310 JOHN BERRY HAYCRAFT. 
structural differences—modern investigators have introduced 
hypotheses to account for it, which entirely imply differences of 
structure along the filament. The reason of this is, if I may 
express an opinion, that his theory has been completely lost 
sight of, and that it was followed by the discovery of startling 
facts, which at first sight seemed to set it on one side. 
In discussing the views of modern inquirers, I shall not, in 
all cases, consider them in the order of their priority, and illusion 
will not be made to much that has been written upon this 
subject, which, indeed, may safely be put on one side. 
The light stripe—dark stripe of Bowman—has been shown by 
Dobie, Busk, and Huxley, to be traversed by a very fine dark 
band, or rather line “ Querlinie” dividing it into two equal parts. 
We shall speak of this as Dobie’s line, or the dark stripe in the 
centre of the light. (Fig.1, p, woodcut.) Then, again, the 
dark stripe is traversed in its centre by a lighter band called 
Hensen’s stripe.’ (Fig. 1, #, woodcut.) Other bands border 
this stripe, but as they are certainly not to be seen in all speci- 
mens, however well prepared, and as we shall presently account 
for them, they need not trouble us here. 
As early as the year 1839, Boeck showed that muscle refracts 
light doubly, which statement was, however, modified in 1857 
by Briicke. The latter examined muscles prepared in alcohol 
by polarised light, and found that the dark stripe (dark in 
ordinarily non-polarised light) appeared luminous in the dark 
field of the microscope, and that the light stripes were dark 
when the Nicols were crossed. The dark stripes, therefore, 
appeared to be doubly refracting (anisotropous), and the light 
stripe singly refracting (isotropous), the fibre consisting of 
singly and doubly refracting discs alternating one with another. 
These observations he verified by an examination of the fibre 
with thin plates of selenite and mica. The views of Briicke 
have, in their turn, received considerable modifications which 
will be understood by reference to a diagram. Fig. 2 expresses 
very well the results of my own observations, which are in accord- 
ance, | find, with those of other observers. (See the ‘Handbuch 
der Physiologie,’ by Dr. L. Hermann, 1879, p. 20.) The black 
part of the diagram corresponds with the portion of the muscle 
which singly refracts light (isotropous), while the light shaded 
parts correspond with the anisotropous substance. 
This diagram does not, it will at once be seen, correspond 
with the views held by Briicke, for the great mass of the light 
stripe, with Dobie’s line in the centre of it is anisotropous, the 
1 This stripe was also described by Dobie in the ‘Annals of Natural 
History for 1849,’ and it may be called Dobie’s light stripe. as 
