ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 967 



" For this purpose we may select the large muscles of the thigh 

 of a rabbit ; stretch them ever so little upon a piece of wood, and 

 place them for some days in 50 per cent, alcohol. A high power is 

 required for their examination. I have been in the habit of using a 

 gVi^ch of Gundlach, a very perfect lens ; a ^^^-inch will, however, 

 do. A small bundle of fibrils should be selected in preference to a 

 whole fibre for examination. 



" On focussing, it becomes at once apparent that on varying the 

 adjustment ever so little, you may bring into focus the tops of the 

 ridges or the bottoms of the valleys which lie between them. Now 

 this slight alteration is sufficient entirely to change the optical 

 appearances. 



" First raise the lens until the fibre be out of focus, and is only to 

 be seen as a dim streak running across the field, then bring it down 

 until its form and the cross markings are distinctly to be seen (the 

 border is now not quite distinct, on a level with the horizontal axis of 

 the fibre). In this position, alternating light and dark bands are 

 made out, but no vestiges of Hensen's stripes or Dobie's lines. The 

 dark band corresponds with the valley and the light one to the ridge, 

 or crest. ... If the lens be now lowered ever so little, the stripes 

 are reversed, a most cmdous point, which was noticed by Bowman, 

 but afterwards lost sight of. The dark band now corresponds with 

 the ridge, and the bright band with the valley. This is the focussing 

 in which it is usually described, and in this position Dobie's line and 

 Hensen's stripe are to be seen, as a rule, in uncontracted fibres. 



" Between these two positions of the lens there is generally a well- 

 marked intermediate one. The crests and valleys are both bright, 

 and equally so, although the slightest movement of the fine adjuster 

 will make either one or the other the darker ; on the slopes, as it 

 were, there are, however, narrow shaded bands. The fibre is now 

 quite clear and distinct, and the longitudinal fibrillation is now best 

 made out — if it can be seen at all — and yet there is no sign of either 

 Hensen's or Dobie's stripes. These being the observed appearances 

 (and they may be verified without very much trouble), I shall 

 calculate theoretically the appearances which a homogeneous fibre of 

 such a shape should present when examined by transmitted light, so 

 as to see whether our observed effects tally with what may be theoreti- 

 cally calculated. 



"Parallel rays of light pass upward through the fibre, and in 

 their course are altered in direction. The substance of the fibre being 

 of higher refrangibility than the fluid in which it is mounted, the 

 thicker parts which correspond to the ridges will act like converging 

 lenses, causing the rays of light to come to a focus, diverging again. 

 The thinner parts (the valleys) will, on the other hand, act as 

 diverging lenses, causing the rays to spread out. Now it is evident 

 that when the objective is arranged to focus those rays which have 

 passed through the fibre and converge over the ridges, at that same 

 position the rays above the valleys will be diverging. This will pro- 

 duce a difference in the appearance, for the converging rays will give 

 a bright band, while the position of those rays which diverge will 



