210 The Bird 
reflects the black pigmented tissue at the back of the eye- 
ball. The image passes through this lens and is thrown 
upon the curtain of jet, and here the brain nerves find it 
and know it—how, we cannot even guess. 
If the eye-camera of the bird has no long bellows to 
focus out and back, it has something infinitely better— 
the coloured ring or iris which surrounds the pupil. We 
Fig. 151.—Head of living Sloth. 
are all familiar with the way the oval iris of a cat nar- 
rows to a slit in bright sunlight and broadens at night 
to let in all the light possible. Look closely at the eye 
of an owl or parrot, even in broad daylight, and the 
circle of the iris will be seen to contract and enlarge at the 
will of the bird. We have always been inclined to pity 
the poor “blind” owl during the daytime, but the truth 
is that, because of this power of adjustment, almost 
