202 Dr. J. B. Haycraft. 



glass t'n a ray of parallel light. Rods of faintly tinted glass of the 

 same shape as the muscle fibres, having tiny globules Dobie's lines 

 and broader swellings for the dim stripes, when examined under the 

 microscope, or in the field of a lantern, give as strong colour differentia- 

 tions as any muscle fibres, the constrictions coming out quite colour- 

 less, while the dim band and Dobie's line are sharply brought out by 

 their deepened colour.* 



One of the chief faults of which I was guilty when publishing 

 these results in 1880 was that I did not sufficiently lay stress on the 

 appearances presented by a coloured or colourless varicose thread of 

 glass when placed in the path of a parallel ray of light. It is quite 

 different from the same thread when examined in diffuse daylight, 

 for in the latter case a hundred images fall upon the retina at the 

 same time, and the striping and colour differentiations are confused. 

 One can see little appearance of striping, and if the glass is coloured 

 it may appear very much of the same tint ; place it in a lantern, or 

 even lay it down on a piece of white paper, and the picture is quite 

 different. As one is accustomed to view objects in diffuse daylight, 

 one is not prepared to interpret correctly the character of such an 

 object when viewed through a microscope : the clear well-defined 

 bands and colour differentiations of a muscle fibril are not the ap- 

 pearances of a varicose thread as seen in diffuse daylight, but they 

 are those of a similar fibre observed in parallel light when practically 

 a single image falls upon the retina. 



Lastly, we come to the action of polarised light, and here at once 

 the phenomena by no means prove structural differentiation, along 

 the fibre. There are many questions which lead to complication. We 

 have the varicosity of the fibril, which will alter the path of the 

 polarised beam and produce apparent differences along the fibre when 

 there may be in reality none at all. Then we have as a complication 

 the interfibrillar substance, which is simply refracting, and which is 

 chiefly lodged in the neighbourhood of the clear stripe. I was not 

 prepared to say, under these circumstances, what is the action of 

 polarised light on the fibrils, nor do I wish to commit myself now : it 

 is sufficient to say that, even if we grant that alternating singly iso- 

 tropous and doubly refracting anisotropic bands exist along a fibril, 

 it does not follow that these are bands of more solid and less solid 

 material : the whole difference may be due to molecular tension. A 

 fibre of such a shape, as was pointed out to me both by Professor 

 Stokes and by Professor P. G. Tait, is almost bound to possess altern- 

 ating parts in different conditions of molecular tension, and give the 

 familiar appearances when examined by polarised light. 



* In doing this experiment, onlj faintly tinted glass must be used, and, aa this is 

 difficult to obtain, I generally use hollow varicose tubes of white glass filled with 

 coloured fluid. 



