feature in bmocular vision, mentioned in paragriph VI. above, 

 has not been explained hitherto, and has been but rarely perceived. If 

 an object possessing three dimensions in space be held within a short 

 distance and viewed alternately by the right and left eyes, the retinal 

 images of it at these different standpoints are necessarily dissimilar. 

 On this principle depends the whole art of stereoscopy, as illustrated 

 with the instruments and stereographs in ordinary use. Each of the 

 latter consists of two pictures of an object, taken from different points, 

 so as to secure dissimilarity. The binocular combination of their 

 retinal images hence presents the appearance of solidity, independently 

 of any perspective effect secured by art. 



If a large plane surface, on which are drawn similar figures regularly 

 recurring at equal distances apart, be appropriately viewed wnth very 

 strong cross vision, so that a phantom image, reduced in size, is seen in 

 mid-air, the latter appears slightly curved, with the convexity toward 

 tke observer. This mere fact was first noticed by Sir David Brew- 

 STER, but on his theory of visual triangulation, the phantom image 

 should be a plane, parallel to the given plane. The appearance of any 

 convexity would disprove his theory, and Brewster undertook no ex- 

 planation of what was to him, under the circumstances, probably not a 

 striking phenomenon. This long-forgotten peculiarity of the phantom 

 image, or at least its curvature in one direction, has lately been redis- 

 covered by Professor Le Conte, and its curvature in all directions by 

 myself. It was at first regarded as one of the consequences of strong 

 convergence of visual lines. By using, instead of a large plane surface, a 

 card on which the two pictures are perfectly similar, such as a pairof series 

 of concentric circles, and cutting the two halves apart, the horizontal vis- 

 ual lines may be made parallel while the two small cards are oppositely 

 inclined to them, at any desired angle, by rotation about a pair of verti- 

 cal axes. Each plane may thus bear to the visual line that meets it the 

 same relation that would result from strong convergence or divergence 

 in viewing a single continuous plane. The binocular effect is that the 

 combined surface appears convex or concave, at will, by varying the 



