48 THE AMERICAN MONTHLY [Feb 
viously nothing to do with the color in question, because 
it would require a far greater aperture than .45 N.A. to 
develop upon a dark ground, any color arising from the 
diffraction of so fine a grating ; and this question is quite 
independent of that concerning the different kind of illu- 
mination required to develop the color, a point of which 
we have as yet found no explanation. If a Hyalodiscus 
subtilis whose structure is about 70,000 per inch, or twice 
as fine as that of Hyalodiscus stelliger, be examined on a 
dark ground with a lens of .25 N.A. no color will be per- 
ceived, while the H. stelliger under similar conditions will 
be brightly colored ; if the aperture be increased to .5 or 
.6 the H. stelliger will be resolved, while the color of the 
H. subtilis will be an intense blue. Now the resolution of 
the H. subtilis may be accomplished with a dry lens of 
.95 N.A., used critically, but as this lens reveals nothing 
of the extremely delicate structure we are considering on 
Actinocyclus, it stands to reason that the color, observed 
in Actinocyclus with quite a low aperture and with trans- 
mitted light, cannot possibly be caused by this delicate 
structure. To repeat the argument :— 
HYALODISCUS SUBTILIS. 
This diatom when viewed upon a dark ground, with a 
lens whose aperture is .55 N.A., is colored ; the structure 
which gives rise to this color can be resolved by a dry 
lens of .95 N.A. ) 
ACTINOCYCLUS RALFSII. 
This diatom when viewed by transmitted light, with a 
lens whose aperture is .25 N.A., is colored ; the color re- 
mains when the coarse structure on the diatom is resolv- 
ed;adry lens of .95 N.A., however critically used, is 
quite unable to resolve the fine veil on this diatom. If 
this fine veil were the diffractor which caused the color- 
ation of this diatom, it would require a lens with an aper- 
ture of at least .55 N.A. to develop the color. 
Finally, all diffraction colors vanish with transmitted 
