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. 
