THE LONG-TAILED TIT. 405 



nines in line, as compared with the standard of the human 

 eye, at five inches' distance ; and the numbers are found by 

 dividing five inches by the focal length of the eye as stated 

 in the first column. The third column is the magnifying 

 powers in surface, and the numbers in it are found by multi- 

 plying those in the second column by themselves. The num- 

 bers in the last column are the magnifying powers in bulk, 

 and they are found by multiplying together the correspond- 

 ing numbers in the second and third columns. In conse- 

 quence of the greater magnifying power, an insect a quarter 

 of an inch long will appear as large to the eye of the long- 

 tailed tit, as the common mouse does to the eye of man, when 

 held as near as it can be seen. 



The microscopic power of the eyes of those little birds 

 which seek for minute prey on the bark of trees, is, therefore, 

 as wonderful as the telescopic range of the eyes of eagles and 

 other birds which soar aloft, and scan a horizon of miles ; and 

 not the least wonderful part of the whole matter is, the ease 

 with which the eye changes from telescope to microscope. 

 The eye of the eagle, which can discern the motion of a small 

 quadruped at the distance of more than a mile, can shorten 

 its focus, so as to be keen and perfect at the distance of a few 

 inches; and the tit, to whose near vision the eggs of flies- 

 must appear as large as musket-bullets do to us, feels no diffi- 

 culty at seeing a bush at the distance of more than a hundred 

 yards. We may admire, but we cannot imitate, that won- 

 derful mechanical skill which can, in an organ not altogether 

 larger than a pepper-corn, produce so many focal lengths in 

 the same crystalline lens, instantly adapt the retina, or sen- 

 tient tissue, to all of them, manage the admission of the 

 proper light by the iris, and remove the colours and correct 

 the aberrations by the other humours all, as it were, in an 

 instant, without effort, and (to use the common, though un- 

 philosophical terms) without will or even conscioibsness on the 



