Re: More on A.pellucida

No Spam, from my memory the first image I have is in Neuhauss, Lehrbuch
der Mikrophotographie 1907. I would have to find it again, but it was
made with 'Cadmium light', 275nm I believe was mentioned. Search for
Neuhauss in the yahoo microscope archives and you will find additional
info on earlier and later work on A.p.


It is
desirable to have as large as possible a magnification upon
the sensitive surface as is practical and combine this with as
small as practical a grain size on film or of a pixel. The latter
of course needs to have a certain size to achieve a good S/N
ratio.

Nowadays with 36 mm film and small CCD or CMOS sizes
there are limits. In the old days when microscopists used
very large photographic plates and large projection distances
these limits were more favorable.

The more empty magnification, the lower the contrast. 36mm fim and 3Mp
cameras are more then adequate.

The problem with A.p. for test specimen is it's variability in
silification (or so I believe) and possibly variation in RI of mounting
media. Even slight differences in refraction index between batches will
have tremendous impact on contrast. This all makes a diatom test slide
a personal subjective test.

Last comment, the use of contrast enhancement techniques like
oblique/single sideband/DIC/phase contrast makes it even more
subjective. Only brightfield will show the quality of an optical
system. As you say, the resolution is there, no problem, but without
contrast (by the optical system OR the test specimen), you cannot see
it.

Oh yes, and be careful with taking sizes from SEM, in respect of tilted
valves, also SEM calibration. I wouldn't trust my own SEM pics from
A.p. within 10%. I'm sure Gary's FESEM would be better in that respect.

René.

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