bird's-eye views. 15 



The' iris of birds is copiously supplied with coloring mat- 

 ter ; the tints vary with diiierent 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 Avith. 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 JVbvceboracensis) . 



The crystalline lens (o) is a trans^jarent 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 cooperative 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, the 

 iris, and the lens, which together form a vertical wall. Both 

 of these chambers are filled with fluid, of difierent density 

 and consistence in each. That in the anterior division is 

 thin and watery, and therefore called the "aqueous humor;" 

 that in the posterior one is more dense and glassy, and is for 



