CAUSE OF STRIATION OF VOLUNTARY MUSCULAR TISSUE, 317 
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 much trouble), we will 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 sub- 
stance 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 (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 refer- 
ence 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 valley will be converging and will 
appear bright. 
The condition seen in Fig. 3, 4, 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. Dobie’s line might, of course, 
be the reverse of this, no rays at this point coming to the eye 
of the observer; but we shall speak of this more hereafter, when 
we shall show that there is some reason for suspecting at this 
point a distinct structure. 
Although it is indispensable to account theoretically for these 
appearances, yet to most persons a simple demonstration will 
carry more conviction than any proof deduced from the laws of 
optics, however well they be understood. Instead of showing 
“what should be,”’ we will study “ what is.” 
For this purpose we will imitate as nearly as possible the 
