4 THE AMERICAN MONTHLY (Jan 
expects a perforation to appear brighter than the sub- 
stance surrounding it; and I would point out that opaque 
materials present in this image the only appearance one 
would expect, viz. black. 3 
Mr. Nelson preferred the “black dot’’ image because 
he was able to see fine differences and minute details in 
individual “black dots’ of Pleurosigma formosum, which 
were quite invisible at any other focus. 
I do not know Mr. Comber’s reasons for regarding the 
true image to exist between the “black” and ‘white dot” 
images. 
The true image, if it exists, must lie at the true focus. 
But if we divide our objects into zones, may not spheri- 
cal aberration allow, within limits, a true focus to each 
zone? 
Now, the conditions under which the “black” and 
‘‘white dot” images are seen as separate and distinct en- 
tities are those in which the objective is divided intozones. 
When an axial cone is used, the objective is divided 
into two concentric zones, the central zone occupied main- 
ly by directly transmitted light, the peripheral zone oc- 
cupied only by indirectly transmitted (diffracted) light. 
An annular cone (full cone and a central stop in the 
condenser) also divides the objective into two zones; but 
in this case the diffracted light occupies the central, the 
directly transmitted light the peripheral zone of the ob- 
jective. _¢ 
A “half-way” annular cone (a two-thirds cone with a 
central stop half the size of the diaphragm opening) di- 
vides the objective into three zones, of which the central 
and peripheral zones are occupied by diffracted light only, 
and the intermediate zone is occupied by direct light. 
Examine an ordinary balsam-mounted diatom (not a 
Pleurosigma) by these three methods, and, when focuss- 
ing down, you will find the positions of the “black dot” 
and ‘‘white dot” images to be as follows: 
