Prof. Wheatstone on the Physiology of Vision. 511 



stance from each other. If the engravings should be less than 

 2^ inches apart, the prisms may be dispensed with by persons 

 who have command over the adaptation of their eyes, particularly 

 if they be short-sighted. 



§19. 



At the date of the publication of my experiments oh binocular 

 vision, the brilliant photographic discoveries of Talbot, Niepce 

 and Daguerre, had not been announced to the world. To illus- 

 trate the phsenornena of the stereoscope I could therefore, at that 

 time, only employ drawings made by the hands of an artist. 

 Mere outline figures, or even shaded perspective drawings of 

 simple objects, do not present much difficulty ; but it is evidently 

 impossible for the most accurate and accomplished artist to deli- 

 neate, by the sole aid of his eye, the two projections necessary to 

 form the stereoscopic relief of objects as they exist in nature with 

 their delicate differences of outline, light and shade. What the 

 hand of the artist was unable to accomplish, the chemical action 

 of light, directed by the camera, has enabled us to effect. 



It was at the beginning of 1839, about six months after the 

 appearance of my memoir in the Philosophical Transactions, that 

 the photographic art became known, and soon after, at my request, 

 Mr. Talbot, the inventor, and Mr. Cohen (one of the first culti- 

 vators of the art) obligingly prepared for me stereoscopic Talbo- 

 types of full-sized statues, buildings, and even portraits of living 

 persons. M. Quetelet, to whom I communicated this application 

 and sent specimens, made mention of it in the Bulletins of the 

 Brussels Academy of October 1841. To M. Fizeau and M.Claudet 

 I was indebted for the first Daguerreotypes executed for the 

 stereoscope. The beautiful stereoscopic representations of sta- 

 tuary, architecture, machinery, natural history specimens, por- 

 traits of living persons, single and in groups, &c, which have 

 recently been produced by M. Soleil and M. Claudet, are now 

 too well known to the public to need more than a slight refer- 

 ence to them. 



With respect to the means of preparing the binocular photo- 

 graphs (and in this general term I include both Talbotypes and 

 Daguerreotypes), little requires to be said beyond "a few direc- 

 tions as to the proper positions in which it is necessary to place 

 the camera in order to obtain the two required projections. 



We will suppose that the binocular pictures are required to 

 be seen in the stereoscope at a distance of 8 inches before the 

 eyes, in which case the convergence of the optic axes is about 18°. 

 To obtain the proper projections lor this distance, the Ciinicni 

 must be placed, with its lens accurately directed towards the 

 object, successively in two points of the circumference of a circle 



