THE LONG-TAILED TIT. 403 



the centre of the eyes to the tip of the bill, is only four- 

 tenths of an inch, while that in the golden-crested wren is 

 one-half more, or six-tenths. Hence it is probable that in 

 its vision, the long-tailed tit is the most microscopic of 

 birds. Now it accords with analogy, and with observation, 

 as far as that can be carried, that the shortest focal length 

 of the eye, that at which minute objects can be distinctly 

 seen, is never much longer (if longer at all) than the distance 

 from the eye to the tip of the bill ; and nearer than that 

 would be useless. 



Hence we have a very easy means of judging of the 

 relative power of vision in those birds which feed upon very 

 small substances, and which, being hard-billed, as the tits 

 are, or not having the bill covered with a sentient mem- 

 brane, or the nasal groove continued to the point of it, as is 

 the case in that most singular of all birds, the apteryx, or 

 wingless bird, of New Zealand, must feed wholly, or chiefly, 

 by sight. 



In the average of human eyes, of good formation, and in 

 a healthy state, the common focal distance for small objects, 

 such as for reading very small print, is about seven inches 

 and a half; and the microscopic distance, such as that at 

 which a very delicate engraving is examined (and that 

 requires a little time for the adjustment of the eye), is about 

 one-third less, or five inches. The bird must often strike 

 its prey without any time for adjustment, and therefore the 

 distance from the eye to the bill may be taken as the ordi- 

 nary short focal length; but call it microscopic, and the 

 magnifying power in line being inversely as the focal longth, 

 the lineal dimensions of an object seen by the long-tailed 

 tit will be, to that of the same seen by the human eye, as 

 five inches to four-tenths of an inch, or as twelve and a half 

 to one. The surface will of course be as the square, or as 

 156-} to 1 ; and the body, or solid of an object, as the cube, 



2 D 2 



