SEC. IV ANATOMY OF BIRDS 271 



position; or both. Tliis is a difficult matter to settle, when such 

 delicate structures are in question. 



The ir/.s', /, Z, or rainbow of the eye, is an exquisite structure 

 hanging like a many-coloured curtain vertically between the two 

 compartments of the eye ; a highly ornamental framework of the 

 eye's window, being both sash and blind to the pupil. It is sus- 

 pended vertically in the aqueous humour, just in front of the lens. 

 Viewed in front, from the outside, the iris appears as a coloured 

 circular band around the pupil, and seems to come to the surface of 

 the eye. But this is not so, for the conjunctiva, the cornea, and 

 the aqueous humour of the front chamber of the eye, are between 

 us and it. It may be likened to the dial-plate of a watch, which we 

 look at without noticing the interposed crystal. Similarly, the pupil 

 of the eye, which shows us our own reflection, diminished to the 

 size of the " eye-baby," may be likened to the round central hole in 

 the dial-plate through which protrudes the shaft that bears the 

 hands of a watch. The " pupil " is the round black spot within 

 the coloured rim of the iris ; but it is not a thing — it is a hole in a 

 thing — the hole in the iris through which we may look and see the 

 black choroid coat behind. The quivering iris is very similar in 

 texture to the choroid, being a delicate tissue of interlacing fibres 

 and vessels ; but it is highly mobilised by circular and radiating sets 

 of contractile fibres, by which the curtain is tightened and loosened, 

 with corresponding change in the size of the central orifice — the 

 pupil. Although the iridian movements are largely automatic, 

 depending upon the stimulus of light, they are to some extent 

 voluntary, as any one may satisfy himself who observes owls in con- 

 finement. During these expansions and contractions of the iris the 

 pupil in birds preserves its circularity ; and even when the move- 

 ment is freest and most voluntary, as in owls, the contracted pupil 

 never appears as a vertical oval figure, or a slit, like that of cats. 

 The round pupil of the great horned owl. Bubo vinjinia/ms, ranges 

 from the diameter of a finger- ring down to that of a small split- 

 pea. The iridian colours are often striking in birds. Though 

 black and brown are the commonest, yellow is quite frequent, red 

 is often seen, blue and green are rarer ; the eyes of cormorants 

 are of the latter colour. The iris is sometimes pure white, as it is 

 in the white-eyed greenlet, Vireo novehoracensis. In the Californian 

 woodpecker, Melanerpes formicivorus, the eyes are indiff"erently {or 

 at difterent ages of the bird, or seasons) brown, bluish, pink, rosy, 

 or yellow. 



The crystalline lens, 0, is a transparent biconvex disk like a com- 

 mon magnifying glass, apparently set in the iris like a mirror in its 

 frame, but really hanging a little back of that structure. It is 

 enclosed in a capsular membrane, n, of extreme delicacy and trans- 



