1902} MICROSCOPICAL JOURNAL. 3 
Images of Diatom Structure. 
W. BALFOUR STOKES. 
The advantage of using a wide solid axial cone of il- 
lumination lies mainly in the blotting out of those per- 
plexing and numerous interference effects so apparent 
when a narrow or oblique beam of light is employed. 
With a wide cone, therefore, difficulties of interpreta- 
tion are greatly reduced; but with many objects we may 
still have two images, from which we must select the 
true one. It is easier, however, to select from two than 
from a dozen. | 
A simple diatomic structure, for instance, if examined 
by means of a narrow or oblique beam of light, will pre- 
sent, at different focal positions, many images or effects, 
not one of which can be called trustworthy; but with a 
wide solid axial cone we obtain two images only, totally 
opposite in appearance and at different focal positions, 
but both of which are conceivably trustworthy repre- 
sentations of the only possible reality. 
These two last images I will describe as follows: 
Silex. Edges and perforations. 
Upper focus .. dark bright. 
Lower focus .._ bright dark. 
The upper image is usually known as the ‘‘white dot” 
image, diatom perforations appearing as white dots. 
The lower image is known as the “black dot’ image, 
diatom perforations appearing as black dots. 
The question is: What is the true PMA? A diver- 
sity of opinion seems to exist. 
Most microscopists would, I think, select the ‘‘white 
dot” image, but Mr. Nelson prefers the “black dot,” 
while Mr. Comber suggests that the true image should 
lie between the two. 
The “white dot” image would be preferred by many 
because, with directly transmitted light, one naturally 
