i88i. 



9 



Trans. N. Y. Ac. Set',- 



WHEATSTONE AND BREWSTER'S THEORY OF BINOCULAR PERSPEC- 

 TIVE. 



For some time after the publication of Sir Charles Wheatstone's 

 essay (') in 1838, on the Physiology of Vision, this subject was studied 

 with much zeal by Sir David Brewster, whose name is permanently 

 associated with the lenticular stereoscope, an instrument now familiar 

 in every household. Although the theories advanced by these two 

 physicists to account for the illusion of binocular relief have since been 

 shown insufficient, their mode of accounting for the estimate of dis- 

 tance as perceived in the st-ereoscope has been quite generally accepted. 

 In 1844, Brewster published an essay ('-) "On the Knowledge of Dis- 

 tance given by Binocular Vision," in which he elaborated and abun- 

 dantly illustrated the idea that the apparent distance of an object is 

 determined by the intersection of visual lines. The stereoscope had 

 already been explained as an instrument by which rays of light from 

 two slightly dissimilar pictures were made to enter the eyes, as if com- 

 ing from a single object into which they are combined in front, and on 

 each point of which the visual lines could be made to meet. Thus, in 

 Fig. r, if rays from the conjugate foreground points, Ai and Ao, be 



3 





'A- 





d(' 



' f I * i 1 ■ 1 . » i 



Fig. I. 



(1) Phil. Transactions, 1838, Part II. 



Reprinted in Phil. Mag-azine, s. 4, vol. III., April, 1852. 



(2) Edinburgh Transactions, vol. XV., Part III., p. 360. 



