vr FORM AND FUNCTION 129 



look at the same object. Moreover it is maintained ' 

 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 — i.e., 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 

 species, 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 -£% of the whole body, in the Falcon 

 J.-, in the Woodpecker T V, in the Peacock y^, in the 

 Goose - 5 } T . In the Apteryx, a night feeder like the Owl, 



1 See Bronn's Thier-Reich, vol. " Aves," p. 434. 



2 Ibid, p. 425. 



K 



