i2 4 THE STRUCTURE AND LIFE OF BIRDS chap. 



sides must be carefully screened off, and of course 

 the box of the camera must be impervious to light. 

 The remaining essential is a sensitive plate at the 

 back of the camera on which the image is formed. 

 All these parts are represented in the eye. The 

 eyeball is the box of the camera ; it is tough and un- 

 transparent, and is called the Sclerotic (SC, a/cXyipb? = 

 hard), only in front it becomes transparent, and is 

 known as the Cornea (C). Side rays are shut out by a 

 circular curtain, the Iris (I), with a hole in the middle, 

 the Pupil, which can be seen opening and contracting 

 to regulate the amount of light admitted. There is a 

 crystalline lens (L) of great elasticity, which by the 

 action of the muscles which suspend it is made more 

 or less convex so as to focus for objects at different 

 distances. In front, between the cornea and the lens, 

 is a fluid called the aqueous humour (AH), and behind 

 the lens is the less fluid vitreous humour (VH). The 

 rays of light that fall upon the eyes are refracted or 

 bent by the curved surface of the cornea, then by the 

 anterior surface of the lens, and again when they pass 

 from the lens into the vitreous humour. The cornea, 

 the aqueous humour, the crystalline lens, and the 

 vitreous humour may be, therefore, looked upon as 

 making one compound lens. But of the component 

 parts the crystalline lens is far the most important, 

 since it alone has the power of accommodation — i.e., 

 of adjusting itself to different distances. 



The sensitive plate at the back of this living camera 

 is called, as I have said, the retina (R). Though thinner 

 than tissue paper, it is made up of nine distinct layers, 

 and it is the hindmost of these on which is formed 



