16 EVENINGS AT THE MICROSCOPE 



we slowly turn the adjustment-screw, the lines grow dim 

 on the central part of the cylinder, while those parts that 

 lie near the edges (speaking according to the optical ap- 

 pearance) come into distinctness. 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; 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 differ- 

 ing 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 

 further 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, with 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 mathematically 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; in- 

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

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



