4 EVENINGS AT THE MICKOSCOPE. 



these, however, are so slight that they can be seen only 

 by very delicate adjustment. We go on turning the 

 screw, and presently another series of transverse lines, 

 having the same characters as the former, but differing 

 from them individually, come into view, at the sides first, 

 and presently in the middle, and then, as we still turn, 

 become dim, and the whole is confused. In fact, our eye 

 has travelled, in this process, from the nearer surface of 

 the hair, right through its transparent substance, to the 

 farther surface ; and we have seen that it is surrounded 

 by these sinuous lines, which the edges or those portions 

 of the hair which would be the edges, if it were split 

 through the middle (for, optically, this is the same thing) 

 show to be successive coats of the surface, suddenly 

 terminated. If we suppose a cylinder to be formed of very 

 thin paper, rolled up, and then, by means of a turning- 

 lathe, this cylinder to be tapered into a very lengthened 

 cone, the whole would be surrounded by lines marking the 

 cut-through edges of the successive layers of paper ; and, 

 owing to the thickness of the paper not being mathe- 

 matically equal in every part, these edges would be 

 sinuous ; exactly as we see in these lines upon the hair. 

 The effect and the cause are the same in the two cases. 



A hair is closely analogous to the stem of a plant; 

 inasmuch as it grows from a root, by continual additions of 

 cells to the lower parts, which, as they lengthen, push 

 forward the ever-extending tip. Indeed, in some of the 

 hairs which we shall presently look at, there is the most 

 curious resemblance to the stem of a palm, with the pro- 

 jections produced by the successive growth and breaking 

 away of leaf-bases around the central cylinder. Inter- 

 nally, too, the resemblance is remarkable ; for, if we 

 split a human hair, and especially if we macerate it, i.e. 

 soften it by soaking it, in weak muriatic acid, we shall 

 find it composed of (1) a thin but dense kind of bark, 

 forming the successive overlapping scales just described ; 



