128 THE STRUCTURE AND LIFE OF BIRDS’ cuHap. 
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.! 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 ina 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 See Fiirbringer’s Morphologie und Systematik der Vogel, p. 
1069. ’ 
