576 BIRD’S-EYE VIEWS. 
The iris of birds is copiously supplied with coloring mat- 
ter; the tints vary with different species, and are often ex- 
tremely: brilliant. Some shade of brown is, perhaps, the 
commonest color. Yellow is very common; red is often seen; 
blue and green are more rarely met with. The eyes of Cor- 
morants are of the latter color. Sometimes the iris is black- 
ish, or black, like the choroid; and it is frequently pure 
white, as in the instance of one of our common birds, the 
White-eyed Greenlet ( Vireo Noveboracensis). i 
The crystalline lens (0) is a transparent bi-convex disk, 
just like a common magnifying glass. It apparently hangs 
on the iris like a looking-glass in its frame, but is really set 
a little further back. In. birds, it is rather flatter, especially 
behind, and. also softer. in consistency, than in some other 
classes. It is enclosed in a very delicate transparent mem- 
brane, its capsule (n), which is in turn set in between two 
layers of a membrane, called “hyaline,” to be presently de- 
scribed. Where the two hyaloid layers separate around the 
rim of the capsule, to form its case, a small space is left, 
that makes a circular tube all around, called the canal of 
Petit (k,k).. The lens is stationary as far as the axis of 
vision is. concerned; but is capable of being moved a little 
forwards and backwards, by the pressure of the humors of 
the eye, which is produced by the codperative action of cer- 
tain. muscular and vascular structures, as we shall see before 
we get through; This movement adjusts the focus for 
vision, exactly as it is adjusted in a telescope, for instance, 
by lengthening or shortening the tube. : 
We can understand, now, that the eyeball is divided into 
two compartments, or “chambers,” as they are called, by the 
inward reflection of the two choroid coats, the hyaloid, fe 
iris, and the lens, which together form a vertical wall. Bo 
of these chambers are filled with fluid, of different density 
and consistence in each. That in the anterior division Z 
thin and watery, and therefore called: the “aqueous h pye 
that in the posterior one is more dense and glassy, and 16 fF 

