3820 JOHN BERRY HAYCRAFT. 
may have been, it is well to collect all evidence possible to show 
that these markings are nothing more than optical effects, to 
which end a very testing experiment was suggested to me by 
Professor Tait. It is evident that if these cross bands are seen 
when parallel, or nearly parallel, rays of light are passing through 
the fibre, by using converging or diverging rays the appearance 
will be altered, and it will be possible by careful adjustment of a 
lens to cause a total reversal of the striping. If a fibre be care- 
fully focussed and a strong biconcave diverging lens be placed 
between the stage of the microscope and the mirror, and cate- 
fully moved about with the fingers, it will be possible entirely to 
alter the fibre, causing a total reversal of the cross bands. On 
withdrawing the lens, of course the fibre resumes its normal 
appearance. I may mention that several lenses were tried 
before one was found which would in at all a satisfactory 
manner show this phenomenon; when successful the experiment 
is very striking. 
In opposition to my view is the one generally accepted, 
namely, that the cross stripings are produced by differences 
along the fibre of chemical composition, and refrangibility. 
Now, suppose that there were along the fibre two alternating 
structures, A ands. Let a represent the bright stripe and B 
the dark stripe. If a has a higher or lower refractive index 
than B, it is evident that although they were immersed in any 
number of fluids of refrangibility varying from the lowest to 
the highest, yet a would always be distinguishable from B, and 
the striping would always be apparent. Then, again, by placing 
the fibres in fluids of indices near to that either of « or B, the 
most striking would be the contrast. If, however, the fibre 
were homogeneous throughout, the striping being merely due to 
the form, then if the fluid and the fibre have the same refrac- 
tive index all striping will disappear. On Professor Tait’s 
suggestion, I tried a series of fluids formed by mixing, in 
various proportions, alcohol, whose refractive index is low, with 
oil of cassia, which is high. In this way I have prepared 
specimens showing almost no cross striz, ‘the fibre appearing 
uniform until after most careful examination. 
Dr. Klein has since shown me some muscular fibres of an 
insect. They were quite smooth and cylindrical, and were un- 
striated. In these specimens there were, on very close exami- 
nation, cross lines separated by comparatively wide intervals, 
It is possible that they represented Dobie’s lines. 
1 More recently my friends, Messrs. Geddes and Beddard have demon- 
strated a very curious condition in the muscular fibres of the Echinus, which 
my views entirely explain. They noticed that in the same fibre some parts 
were cross striped, while in parts no striation was to be seen, Hearing of 
