vI FORM AND FUNCTION 129 
look at the same object. Moreover it is maintained 1 
that the muscles of the Iris in the Falcon may be seen 
to contract without any alteration in the size of the 
pupil ensuing, the outer ring seeming to work separately 
from the inner ; it is suggested that the work of this 
outer ring is to aid in focussing the eye. I have 
watched the eyes of Falcons, Eagles, and other birds of 
prey long and carefully, and I do not feel certain that I 
have seen this. But an eagle in a cage has very little 
need of sudden change of focus. It is far different 
when he swoops from a great height upon his prey, 
and, no doubt, keeps him clearly in view as he falls 
like a thunderbolt upon him. It is certain that the 
Iris in birds is highly muscular ; and, moreover, both in 
birds and in reptiles the muscle is striated, not smooth 
as in mammals. This is evidence that its action is 
voluntary, and; perhaps, that it is more powerful. A 
natural result of the tightening of the belt of muscle 
round the lens would be to round it outwards—ze., focus 
the eye for near objects. On the whole it seems prob- 
able that the Iris in birds is not only a curtain to 
regulate the amount of light admitted, but that it aids 
the ciliary muscles in the work of focussing. 
The size of the eye varies very much in different 
speciés, and, as a rule, the power of sight seems to vary 
in proportion. Here are some figures which bring 
this out clearly.2 In the Owl, the two eyes cleared of 
muscle weigh 3 of the whole body, in the Falcon 
gx, in the Woodpecker 75, in the Peacock s4,, in the 
Goose ;},. Inthe Apteryx, a night feeder like the Owl, 
1 See Bronn’s 7héer-Retch, vol. “ Aves,” p. 434. 
2 Tbid, p. 425. 
K 
