First of all -- thanks again the the many people who took the time to
respond,
and especially to people who wrote quite detailed replies.

The consensus is that there is either dirt or other optical
imperfections somewhere after the objectives. Our scope does not have a
window below the objectives to keep dust out of the body, so there is
ample oportunity for something to get in there. Many people suggested
that there might have been oil. They dont use oil objectives in this
setup, but they do electrophysiology, and it is possible some saline
somehow got down there and damaged the optics. It may not be visible,
but a damaged coating creates a phase object that could easily create
artifacts.

The dependence of the phenomenon on the NA of the condensor was
attributed to the increased depth of field that the low NA creates (from
Barbara Foster).

The people in the lab will have to decide if we are going to go after
this  (serious disassembly of the scope and serious cleaning) or  will
they just live with it.

Many  people replied to say that they have the same problem and simply
have to put up with it.

So this is what I have so far. If we decide to try to solve it, and
succeed, you will all hear about it.

--aryeh
--
Aryeh Weiss                          | email: [log in to unmask]
Department of Electronics            | URL:
http://optics.jct.ac.il/~aryeh
Jerusalem College of Technology      | phone: 972-2-6751146
POB 16031                            | FAX:   972-2-6751275
Jerusalem, Israel                    | ham radio: 4X1PB/KA1PB