Thanks for the responses for my request.  For those
who provided technical background, I appreciate the
comments.  Please be assured I knew what the lenses
were, the glass type, etc.  Our problem is that even
with the correct C-mount on the camera I am getting
significant vignetting of the image area, thus
reducing the usable image in a given field, and at
given magnifications.  The microscope is properly
aligned, with Kohler illumination, the condenser and
aperture are correctly set and aligned.  The camera is
parfocal with the objective lens.  So, having used the
camera on another microscope (Nikon FXA, with all
planapochromats) are finding no vignetting, the
conclusion I have reached is that the Fluotars do not
provide a flat enough field for my purposes.
Unfortunately, the FXA is not available for long-term
use!  One other comment: the tube length on the
objectives on the Aristoplan is 160mm, and on the
Nikon (as well as older Zeiss and Olympus) scopes is 140mm.
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