XXVI 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 



