^°'iQi6^^ 1 r.icwis, Some Considetalions on Sight in Birds. 22"^, 



of the gv<s\n j^^ivinj^^ tlu'iii the cliir. M;uiy similar caM-s cimld be 

 quoted. The vision of iioclurual birds is culianccd l)y tlie size 

 of the eyeball itself, and the convexity of the cornea, which 

 collects more light from an ol)ject than that with less convexity. 

 They present, too, the markedly tubular eye. The pupil in these 

 birds is capable of great dilatation. The i)oorness of vision 

 of these birds in the daytime is accounted for by the fact 

 that the eye is normally focussed for objects comparatively near, 

 and, again, because of the amount of stooping down necessary 

 to exclude the strong light. The eyes of these birds are probably 

 what are known as dark-adapted eyes, and the attempt to sec 

 in bright sunlight has an effect similar to that which we experience 

 on emerging from a dark room into the sunlight. This is not 

 due so much to the contraction of the pupil as to arrangement 

 of the protective jngment around the endings of the optic nerve. 



The power of individual movement of the eyes is greater in birds 

 than in man, extensive divergent movement being possible, while 

 convergent movement is seen as in the human being. But, in 

 spite of this, the amount present is not sufftcient for the needs 

 of the bird, which nearly always moves the head to shift the 

 direction of gaze. 



Of the accessory structures of tlie eye not much need be said. 

 The eyehds present httle differing from mammals, with the ex- 

 ception of the absence of eyelashes and the greater mobility of 

 the lower lid. The third eyelid, known as the nictitating 

 membrane, is well developed in birds, constantly sweeping the 

 surface of the cornea and keeping it free of small particles and so 

 forth. In mammals it is not moved voluntarily, but by pressure 

 exerted by the backward movement of the eye itself. This 

 membrane in birds is moved by two voluntary muscles, which 

 bring it across the eye with lightning-Hke rapidity. In aquatic 

 birds it invests the eye while submerged, and is then transparent, 

 to allow vision without endangering the sensitive surface of the 

 globe. 



We come now to a more interesting, though more difficult 

 problem — that of colour vision. If one accepts the Young- 

 Helmholtz theory, it must be taken that white light consists of 

 the combination of three primary colours — namely, red, green, 

 and violet. Later works seem to incline towards the older 

 division according to Newton — that the primary colours included 

 red, orange, yellow, green, blue, indigo, and violet. In other 

 words, the blue and yellow have as much right to be considered 

 as primary colours as the other three. The existence of colour 

 \ision in animals is, of course, very difficult to determine. It 

 appears, however, that with trained dogs and horses there is no 

 difficulty at all in teaching them to distinguish between the 

 saturated colours. The preference of some birds, notably the 

 Bower-Birds, for objects of a certain colour, and the general 

 evolution of colour in the different species, must point to an 

 appreciation of different shades. Colour sensation must be 



16 



