The Theory of Immersion. 221 



different enveloping layers and the axis cylinder, I still remained in 

 doubt, as it seemed to me unnatural that such a large number of 

 fibrils, as that of the fibrillous layer, could arise from so small a 

 surface. Finally, nearly three years ago, when composing the first 

 part of this paper, I repeated my examinations and met again in 

 the grey substance of the spinal marrow with a number of those 

 axis-cylinder processes, on which I found my previous observations 

 most satisfactorily confirmed. On these, namely, I observed quite 

 distinctly how the fibrils arose from the sheath of the axis cylinder 

 hi/ decrees, until the whole layer was formed (Fig. 22). As, how- 

 ever, the material on which this observation was made, had been 

 lying for some time in a solution of chromic acid, nothing of the 

 tubular membrane could be seen. 



In finally summing up the results of my researches regarding 

 the structure of the douhle-hordered nerve fibre, this will be found 

 to consist of the following parts : 1, of the true nerve fibre, the 

 so-called axis cylinder, consisting of a bundle of granidar fibriU, 

 enclosed within a distinct sheath of their own ; 2, of a semi-liquid 

 substance, the medullary layer, surrounding the axis cylinder; 

 3, of the fibrinous layer, consisting of very fine, delicate and 

 smooth fibrils, and surrounding the medullary layer ; and, 4, of the 

 tubular membrane, or external sheath, a thin, structureless and 

 elastic membrane, enclosing all the other parts. Whether now the 

 thirdly-named part really exists in the hving nerve fibre, or 

 whether it is only produced by coagulation, it must be decided by 

 other, more accurate histological researches than those hitherto 

 made. 



V. — The Theory of Immersion. 



By Eev. S. Leslie Brakey, M.A. 



Part I. 



If immersion lenses have the superiority over dry lenses which has 

 been ascribed to them, it is essential that we should be able to 

 account for the difi'erence, as this may be a guide to us not to look 

 for improvements in a wrong direction. This is what I propose 

 to investigate up to a certain point in the present paper. The 

 material for it was prepared some time ago, but laid aside because 

 the preliminary controversy about apertures stopped the way, so to 

 speak, for the publication of it. This controversy has several 

 times seemed to be on the eve of settlement, but still, by some 

 curious fatality, broke out afresh, as new correspondents com- 

 mencing it brought new objections to be answered. 



In a certain sense, of course, it may still be said to be un- 



