^"'i'qi?^'] Lewis, Some Considevalions on Sight in Birds. 217 



Some Considerations on Sight in Birds. 

 Hy Dk. J. C. Lewis, R.A.O.U., Melboukxe. 

 That continual adjustment, so necessary for life, between internal 

 relations of an organism and the external world would be im- 

 possible were it not for the communion of the sense organs. They 

 stand, as it were, midway l)etween the organism and its sur- 

 roundings, keeping the internal relations aware of and alive to 

 the external happenings and conditions. These functions 

 probably arose with the necessity for adaptation to environment 

 and its ever-changing demands, and in the struggle for existence 

 they are necessary factors for the survival of the race. 



Of the different special senses, hearing and sight stand apart 

 in the degree of speciaHzation ; and this specialization, again, 

 varies greatly in the divisions of the animal kingdom. In the 

 animal world, for example, we find all stages, from blindness to 

 acute \'ision. Where the sight is poor, smell and hearing are, in 

 compensation, extremely acute. The vision of the rhinoceros is 

 limited to some 50 yards or so, and is poor even for that short 

 range ; but the acuteness of the sense of smell makes good the 

 sight deficiency. In birds, specialization of sight reaches its 

 highest degree of development, and, though hearing is fairly acute, 

 the sense of smell is certainly vestigial. One feature of the 

 functions of hearing and sight is the projection of their sensory 

 impulses. Taking sight, we find that light reflected from a distant 

 object is picked up by the cornea and lens and brought into focus 

 at a point on the retina. The stimulation of the numerous endings 

 of the optic nerve sets up an activity which, after passing through 

 many systems of relays, reaches the sight centres in the brain, 

 giving rise to a complex chemical action in the cells, where the 

 myriad impulses are figured out into a hght pattern in the image 

 of the original object. Though the action setting up these 

 impulses originates in the brain, where the image is really 

 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 advantage 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 accommodation, and corresponds roughly to the focussing 

 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 principal convergence is done by the cornea, 

 the outer transparent covering of the eye, the amount of con- 

 vergence depending upon the laws of refraction governing light 

 passing from a less dense to denser media. 



