1902} MICROSCOPICAL JOURNAL. 125 
structure we have an image greatly lacking contrast, and 
exhibiting no broad diffraction edges, in which we may 
glimpse points just within the theoretical grasp of the 
objective, the difficulty of holding these for any length 
of time strongly reminding a telescopist of his fleeting 
glimpses of planetary detail just visible under the most 
favorable atmospheric conditions. This is precisely the 
contrary of what we should expect to see according to the 
deductions from the diffraction theory summarized in 
“Carpenter,” for these would make us feel confident that 
in the instance of the A. pellucida the central narrow 
incident pencil should produce the most truthful picture, 
and one more nearly approximating to the ultimate struc- 
ture in exactness, than would be the case with a large 
pencil. But it is found by practice, that with N. A. -985 
and a large 5-6 or 6-7 cone, with light passed through a 
copper acetate screen, the stria, running 94,000 to an in. 
are discernible and clearly separated, while they are abso- 
lutely invisibl with a much narrower pencil. The Abbe 
limit of resolution for N.A.-99 being 95,446 lines to the 
inch with oblique white light (line E), and 103,458 with 
blue light (line F), therefore, if this observation is cor- 
rect, we may feel confident that it is not the narrow, but 
the very large axial cone, which affords the maximum 
resolution, and the nearest possible approach to the com- 
plete rendering of the object that the aperture of the ob- 
jective will permit.—Quekett Club. 
Evolution as Seen in Amphiprora, One of the Bacillaria. 
ARTHUR M. EpWARDs, M. D., F.L.S. 
Amphiprora was a group called a genus of Diatom- 
aeew by C. G. Ehrenberg and published in his Abhand- 
lungen, for 1843. I have already said in my paper in 
the American Journal of Science for 1891 that it is not 
an Amphiprora as now understood, but a Navicula of 
the Pinnularia type with pinnules and not striw. The 
