CONSIDEEATTOISrS ON SIGHT IN BIRDS-^LEWIS. 343 



pigeons, feeding them on ca board on wheat, among which a per- 

 centage of the grains have been stuck by adhesive substance. One 

 mistake is sufficient to prevent them again making the error, small, 

 slight alteration from the natural position of the grain giving them 

 the clue. Many similar cases could be quoted. The vision of noc- 

 turnal birds is enhanced by the size of the eyeball itself and the con- 

 vexity 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 gi^eat dilatation. The 

 poorness of vision of these birds in the daytime is accounted for by the 

 fact that the eye is normally focused 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 see in bright 

 sunlight has an effect similar to that which we experience on emerg- 

 ing 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 pig- 

 ment 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 con- 

 vergent movement is seen as in the human being. But, in spite of 

 this, the amount present is not sufficient for the needs of the bird, 

 which nearly always moves the head to shift the direction of gaze. 



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

 The eyelids present little differing from mammals, with the excep- 

 tion 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, etc. 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- 

 like 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 prob- 

 lem — that of color vision. If one accepts the Young-Helmholtz 

 iheory, it must be taken that white light consists of the combina- 

 tion of three primary colors, namely, red, green, and violet. Later 

 works seem to incline toward the older division according to New- 

 ton — that the primary colors 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 colors as the other three. 

 The existence of color vision in animals is, of course, very difficult to 



