1897] MICROSCOPICAL .TOUKJVAL. 81 



att'ords .-i better test tliau a faint striatiou just glimpsed 

 with a lens barely [)ossessing the necessary aperture to 

 resolve it. 



N. lyru. — Two nights ago, 1 saw one valve in balsam 

 beautifully dotted with a 1 in. on a dark ground. Another 

 valve, however, was so fine, that it required a wide- 

 augled h in. to do it. 



Ohe of the best diatoms to work on with the higher 

 powers is the large N. rho/riboides, found in "Sozodont" 

 tooth-powder (discovered in this material by G. Main- 

 laiid, K 11..M.S.); it is very constant in fineness, the trans. 

 strisB being 60,000 per inch. Zeiss a[)Ochromatic ^in. 

 crosses it. 



The best test for low-power lenses, say, from 1^ to ^ 

 or 4-10 of -n N.A. is a balsam-mounted diatom with dark 

 ground illumination by Abbe's achromatic condenser and 

 central stop. The stop should be just of a sufficient size 

 to give a ])erfectly dark ground, and no larger. This 

 test consists in the freedom from scattered light about 

 the diatom. A coarse N. lyra does very well; the clear 

 structureless parts of the diatom should be free from 

 scattered light from the neighbouring parts that have 

 structure. Of course, the lenses must be accurately ad- 

 justed by the alteratior. of tube-length. For the higher 

 powers a bright field should be used from a % axial cone, 

 and the finer forms of Lyra, or P. formosum, or ihe 

 larger N. rhot/i/joides iive suitable. These may be mounted 

 in balsam, or better, styrax; or, better still, in quinidine. 

 yuinidine is the best medium, but for some reason or 

 other it is very difficult to work with. I have one of the 

 first slides prepared in this medium which is still excel- 

 lent; but most of the others in my possession have gone 

 bad. The fact that one of the early slides is perfect 

 shows that mounts in this medium are possible. Why 

 they cannot be multijilied is a mystery 1 am unable to 

 fathom. 



