INTRODUCTION. 
5 
them with long' legs, bare of feathers even above the knees ; their 
toes, unconnected by webs, are only partially furnished with mem- 
branous appendages, just sufficient to support them on the soft and 
boggy grounds they frequent. To this tribe belong the Cranes, 
Snipes, Sandpipers, Woodcocks, and many others. 
In comparing the senses of animals in connexion with their in- 
stinct, we find that of sight to be more extended, more acute, and 
more distinct in birds, in general, than in quadrupeds. I say, in 
general, for there are some birds, such as the Owls, whose vision is 
less clear than that of quadrupeds ; but this rather results from the ex- 
treme sensibility of the eye ; which, though dazzled with the glare of 
full day, nicely distinguishes even small objects, by the aid of twi- 
light. In all birds the organ of sight is furnished with two mem- 
branes, an external and internal, additional to those which occur 
in the human subject. The former ( membrana nictitans) or external 
membrane, is situated in the larger angle of the eye ; and is, in 
fact, a second and more transparent eye-lid, 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 receiving 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 per- 
fect, 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. distance 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 contract, 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 answering, as it were, the purpose of a self-adjusting 
telescope, with a shade for examining the most luminous and daz- 
zling 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. 
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