406 IXSECTIVOILE. 



part of the owner. But curious as the subject is, and wide 

 and wonderful as is the field of investigation to which (if 

 properly followed out) it would lead, we must leave it, with 

 our warmest recommendation, to the attention of even the 

 common reader (to the scientific it needs no recommenda- 

 tion), and return to the individual bird in which the micro- 

 scopic power is so very great. 



But though the vision of these little birds is thus remark- 

 ably nice and microscopic, capable of discerning clearly objects 

 which to our eyes are not at all visible, yet it is not to be 

 supposed that they depend wholly upon the eye in all their 

 motions. Eyes have, no doubt, a wonderful flexibility in 

 their focal length, so that the same eye is to a very consider- 

 able extent both a microscope and a telescope. But still 

 their flexibility, or power of adaptation, in that way, has a 

 limit ; and as the focal length of the long-tailed tit's eye is 

 not a twelfth part of that of the human eye, we may suppose 

 that it cannot see so well at the distance of half a mile as man 

 does at the distance of twelve miles. 



Indeed the estimate should be taken somewhat differently; 

 the mean range of near distance in the human eye is probably 

 about half the length of the arm say about fifteen inches, as 

 that is the medium point to which the eye guides the hand ; 

 and thus the acting focal length of the human eye is between 

 thirty and forty times that of the long-tailed tit's eye, so that 

 one furlong is as telescopic to it as five miles are to man ; and 

 that man would be sharp-sighted who could distinguish a loaf 

 of bread from a grey stone at the distance of five miles. 



But the tits find out carrion at long distances, probably at 

 the distance of several miles, and therefore we must suppose 

 that they are, on their long flights, guided by some other 

 means than sight. What these means are, whether scent or 

 some other species of perception, to which, as a sense, we can 

 give no name, we do not know, and it is neither useful nor 



