Mr. J. Le Conte on Binocular Vision. 29 



viously formed on this subject. My experiments completely 

 confirmed the results of Dove. I found that binocular relief, 

 both by combination of stereoscopic diagrams (whether in the 

 stereoscope or by the naked eye) and by viewing natural 

 objects, is indeed perfectly distinct by the light of the electric 

 spark; but I also observed in all cases the doubling of the 

 nearer lines or objects while regarding the more remote, and 

 vice versa. 



Between the two rival theories, then, the matter stands 

 thus: — Wkeatstone is right in so far as he asserts immediate 

 perception of relief, but is wrong in supposing any mental 

 fusion of the two images. Briicke is right in asserting that 

 the perception of binocular relief is a judgment based upon 

 double images of all parts of the object or scene beyond or on 

 this side the point of sight, but is wrong in supposing that 

 change of optic convergence and successive trial combination 

 is a necessary part of the evidence on which judgment is based. 

 My own view, or theory if I may so call it, has already been 

 published*. It is an attempt to unite what is true in the two 

 preceding views. I quote from a previous paper : — " All 

 objects or points either beyond or on this side the point of 

 sight are doubled, but differently — the former homonymously , 

 the latter heteronymously ; the double images of the former 

 are united by less, of the latter by greater convergence. ISTow 

 the observer knows instinctively and without trial, in any case 

 of double images whether they will be united by greater or by 

 less convergence, and therefore never makes a mistake, or 

 attempts to unite by a wrong movement of the optic axes. 

 In other words, the eye (or the mind) instinctively distinguishes 

 between homonymous and heteronymous images, referring the 

 former to objects or points beyond, and the latter to objects or 

 points on this side the point of sight. The mind therefore per- 

 ceives relief instantly by means of double images in the manner 

 just explained ; but the perception is doubtless made clearer by 

 changes of optic convergence, by ranging the eyes back and 

 forth from foreground to background and vice versa, and the 

 successive combination of different parts of the object or pic- 

 tures, as maintained by Briicke." 



I am, Gentlemen, 



Yours very respectfully, 



Berkeley, California, JOSEPH Le CoNTE. 



November 17, 1877. 



* Anier. Journ. vol. ii. pp. 1, 815, 417 ; Archives des Sciences, vol. xli. 

 p. 394 (1871). 



