Mr. J. Le Conte on Binocular Vision, 27 



polarity of a seemingly neutral pyroelectric crystal (that is 

 to say, a crystal at the surface of which there is an electrifi- 

 cation neutralizing for external space the force due to its in- 

 ternal electric polarity) , the same cooling and heating effects 

 will be produced by moving it in an electric field, as similar 

 motions would produce in a similar crystal which, by having 

 been heated in hot water, dried at the high temperature, and 

 cooled, is in a state of pyroelectric excitement.] 



Yacht 'LallaRookh,' 

 Largs, Sept. 13, 1877. 



III. On Binocular Vision 



To the Editors of the Philosophical Magazine and Journal. 



Gentlemen, 



IN Mr. Thompson's excellent article on the-Chromatism of 

 the Eye, in the July Number of your Journal, I notice 

 the following sentence : — " In Wheatstone's classical research 

 of 1838 it was demonstrated how great is the capacity of the 

 brain to combine two slightly differing retinal images." 

 From this sentence I conclude that Wheatstone's theory of 

 binocular relief is still held by many scientists : I am con- 

 firmed in this conclusion by finding the expression " mental 

 fusion" ot " conscious fusion" of dissimilar images in nearly 

 all works on the subject, even the latest, viz. Hermann's ' Phy- 

 siology.' Now, as I am quite sure that this theory is not only 

 untenable, but positively hurtful to science by discouraging 

 that careful analysis of visual perception so necessary to a true 

 theory and yet so difficult to most persons, I have thought it 

 would not be amiss to state briefly what I conceive to be the 

 present condition of science on this subject. 



In all investigations on binocular vision we are met at the 

 very threshold by the difficulty which most persons find 

 in analyzing what I would call visual judgments, i. e. judg- 

 ments which by long habit and inherited tendency seem at 

 first to be direct sensuous perceptions incapable of further 

 analysis. It is difficult to convince many persons that they 

 ever see double images at all ; and yet they, of course, every 

 day form judgments based on the existence and the uncon- 

 scious perception of such double images. It is difficult to 

 convince most persons, even the thoughtfully observant, that 

 in regarding a stereoscopic scene there is no complete fusion 

 of the two pictures, but that, when the eye is fixed on the 

 foreground, objects in the background are double, and vice 



