XX INTRODUCTION. 
brane, is situated in the larger angle of the eye, and is, in 
fact, a second and more transparent eyelid, whose motions are 
directed at pleasure, and its use, besides occasionally cleaning 
and polishing the cornea, is to temper the excess of light and 
adjust the quantity admitted to the extreme delicacy of the 
organ. The other membrane, situated at the bottom of the 
eye, appears to be an expansion of the optic nerve, which, re- 
ceiving more immediately the impressions of the light, must be 
much more sensible than in other animals; and consequently 
the sight is in birds far more perfect, and embraces a wider 
range. Facts and observations bear out this conclusion ; for a 
Sparrow-hawk, while hovering in the air, perceives a Lark or 
other small bird, sitting on the ground, at twenty times the dis- 
tance that such an object would be visible to a man or dog. 
A Kite, which soars beyond the reach of human vision, yet 
distinguishes a lizard, field-mouse, or bird, and from this lofty 
station selects the tiny object of his prey, descending upon it 
in nearly a perpendicular line. But it may also be added that 
this prodigious extent of vision is likewise accompanied with 
equal accuracy and clearness; for the eye can dilate or con- 
tract, be shaded or exposed, depressed or made protuberant, 
so as readily to assume the precise form suited to the degree 
of light and the distance of the object; the organ thus answer- 
ing, as it were, the purpose of a self-adjusting telescope, with a 
shade for examining the most luminous and dazzling objects ; 
and hence the Eagle is often seen to ascend to the higher 
regions of the atmosphere, gazing on the unclouded sun as on 
an ordinary and familiar object. 
The rapid motions executed by birds have also a reference 
to the perfection of their vision; for if Nature, while she en- 
dowed them with great agility and vast muscular strength, had 
left them as short-sighted as ourselves, their latent powers 
would have availed them nothing, and the dangers of a per- 
petually impeded progress would have repressed or extin- 
guished their ardor. We may then, in general, consider the 
celerity with which an animal moves, as a just indication of 
the perfection of its vision. A bird, therefore, shooting swiftly 
