342 



SCIENCE 



[N. S. Vol. XXX. JSTo. 767" 



was maintained as early as 1671 by Cherubin 

 d'Orleans wbo introduced the theory of corre- 

 sponding retinal points to account for the 

 perception of a single field of view through a 

 double telescope. The geometric theory of 

 visual triangulation, which has been so much 

 associated with the name of Brewster, ante- 

 dates his work by nearly a century, having 

 been introduced by Wiedenburg in 1735. If 

 any one man is worthy to be considered the 

 father of physiological optics it is Eobert 

 Smith, whose " compleat system of opticks " 

 was published at Cambridge in 1738. New- 

 ton had died in 1727, with apparently little 

 knowledge of the eye other than as a mere 

 recipient of light. Smith recognized the ad- 

 vantage of two eyes in securing a more com- 

 prehensive view of a near object and thus per- 

 ceiving spatial depth, and he discovered the 

 important role of play of the eyes in com- 

 pleting this perception. To him is due 

 the record of the first binocular experiment 

 with the unaided eyes, that of holding 

 a pair of compass dividers at arm's length, 

 and adjusting the opening so that 

 retinal fusion is secured for the 

 images of opposite terminal points 

 when the visual lines are made par- 

 s' allel. This is now a familiar mode 

 ;• of measuring interocular distance. 

 ; ; He modified the experiment by 

 ; ; substituting cross vision and thus 

 ; ; securing the appearance of a three- 

 • • legged compass, the middle leg be- 

 ,' '; ing seen binocularly, the others 

 ;■ ; monocularly. It was but a short 

 : ", step from this to the first recorded 

 • • stereoscopic drawing (shown in the 

 ■ • accompanying diagram). Let the 

 • '. observer's eyes, above B and L 

 jj jj respectively, be initially directed 

 to the point, P, of a vertical pin 

 pressing on the paper. Its projection for the 

 right eye is the horizontal line Pa ; for the left 

 eye. Ph. Now change the point of fixation to 

 the head of the pin, which is monocularly 

 aligned with a and h respectively. On sud- 

 denly lifting the pin away it is apparently re- 

 placed by a vertical black line in space, which 



is the binocular image of Pa and Ph, while the 

 horizontal monocular images are still seen ad- 

 ditionally. The scale of the diagram is neces- 

 sarily rather small. Neither Eobert Smith 

 nor any of his successors for nearly a century 

 applied this stereoscopic experiment to more 

 complex drawings, but through them the way 

 was well prepared for the inventor of the re- 

 flecting stereoscope, Wheatstone, a little over 

 seventy years ago. 



Von Rohr gives an interesting account of 

 the work of Wheatstone to whom is due the- 

 discovery that, since the two retinal images of 

 an object occupying three dimensions in space 

 are sensibly different, the corresponding 

 images on a larger scale may be made on 

 paper or other suitable surface, either by 

 hand or by the new art of photography just 

 then in its infancy; and that these may be 

 suitably arranged for simultaneous binocular 

 observation with the attainment of virtual re- 

 lief in space. This was distinctly conceived' 

 by him in 1833 but not published until 1838. 

 Others had understood the essential conditions 

 of binocular vision, but he was the first to re- 

 produce them artificially and bring them under 

 control. His own discovery of these condi- 

 tions was likewise independent. 



Wheatstone soon had his critics, one of 

 whom, Briicke, stoutly contested his views, 

 laying the utmost stress on corresponding- 

 retinal points, motion of the eyes, *" and espe- 

 cially variations of convergence. Another 

 critic, Tourtual, in 1841, used a modification 

 of Wheatstone's stereoscope and obtained re- 

 lief effects from a pair of identically similar- 

 drawings by slight opposite rotation of these- 

 about parallel vertical axes. This seemed to 

 contravene Wheatstone's claim that the pair of 

 pictures must be different. Just forty years 

 afterward the same discovery was indepen- 

 dently made by the present writer, who was 

 ignorant of Tourtual's work or his existence;- 

 and it was published by him as " a new mode- 

 of stereoscopy."^ He did not, however, find his 

 discovery to contravene Wheatstone's views. 

 On the contrary it was a novel confirmation 

 of them, by which either orthoscopic or pseu- 



' American Journal of Science, April, 1882. 



