28 Mr. J. Le Conte on Binocular Vision. 



versa. It is still more difficult, nay, almost impossible, even 

 for those accustomed to analyze these visual perceptions, to 

 perceive a similar partial doubling of images in regarding an 

 actual solid object. Yet by careful analysis we may convince 

 ourselves of all this. Nothing can be more certain than the 

 fact that the complete fusion of dissimilar images never takes 

 place, and that, if we think otherwise, it is only because we 

 do not observe and analyze carefully. 



From early boyhood I have accustomed myself to make ex- 

 periments on binocular vision, and have thus gradually acquired 

 an extraordinary aptitude in decomposing complex visual 

 judgments into their component sense-impressions. In com- 

 binations of stereoscopic pictures, whether in the stereoscope 

 or with the naked eye by squinting (i. e. whether beyond or 

 on this side the plane of the pictures, and therefore whether 

 the binocular relief be natural or inverted), I always distinctly 

 perceive the doubling of parts of the scene or object when a 

 nearer or a more distant part is regarded. Also in viewing 

 natural objects, even such objects as small cones or prisms, I 

 always perceive the doubling of the nearer parts while regard- 

 ing the further parts, and vice versa. Wheatstone's theory 

 therefore seems true only to the unobservant or unpractised: 

 it is a popular explanation, not a scientific theory ; it cuts, but 

 does not loose the Gordian knot. 



Briicke and Brewster, by more refined observation and 

 more careful analysis, easily perceived that there was in reality 

 no mental fusion of two dissimilar images. Their view is that, 

 in regarding a solid object or two stereoscopic pictures, the 

 eyes are in incessant unconscious motion, by greater and less 

 optic convergence successively combining the different parts 

 of the two images, and thus by ranging back and forth reach 

 by trial the distinct perception of binocular relief. 



This theory is undoubtedly a great advance upon Wheat- 

 stone's. It is really a scientific theory, since it is based upon 

 an analysis of our visual judgment. It is also in part a true 

 theory : but it is evidently not the whole truth ; for successive 

 trial-combinations of different parts of the two images are not 

 necessary to the perception of binocular perspective. The ex- 

 periments of Dove have proved that binocular relief is dis- 

 tinctly perceived even by the light of an electric spark, which 

 lasts only 2 4oo"o °f a secon d — a time too short to allow any 

 change of optic convergence. 



I have repeated these experiments of Dove with great care, 

 with the double object of testing their accuracy and at the 

 same time of testing the truth of a view which I had pre- 



