HAIRS, FEATHERS, AND SCALES 15 



also a number of fine 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 get the hair to 



be exactly in 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 magni- HUMAN HAIR. 



fying power, each line represents l-10,000th of an inch, 



the hair is 80-10,000ths,=^d f an inch, in diameter. 



In all branches of natural history, but perhaps pre-emi- 

 nently in microscopic natural history owing to its greater 

 liability to error from illusory appearances we gain much, 

 information on any given structure by comparing 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, tinder the 

 same magnifying power, specimens of the hair of other 

 animals. In order, however, to explain it, I must antici- 

 pate those observations. 



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

 having a light brown tinge, and marked with a great num- 

 ber of delicate lines, having a general transverse direction, 

 but very irregularly sinuous in their individual courses. 

 These lines we perceive to be on the surface; because, if 



