318 THE AMERICAN MONTHLY [Dec 
ture. The point to which I now beg to call your atten- 
tion is the accomplishment of the resolution of normal 
specimens of this diatom by means of Zeiss’s dry 4 mm. 
apochromat, and 5-6ths solid axial cone from Powell’s ad- 
justable apochromatic condenser. 
I was led to attack the A. pellucida with the above 
specified optical arrangement through having remarked 
the great strength of the resolution yielded by some real- 
gar-mounted specimens under the Zeiss 3 mm. of N. A. 
1-4 and a solid axial cone of about N. A. 1:2 from an oil- 
immersion condenser. I must confess that the exact 
theoretical resolving limit of an object glass of N. A. -95, 
as given in the table on page 85 of Carpenter’s ‘‘The Mi- 
croscope and its Relations’ (Seventh Edition, Edited by 
Dallinger, 1891), had at the time escaped my memory, 
otherwise it is extremely improbable that any such at- 
tempt would have been made. 
It was found, however, that in actual practice the 4 
mm., used in conjunction with a 27 compensating ocular, 
with which eyepiece the image remained perfectly sharp, 
would steadily show the fine transverse striae on realgar 
mounts, although the lineation was much fainter than 
that revealed by oil-immersion lenses of large aperture. 
The resolution of valves in realgar having been ac- 
complished, dry and balsamed specimens were next ex- 
amined, and to my very considerable surprise, both prov- 
ed resolvable with the 4 mm. and 5-6ths axial cone. In 
balsam the striae appear as extremely faint, but clean, 
grey lines of great fineness, Although most faint and 
difficult, they have been held with perfect certainty for 
short intervals, slightly averted vision proving of materi- 
al assistance in this instance. 
In order to satisfy myself that the true striae are in- 
deed rendered visible by the 4 mm., a valve has been 
first arranged to exhibit them nnder the lens, an oil-im- 
