VISION. 235 



the idea that it is chiefly destined to absorb the 

 rays of light when they are too strong or dazzling ; 

 if this be the fact, it may serve the eagle in good 

 stead when gazing, if he ever do so, on the sun. 

 It is the opinion of others that it serves to assist in 

 producing the internal changes of the eye ; but this 

 has been opposed by Crampton, who has shown 

 that the changes in question, at least in the ostrich 

 and several large birds, are produced by a peculiar 

 circular muscle in the eyeball. 



This singular membrane has been long known, 

 and is well described by the French Academi- 

 cians, who remark that, in the eyes of the turkey, 

 the optic nerve, which is situated very near the 

 side, spreads into a round space, from the circum- 

 ference of which a number of black filaments are 

 sent off to form, by their union, a membrane pecu- 

 liar to the eye of birds. In the eyes of the ostrich 

 they describe the optic nerve as dilated into a sort of 

 funnel of a similar substance. From this funnel a 

 folded membrane takes its origin, forming a sort of 

 purse drawn to a point. This is covered with a 

 black pigment easily rubbed off. 



Buffon is of opinion, that on account of this ex- 

 pansion of the optic nerve, birds must have a vastly 

 more perfect sight than other animals, embracing 

 also a much wider range. Hence it is that a spar- 

 row-hawk, while he hovers in the air, espies a lark 

 sitting on a clod, though at twenty times the distance 

 at which it could be perceived by a man or dog. 

 The kite, which soars to so amazing a height as 

 totally to vanish from our sight, can yet distinguish 

 small lizards, field-mice, and birds, and from this 

 lofty station he selects his prey. This prodigious 

 extent of vision is, moreover, conjoined with equal 

 accuracy and clearness, inasmuch as the eye can 

 dilate and contract ; can be shaded or uncovered ; 

 depressed or protruded ; readily assuming the pre- 

 cise condition adapted to the distance of an object 

 and the quantity of light. 



