92 Wild Birds and their Haunts 



dazzling. If this be the fact, it may serve the eagle in 

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



It is the manifest 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 bya peculiar circular muscle in 

 the eyeball. Buffon is of opinion that, on account of this 

 expansion 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 sparrow-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 un- 

 covered, depressed or protruded, readily assuming the 

 precise condition adapted to the distance of an object 

 and the quantity of light. 



Had they, indeed, been formed with eyes like the mole, 

 incapable of seeing more than a few inches' distance, they 

 would have been in constant danger of dashing against 

 every intervening obstacle. " Indeed," remarks the 

 same writer, ' ' we may consider the celerity with which 

 an animal moves as a just indication of the perfection of 

 its vision. A bird, for instance, that shoots swiftly 

 through the air must undoubtedly see better than one 

 which slowly describes a tortuous tract. Among quad- 

 rupeds, again, the sloths have a very limited sight." 



It may accordingly be inferred that birds have more 

 precise ideas than slow-moving caterpillars of motion and 

 its accompanying circumstances, such as those of relative 

 velocity, extent of country, the proportional height of 

 eminences, and the various inequalities of hill and dale, 

 mountain and valley. 



