2l8 Lewis, Some Considerations on Sieht in Birds-. \ ^T" ■, 



" L ist April 



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 

 mammals. 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 focussed 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 

 focussed 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 further differs in that it is a positive 

 process, relaxation of the muscle focussing 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. 



There is little to be said of the iris in birds apart from the fact 

 that the movement of this curtain or diaphragm is voluntary, 

 the pupil widening or closing at will. Apart from the voluntary 

 action, closing of the pupil or a stopping-down process occurs in 

 the presence of strong light, and is, therefore, reflex in nature, 

 widening of the pupil being noticed in weak Hght and also for 

 distant vision. 



The retina — the sensitive plate, as it were, of the eye — consists 

 of a layer of fine nerve endings which in most animals conform to 

 two well-marked types, rods and cones. In birds it has been for 

 a long time thought that this layer consisted of rods only, but 

 closer examination shows that cones are present, though very 

 much reduced in number. There is also a belief existent, with 

 perhaps some reason, that the function of the cones is associated 

 with differentiation of colours, or the formation of visual purple, 

 while rods determine movement, form, and shape. This is the 

 layer which is stimulated by the photo-chemical action of Hght, 

 the sensitizing substance being found in the external layer of the 

 retina, and called, for convenience, \asual purple. It is believed 

 that this substance changes under the effect of light, and the 

 chemical changes effected act on and stimulate the nerve endings. 



