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 
