436 Mr. W. Le Conte Stevens on Wheatstone and 



have the credit of this law taken away by a careless sentence 

 like the above. 



1 do not think there is any thing I need add, except that I 

 do not follow the argument by which Prof. Thompson arrives 

 at the result that the beats of the lower notes are not subjec- 

 tive. There seems to me to be no proof whatever of this 



result. ^ T , -, 



lours truly, 



R. H. M. Bosanquet. 



LIII. On Wheatstone and Brewster's Theory of Binocular 

 Perspective. By W. Le Conte Stevens*. 



IN 1838 Sir Charles Wheatstonef announced his invention 

 of the reflecting stereoscope, by means of which slightly 

 dissimilar pictures of the same object could be simultaneously 

 viewed in such a manner as to produce the illusion of bin- 

 ocular relief. The essential object attained by using this 

 instrument he expresses by saying J, " the two pictures (or 

 rather their reflected images) are placed in it at the true con- 

 course of the optic axes." The apparent distance of the com- 

 bined image is thus determined by the intersection of these 

 axes. In a subsequent paper§, published in 1852, Wheatstone 

 described a number of experiments with his stereoscope, by 

 which he had investigated the effect of varying the conver- 

 gence of visual lines. The result he expressed by saying |j, 

 " The perceived magnitude of an object diminishes as the incli- 

 nation of the axes becomes greater while the distance remains 

 the same ; and it increases, when the inclination of the axes 

 remains the same while the distance diminishes." In a sub- 

 sequent paragraph, however, speaking of the change in appa- 

 rent magnitude of the image produced by varying the optic 

 angle, he says% " and yet, if we attentively regard it in any 

 fixed position, it is perceived to be at a different distance." 

 That Wheatstone had not given up the idea of apparent dis- 

 tance expressed in his first paper, however, is shown by his 

 subjoining** a table of inclinations of the optic axes which cor- 

 respond to different distances; it also shows the angular posi- 

 tions of the camera required to obtain binocular pictures 



* Abstract of a memoir read before the New York Academy of Sciences, 

 Oct. 24, 1881. Communicated by the Author. 



f Wheatstone, " Contributions to the Physiology of Vision," Phil. 

 Trans. 1838, part ii. Also reprinted in the Philosophical Magazine, April 

 1852, p. 241. 



X Ibidem, p. 245 (1852). § Ibidem, p. 504. || Ibidem, p. 507. 



If Ibidem, p. 508. ** Ibidem, p. 512. 



