1909] MICROSCOPICAL JOURNAL 5 
‘white dot.” 
“black dot.”’ 
“black dot.” 
‘white dot.” 
) “black dot.” 
“white dot.” 
‘black dot.” 
There is, as the last method shows, an image for each 
zone of the objective, and on examining the results we see 
that apparently the ‘white dot” image is formed by the 
direct light, and the “black dot” is formed by diffracted 
light. | 
What would be the real effect of spherical aberration ? 
I have been told that most objectives are spherically un- 
der-corrected. If this be so, the foregoing suggestion is 
strongly supported: for one must bring an under-cor- 
rected objective nearer to the object in order to utilize 
the periphery of the lens. Compare this with our three 
previous experiments, and agreement will be found. 
To return to our question: Which is the true image ? 
Let us deal with Mr. Nelson’s deductions from the ex- 
amination of Pleurosigma formosum. This diatom dif- 
fers from the majority of diatoms in this respect: When 
an axial cone is employed, the “black dot’’ image is above 
the “white dot,” the very reverse of what occurs with the 
great majority of diatoms. This, in my opinion, points 
to a difference of peculiarity in the structure of P. for- 
mosum. . 
I suggest that the perforations of this diatom are more 
or less silted up with foreign material. When the late 
Mr. C. Haughton Gill charged the minute perforations in 
diatom valves with opaque metallic sulphides, the ‘“‘white 
dot” image of the perforations was changed to a “black 
dot” and the “‘black dot” to a ‘“‘white dot”; but no such 
changes took place in the appearances of P. formosum, 
the “black dot’? image being merely intensified. Does 
Axial cone 
Full annulus 
Two-thirds annulus 
