204 RECEXT PROGRESS OF THE THEORY OP VISION. 



the eyelids as 'the white of the eye.' This globular 

 chamber is lined with a delicate coat of winding blood- 

 vessels covered inside by black pigment. But the apple 

 of the eye is not empty like the camera : it is filled with 

 a transparent jelly as clear as water. The lens of the 

 camera obscura is represented, first, by a convex trans- 

 parent window like a pane of horn (the cornea), which 

 is fixed in front of the sclerotic like a watch glass in front 



of its metal case. This union and its own firm texture 

 make its position and its curvature constant. But the 

 glass lenses of the photographer are not fixed ; they are 

 moveable by means of a sliding tube which can be ad- 

 justed by a screw (Fig. 27, r), so as to bring the objects 

 in front of the camera into focus. The nearer they are, 

 the farther the lens is pushed forward ; tlie farther off, 

 the more it is screwed in. The eye has the same task 

 of bringing at one time near, at another distant, objects to 

 a focus at the back of its dark chamber. So that some 



