1881.] the Striation of Voluntary Muscular Tissue. 369 



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

 or crest. This was the focus in which Bowman described his prepara- 

 tions as far as I can gather from the paper. If the lens be now lowered 

 ever so little, the stripes are reversed, a most curious 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. (Fig. 

 3, c.) 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, which is depicted in fig. 3, b. 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, which are shown in fig. 3, b. 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 theoretically calculated. 



Parallel rays of light pass upwards through the fibre, and in their 

 course are altered in direction (see fig. 4). 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 con- 

 verging lenses, causing the rays of light to come to a focus (A A' A"), 

 diverging again. The thinner parts (the valleys) will, on the other 

 hand, act as diverging lenses, causing the rays to spread out, as 

 may be seen on reference to the diagram. 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 (see fig. 4). This will 

 produce a difference in the appearance, for the converging rays will 

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

 appear darker. Alter the focus by screwing the lens up or down, and, 

 provided the fibre can still be seen, this state of matters will be 

 reversed ; for after converging, the rays above the position of the 

 ridges will now be diverging, while at the same time those over the 

 valleys will be converging aud will appear bright. 



The condition seen in fig. 3, 6, which is intermediate between the 

 low and high focussed picture of the fibre, would be obtained by 

 shifting the lens half-way between these two positions. Hensen's 

 stripe is no doubt due to rays passing through the centre of the ridges 

 suffering little refraction in their course, and thus causing a brightness. 



