ZOOLOGY AND BOTANY, MICEOSCOPY, ETC. 969 



" We see, therefore, that a muscular fibre presents just those 

 appearances which a transparent body of uniform texture and of 

 similar shape would possess. However conclusive these proofs 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 

 searching 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 con- 

 verging 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 carefully focussed, and a strong bi-concave 

 diverging lens be placed between the stage of the Microscope and the 

 mirror, and carefully 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 phe- 

 nomenon ; 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 and B. 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 A or B, the more 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 

 refractive index, all striping will disappear. On Professor Tait's 

 suggestions, 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 stride, the fibre appearing uniform until after most 

 careful examination. . . . 



" The position that we have reached is this : a muscular fibre 

 presents such cross markings, varying with shifting the lens up or 

 down, as a filament of homogeneous structure and similar shape. I 

 have shown this experimentally, and have illustrated it by simple 

 experiments, which it is in the power of any one to test. This being 

 the case, I have searched to find if there be reason to assert any want 

 of uniformity along the fibre, using various methods of staining. This 

 I have failed to do, and have shown that the views commonly held are 

 to be explained simply by the shapes of the fibres. . . . 



" A fibril is structureless throughout its entire length, except that, 

 perhaps, there may be membranes, or lines of fission, or layers of 

 cement at the positions of the lines of Dobie ; this we leave an open 

 question. In using the word ' structureless,' I must not be mis- 



