316 JOHN BERRY HAYCRAFT. 
as well to give a full description of what is actually to be seen, 
for this has yet not been stated. 
With a structure of complicated figure, such as the one we 
are considering, it is obvious that there is no one focus in which 
it may be described. ‘There is one pretty definite focus for a 
single speck or thin film, but even when examining a simple 
cylinder, it is evident that when the borders of it are clear and 
distinct, the upper surface is slightly out of focus. We shall 
see, that in the case of the muscle, although there is one 
position of the lens when the parts are very distinctly seen, and 
in which they have mostly been described, yet that on slightly 
altering the focus, the appearance is changed. These changes 
we must carefully study. 
For this purpose we may select the large muscles of the thigh 
of arabbit; 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 3; inch 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. (Fig. 3, a.) The dark band corres- 
ponds with the valley and the light one to the ridge, or crest. 
‘This was the focus in which Bowman described his preparations 
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 focus- 
sing 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 
