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THEORY OF THE EYE. © 293 
camera obscura ; the retina is the ground of the picture, 
the crystalline replaces the glass lens.* 
This assimilation, generally adopted since Kepler’s 
time, remains open only to one difficulty ; the camera 
obscura, like an ordinary telescope, requires to be brought 
to a proper focus according to the distance of objects. 
When objects are near it is indispensable to increase the 
distance of the picture from the lens; a contrary move- 
ment becomes necessary as they become more distant. 
To preserve to the images all the distinctness which is 
desirable, without changing the position of the surface 
which receives them, is therefore impossible: at least, 
always supposing the curvature of the lens to remain 
invariable ; that it cannot increase when we look at near 
objects, or diminish for distant objects. 
* The author seems to have left this illustration incomplete. Kep- 
ler’s suggestion of the identity of the eye with the camera obscura, 
after all, does not touch the difficulty of the inversion of the image. 
Nor has it been considered as completely cleared up even till much 
later times. The solution which, it is believed, is now most generally 
assented to is this. It is a law of our constitution, dependent on some 
physiological principle unknown, that we refer impressions on the 
retina to objects existing, or believed to exist, in the rectilinear direc- 
tion from which the impression comes to the retina. Consequently, 
as rays cross at the pupil, an impression arriving at (r) in the direction 
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of the arrow, will convey the idea of an object existing at (g); in other 
words, a ray falling on the wpper part of the retina suggests an object 
lying below, or an inverted image suggests an erect object. 
