126 THE AMERICAN MONTHLY [May 
naphthaline and balsam. Dotted structure on outer zone 
well seen, although faint and difficult near the edge of 
the disc. In balsam mounts the structure appears still 
fainter, but nevertheless may be traced nearly to the out- 
er edge, where it runs at about 76,000 to the inch. 
Surirella gemma Ehrbg. In realgar the beading has 
been seen beautifully defined with the valve arranged 
longitudinally on the sharply focussed edge of the lamp 
flame. Specimens mounted dry, in balsam, and in quin- 
idine, have been also examined, but their complete reso- 
lution has proved a much more difficult matter. 
Colletonema vulgare. Moller’s balsam type slide. This 
has been most carefully studied with the 4mm. The res- 
olution is very faint, and requires particularly exact focal 
adjustment, but when once seen it can be held fairly stead- 
ily without any great difficulty. Dr. Van Heurck writes 
of this-diatom, “Stries fines, délicates, les moyennes fai- 
blement radiantes, les terminales paralléles, environ 34 en 
1 c.d.m.; les stries médianes plus fortes, plus écartées, 24 
en 1 c.d.m. et plus radiantes.”’ 
Navicula major. Moller’s balsam type slide. The full 
resolution of the structure of the bands on the hoop of 
this diatom is by no means easy, even with the Zeiss 3 
mm. apochromat of N.A. 1:4. Notwithstanding this, the 
resolution is carried very far by the 4 mm., the stri@ ap- 
pearing remarkably black, crisply defined, and well sep- 
arated, their beaded nature being quite recognizable, al- 
though not so fully revealed as‘with the oil-immersion. 
On this specimen the striz alone are just visibly separa- 
ted by the 12 mm. apochromat, 5-6ths axial cone, and a 
Huyghenian eyepiece magnifying about 45 times, the 27 
compensating ocular not proving sufficiently powerful for 
the purpose with this objective. 
ComMENTsS BY Dr. E. J. Sprrra.—To enable an object 
consisting of lines separated by minute intervals, or dots, 
or any small structures, to be seen, two conditions were 
