46 THE AMERICAN MONTHLY [Feb 
will have become green, and so on. If a lens of still 
greater aperture be employed, those that were originally 
green will become colorless and will be resolved, and the 
colors of the others will be lowered a stepin the gamut. 
This law, which holds good with other diatoms, quite 
breaks down with the Actinocyclus Ralfsii, for if we ex- 
amine one on a dark ground with a low power those parts 
which were brilliantly colored blue with transmitted light 
now become a golden yellow. Again, all other diatoms 
lose their color when the structure which gives rise to it 
by diffraction is resolved, but with A. Ralfsii the color 
remains, although the structure is resolved, and lastly 
other diatoms when viewed by axial transmitted light 
appear white, while this is brilliantly colored, provided 
that a lens of suitable aperture be employed to examine 
it. The color in this diatom is visible with transmitted 
light, provided that the aperture of the objective used does 
not greatly exceed .45 N. A.; the power of the objective 
or eyepiece is of no consequence, the aperture of the lens 
is the sole determining factor in the case, as may be 
proved by manipulating an iris diaphragm at the back of 
the objective. 
There is a slide in my cabinet which contains both an 
Actinocyclus Ralfsii and a Hyalodiscus stelliger. This last 
diatom has an ordinary sieve-like structure of about 35,- 
000 perinch. Now, these two diatoms act in precisely con- 
trary manners, for on a light field with ordinary trans- — 
mitted light the Actinocyclus is brilliantiy colored while 
the Hyalodiscus is colorless; but on a dark ground the 
Hyalodiscus is colored, and the Actinocyclus colorless. In 
short, the Hyalodiscus follows the rule of all other dia- 
toms, e. g., the Pleurosigme, Naviculae, etc., and behaves 
precisely like them. In Actinocyclus Ralfsii the only part 
which follows this general diatomic rule is the narrow 
margin which, with transmitted light, is a golden yellow. 
(Thsi color may be somewhat erroneously described, as its 
