124 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, oxAnpos = 
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 (1), 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—ze., 
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 
