1901) MICROSCOPICAL JOURNAL. 45 
When, however, this diatom is viewed in a critical man- 
ner with a wide-angled oil-immersion lens all its lovely 
color vanishes and its beautiful rays become so inconspic- 
uous as to be hardly noticeable; in spiteof this, however, 
its interest to a scientist will be rather increased than 
diminished. It is not difficult to account for the loss of 
the rays, for when the diatom is examined under a low 
power, the dots, or more accurately the minute perfora- 
tions in the silex, are so closely approximated to one an- 
other that they appear to run together and form rays, but 
when this structure is examined under a higher power of 
greater aperture, these dots are so widely separated that 
they cease to give this appearance of lines or rays. 
The reason for the loss of the color is not quite so ob- 
vious, for the color may be produced in a variety of ways 
e. g. by polarization, by the unequal refraction of light, 
by diffraction, by the varying thickness of transparent 
thin plates, and lastly by pigments. Now we know that 
exceedingly minute objects, such as bacteria and micro- 
- cocci, when stained by pigments do not lose their color 
when examined by high powers ; but on the other hand, 
objects such as diatoms, which owe their color to the dif- 
fraction of light by their minute structure, change their 
color from violet to red and finally lose it altogether as 
the power, or rather the aperture, of the objective is in- 
ereased. It is an instructive experiment to examine with 
dark ground illumination and a low-power objective, say 
one inch or % inch of aperture .25 to .3 N. A., aslide con- 
taining various species of Plewrosigma that have differ- 
ent degrees of fineness of structure; the coarser forms 
will appear ruddy, those a little finer,greenish, those still 
finer blue, and some finer still, will appear violet or indigo 
Now if the lens be changed for one whose aperture is .4 
N. A., those that were ruddy will be colorless, and the 
structure that gave rise to the color will be resolved, those 
that were green will be ruddy, and those that were blue 
