80 ON THE EELATION OF OPTICS TO PAINTING. 



produce displacements of the visual image, different 

 when he stands before objects from those when he 

 stands before the image, I could speak of only one 

 eye for which equality of impression is to be estab- 

 lished. We however see the world with two eyes, 

 which occupy somewhat different positions in space, 

 and which therefore show two different perspective 

 views of objects before us. This difference of the 

 images of the two eyes forms one of the most im- 

 portant means of estimating the distance of objects 

 from our eye, and of estimating depth, and this is 

 what is wanting to the painter, or even turns against 

 him; since in binocular vision the picture distinctly 

 forces itself on our perception as a plane surface. 



You must all have observed the wonderful vividness 

 which the solid form of objects acquires when good 

 stereoscopic images are viewed in the stereoscope, a 

 kind of vividness in which either of the pictures is 

 wanting when viewed without the stereoscope. The 

 illusion is most striking and instructive with figures in 

 simple line ; models of crystals and the like, in 

 which there is no other element of illusion. The 

 reason of this deception is, that looking with two eyes 

 we view the world simultaneously from somewhat 

 different points of view, and thereby acquire two dif- 

 ferent perspective images. With the right eye we see 

 somewhat more of the right side of objects before us,, 



