HAIRS, FEATHERS, AND SCALES. 3 



lines drawn parallel to each other, exactly like those on 

 an ivory rule or scale, with every fifth line longer than the 

 rest, and every tenth longer still. This is the micrometer, 

 or scale by which we measure objects ; and the difference 

 in the length of the lines, you will readily guess, is merely 

 a device to facilitate the counting of them. By moving 

 the stage up or down, or to either side, we easily bring 

 the hair exactly into the centre of the field; and now, 

 by adjusting the eye-piece, we make the scale to lie 

 directly across the hair, at right angles with its length. 

 Thus we see that its diameter covers just thirty of the 

 fine lines ; and as, with this magnifying power, each 

 line represents l-10,000th of an inch, the hair is 30- 

 10,000ths,;zg-J-gTd of an inch, in diameter. 



In all branches of natural history, but perhaps pre- 

 eminently in microscopic natural history, — owing to its 

 greater liability to error from deceptive appearances, — we 

 gain much information on any given structure by com- 

 paring it with parallel or analogous structures in other 

 forms. Thus we shall find that our understanding of the 

 structure of this hair will be much increased when we have 

 seen, under the same magnifying power, specimens of the 

 hair of other animals. In order, however, to explain it, I 

 must anticipate those observations. 



"What we see, then, is a perfectly translucent cylinder, 

 having a light brown tinge, and marked with a great 

 number of delicate lines, having a general transverse 

 direction, but very irregularly sinuous or winding in 

 tlreir individual courses. These lines we perceive to be 

 on the surface ; because, if we slowly turn the adjustment- 

 screw, the lines grow dim on the central part of the 

 cylinder, while those parts that he near the edges (speak- 

 ing according to the optical appearance) come into dis- 

 tinctness. Presently the edges of the cylinder become 

 sharply defined, and are seen to be cut into exceedingly 

 shallow saw-like teeth, about as far apart' as the lines ; 

 b 2 



