220 Lewis, Some Considerations on Sight in Birds. [lif'^prii 



only in birds. They appear to exert no effect on colour vision, 

 as they are in no way identical in composition with the visual 

 purple or sensitizing substance. 



The numerous fibres from the endings of the rods and cones 

 collect to form the optic nerves. The nerve from each eye con- 

 verges and meets at what is known as the optic chiasma, where 

 they unite and again separate. In all animals where binocular 

 vision takes place, or, to be more correct, where there is total 

 binocular vision, there is partial decussation of the fibre. Those 

 fibres leading from the right half of the right eye pass to the right 

 side of the brain, while the fibres from the left side of the right eye 

 cross over at the chiasma to the left side of the brain. 



The amount of decussation varies accordingly with the power 

 of binocular vision. In some animals where partial binocular 

 vision is possible, though not usual, as in the horse and some 

 rodents, only a few fibres do not decussate. In animals incapable 

 of any binocular vision complete decussation takes place. This 

 latter condition is found in birds, or nearly all birds, the fibres 

 entirely crossing over at the chiasma. One must first get a grasp 

 of the true meaning of binocular vision to appreciate the difference 

 between pure binocular vision and seeing the same object with 

 both eyes. If we hold a piece of paper between the eyes so as to 

 view, say, a red area with the right eye and a yellow area with the 

 left, we do not see the two separate coloured spots, but a spot of 

 the colour 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 falUng 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 tnie 

 binocular \nsion. 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 stei'eoscopic in effect. 



The advantage of obser\dng 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, &c., 

 where acuity and concentration are so necessary for their exist- 

 ence. In man the stereoscopic vision gives him the judgment of 

 distance, and it is chiefly by this, and to a smaller extent by 

 accommodation, that distance is accurately estimated. On the 

 other hand, birds, or most birds, have to depend upon accommoda- 

 tion for their judgment of distance, possibly by the focussing move- 

 ment of the lens brought about by the action of Crampton's 

 muscle, the pull being so strong in some specie? 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 

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



