CONSIDERATIOlSrs ON SIGHT IN BIRDS — LEWIS. 341 



right eye and a yellow area with the left, we do not see the two sepa- 

 rate colored spots, but a spot of the color equalling the blending of 

 the pigments ; this is due to a superimposing of the images registered. 

 In animals and birds where the axes of the eyes are not parallel it 

 means that the image of an object falling on the right half of the 

 right eye falls on the left half of the left eye. Only in animals 

 where the axes of the eyes are parallel do the images fall on the same 

 half of each eye, notably in human beings and monkeys, thus making 

 possible true binocular vision. In other words, in birds, with the 

 possible exception of some of the birds of prey and some nocturnal 

 birds, the sight or visual field consists of two separate views not 

 capable of being superimposed and not stereoscopic in effect. 



The advantage of observing the same object with both eyes is that 

 it permits of greater concentration once an object or victim has been 

 perceived, and it is thus found in eagles, hawks, etc., where acuity and 

 concentration are so necessary for their existence. In man the stereo- 

 scopic vision gives him the judgment of distance, and it is chiefly by 

 this and, to a smaller extent, by accommodation, that distance is ac- 

 curately estimated. On the other hand, birds, or most birds, have 

 to depend upon accommodation for their judgment of distance possi- 

 bly by the focusing movement of the lens brought about by the action 

 of Crampton's muscle, the pull being so strong in some species that 

 a ring of bony lamina? is provided in the sclerotic coat near the 

 corneal margin to prevent alteration in shape of that part of the eye. 



Monocular vision has a great advantage of giving a far more ex- 

 tensive scope of vision. It is a valuable asset for the birds which 

 must maintain a constant lookout for the approach of danger, and 

 for that reason it is found mainly in those birds of poor defense, 

 whose safety lies in speedy detection and evasion of their enemies. 

 In these birds there is the range of two extensive visual fields, each 

 being equally recorded and scrutinized. The moment an object of 

 interest is detected the bird does not direct both eyes toward it, but 

 there is a concentration of one eye, the vision of the other being sup- 

 pressed at will. In some diseases of man, where the axis of one eye 

 has departed from the parallel of the other, each eye sees a field 

 which does not correspond with the other, yet diplopia, or double 

 vision, is not present, as the one or the other field of vision is sup- 

 pressed according to the automatic concentration in one or the other 

 eye. Note a group of pheasants or pigeons watching the same ob- 

 ject; one eye only will be directed toward the position. Watch a 

 fowl or a pigeon gazing upward at a hawk ; one eye will be skyward, 

 the other toward the ground. In such cases the vision of the down- 

 ward eye is being suppressed. If suppression were not possible in 

 birds a position similar to diplopia would be present. An idea of 

 this condition can be gained by pressing one's eye, thus shifting the 

 73839°— SM 1916 23 



