338 ANKUAL REPORT SMITHSONIAN INSTITUTION, 1916. 



synthetized, the sensation is projected to the object from which the 

 light is reflected. A similar projection occurs with the function of 

 hearing, though perhaps not so definite in its localization. 



If we consider the eye as an optical apparatus, looking at it from 

 a mechanical point of view, we find that it can be likened with advan- 

 tage to a camera, the convergence of rays being brought about by 

 the lens and the cornea, the retina taking the place of the sensitized 

 plate. This convergence of the diverging rays of light into focus 

 on the retina from objects at varying distances is termed accommo- 

 dation and corresponds roughly to the focusing of a camera. The 

 process of accommodation differs greatly in the different classes of 

 the animal kingdom. In terrestrial forms, where there is media of 

 very much less density outside the eye — namely, the air — the princi- 

 pal convergence is done by the cornea, the outer transparent covering 

 of the eye, the amount of convergence depending upon the laws of 

 refraction governing light passing from a less dense to denser media. 



Though the lens also acts to a lesser extent in the same way, the 

 corneal convergence is the more important in these forms, the special 

 important function of the lens being the alteration of focus. On the 

 other hand, in aquatic forms, such as fish, no corneal convergence, 

 or almost none, is present, the media — namely, sea water, or even 

 fresh water — being of practically the same density as the media of 

 the eye itself. In these forms convergence must, therefore, be 

 brought about by the lens only, and for that purpose a spherical lens 

 is present. 



The physiology of accommodation in birds is remarkably com- 

 plicated, differing in many respects from that found in the mam- 

 mals. In the latter, or to be more correct, in the terrestrial forms 

 alteration of focus is brought about by alteration in the shape of 

 the lens. This structure when focused for near objects becomes more 

 convex, particularly on the anterior surface. There is no change in 

 shape of the transparent front part of the eye. In birds, on the 

 other hand, with the exception of some of the night fliers, though like 

 in man and other animals, the eye is normally focused for distance, 

 accommodation is a more complex process, there being change in 

 shape both of the lens itself and of the eyeball as a whole. It fur- 

 ther differs in that it is a positive process, relaxation of the muscle 

 focusing the eye for nearer points. 



In birds there are found two main types of eyes, though inter- 

 mediate forms exist — namely, the tubular eye, with rounded lens, 

 which allows for a normal near vision such as in the night-flying 

 birds; and the other, the almost spherical eye, with flattened lens, 

 characteristic of high-soaring birds of prey, and consequently 

 adapted for distant vision (pi. 1). 



