122 THE AMERICAN MONTHLY [July 



altering the image of the exclamation-marks. Using 

 successively larger annuli of light, this test becomes far 

 more efficient and severe. It was accordingly tested upon 

 A. pellucida mounted in arsenic by Dr. Van Heurck. All 

 the transverse striae in the diatom were most easily re- 

 solved with a central, solid, unstopped full aplanatic cone 

 of over 0-90 from a dry condenser. The larger features 

 were of course also quite correctly and sharply imaged. 

 But this is not nearly the limit. Owing to some astig- 

 matism and other defects, my vision is very coarse and 

 imperfect in these matters, and for me to see the strisB 

 means much more for many other observers. The first 

 valve Mr, E. M. Nelson showed me in balsam as 

 "strongly" resolved, was to my sight quite unresolvable, 

 and he had to search for another, which I was able to see. 

 This diatom is one of the most variable in resolvability of 

 the whole list, quite apart from the mere coarseness of 

 striation. That is no difficulty at all. Since that exper- 

 iment Mr. E. M. Nelson has shown A. pellucida dearly 

 resolved into striae mounted in balsam, as well as " dry," 

 with a similar cone of over 0*90 from Powell's apochro- 

 matic condenser, and a Zeiss ^ apochromatic of 1"40. This 

 latter lens was probably one of the finest ever made, 

 and the mere strisB were not all it had to tell us, using 

 no arrangements beyond the 0-90 full cone, and Giffard's 

 green light-filter. On a dry valve, it clearly displayed 

 where bits of coarser upper membrane with their blacker 

 lines were overlying the lower, as is more often seen in 

 A. Lindhei7nerii. And on a strong valve in quinidine, 

 carefully adjusting for what may be termed the " white " 

 focus, each of the striae could be seen outlined at both 

 edges, the ontlines being a series of small convex curves, 

 scalloping out the stria into partly-defined oval beads. 

 The divisions or narrower necks between these partly- 

 defined ovals did not lie in longitudinal rows, but oc- 

 curred with a considerable degree of irregularity. Sueli 



