VISION. 



77 



Every picture consists of points of light of different 

 degrees of intensity and varieties of colours, and the 

 appearance of form is nothing but the grouping of 

 points of light and shade and colour. That the 

 mind, then, which is only affected by the external 

 world through the medium of nerve-terminations, 

 may appreciate the landscape, it is necessary that 

 it shall receive a separate sensation from the light 

 emanating from each of a large number of points, 

 and that the relative positions of these sensations 

 shall be such that they may be recognized in 

 positions corresponding with the points in the 

 landscape to be represented. 



In the eye these requirements are furnished. 

 In it a camera or dark chamber of notable size 

 exists similar to that which a photographer uses, 

 having a lens in the fore part, and a sensitive 

 curtain at the back. All the difference between 

 this camera and a photographer's is, that this 

 is globular and the optical arrangements are more 

 perfect. When the photographer looks in at the 

 back of his camera, he sees on the ground glass 

 plate the image depicted which he wishes to photo- 

 graph, placed upside down, but faithfully delineated 

 in all its colours ; and such an inverted landscape 

 is formed in like manner in the back part of each of 

 our eyeballs. And as the photographer adjusts the 



