ON THE KELATION OF OPTICS TO PAINTING. 81 



and also somewhat more of those behind it, than we 

 do with the left eye ; and conversely we see with the 

 left, more of the left side of an object, and of the back- 

 ground behind its left edges, and partially concealed 

 by the edge. But a flat picture shows to the right eye 

 absolutely the same picture, and all objects represented 

 upon it, as to the left eye. If then we make for each 

 eye such a picture as that eye would perceive if itself 

 looked at the object, and if both pictures are combined 

 in the stereoscope, so that each eye sees its correspond- 

 ing picture, then as far as form is concerned the 

 same impression is produced in the two eyes as the 

 object itself produces. But if we look at a drawing or 

 a picture with both eyes, we just as easily recognise 

 that it is a representation on a plane surface, which is 

 different from that which the actual object would show 

 simultaneously to both eyes. Hence is due the well- 

 known increase in the vividness of a picture if it is 

 looked at with only one eye, and while quite stationary, 

 through a dark tube ; we thus exclude any comparison 

 of its distance with that of adjacent objects in the 

 room. For it must be observed that as we use differ- 

 ent pictures seen with the two eyes for the perception 

 of depth, in like manner as the body moves from one 

 place to another, the pictures seen by the same eye 

 serve for the same purpose. In moving, whether on 

 foot or riding, the nearer objects are apparently dis- 

 ii. O 



