128 THE STRUCTURE AND LIFE OF BIRDS CHAP. 



any such effect. It is wanting, so far as is known, in 

 only one bird — the New Zealand Apteryx. It is found 

 in some reptiles, but always less developed than in 

 birds. It is odd that it does not interfere seriously 

 with the access of light to the retina. Besides the 

 central " yellow spot," which however is not absolutely 

 central, birds have a second similar spot more towards 

 the outer side of the eye. It has been thought that, 

 of the four spots thus possessed by the two eyes, two 

 are used together for binocular, and two separately for 

 monocular vision. The retina of a bird or a reptile 

 contrasts with that of a man in another point : the 

 cones exceed the rods in number. 1 The nictitating 

 membrane most people have heard of ; but it is often 

 imagined that it is the privilege of the eagle alone to 

 possess it, and that its object is to enable him to gaze 

 at the sun. As a fact, it is found in all birds and 

 reptiles. Watch the eye of any bird, and before long 

 you will see a film pass over it and in a moment vanish. 

 This is the nictitating membrane, which lies in the 

 front angle of the eye, and can be found without much 

 difficulty when the bird is dead. Some birds seem to 

 have great power of moving the Iris, a movement that 

 in most human beings is always involuntary, though 

 sometimes it is caused by nerves which, except for the 

 force of habit, are believed to be subject to our will. If 

 a Parrot's eye be watched, the pupil may be seen to 

 contract till it is quite small, though the light remains 

 as it was, and though the bird, apparently, continues to 



1 Sec Fiirbringer's Morphologic mid Systematik dcr Vogel, p. 

 1069. 



