IIAIKS, FEATHEKS, AND SCALES. 



with this eye-piece micrometer, we will first of all 

 measure its diameter. 



You see, crossing the bright circular field of view, 

 a semi-pellucid cylindrical object ; that is the hair. 

 You see 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 



||i!|-lil*« 



nUMAM HAIK. 



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,= ^i^rd of an inch, in diameter. 



In all branches of natural history, but perhajis j)re- 

 eminently in microscopic natural history, — owing to its 

 greater liability to error from illusory appearances, — we 

 gain much information on any given structure by com- 

 paring it with parallel or analogous structures in other 

 forms. Tims we shall find that our understandinjr of 

 the structure of this hair will be nmch 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 ob- 

 servations. 



