OK, Gisela, since you are using confocal, then as Barbara Foster suggested, you might check your pinhole to make sure it's as small as possible, to exclude as much of the "bent" out of focus light as possible from your detector as it scans.  If you have a wide pinhole you give up the advantages of the confocal mode.
 To accomplish that you should be imaging as close to the cover slip as possible (assuming you are using one), because brightness falls off as you penetrate into the sample, also to minimize other effects of refractive index mismatch. 

Your 20 micron bubbles are so large you really CAN'T always image close to the cover slip if you want to look at their equators.  (Close usually means 10 microns or preferably less from the coverslip surface.)   Working this far away will exaggerate "RI mismatch" artifacts that you should be aware of.  (As they say, "read the book" --Jim Pawley's & others.)  The list may have further suggestions so I'll send this back to the list  --sorry I'm not real familar with this problem.    (Can you whip up a batch with very small bubbles to see if it looks better closer to the cover slip?) 

You haven't described your microscope setup.  If you are using an oil immersion lens you may very well get better results by switching to water or air immersion. You definitely should do the comparison to be sure.  What is the composition of your foam?  Also I am just curious what fluorescent probes you are using.  

Richard
(p.s.   I work on "biliquid foam" made of phospholipids and triglyceride filled with water instead of air; this has much smaller bubbles.  Unfortunately I'm not doing microscopy any more, more formulation work.)

>>> [log in to unmask] 4/24/01 2:40:11 PM >>>
 . . .
In terms of sample prep, the only way to get rid of the rings is to have a
better match of refractive index inside and outside the bubble ...
typically an impossibility in a heterogenous sample such as foam.
In terms of the microscopy, try making your pinhole smaller so that you get
thinner optical sections.  . . .

Barbara Foster

>>> "Gisela Richardson" <[log in to unmask]> 4/27/01 12:54:16 AM >>>
Richard,

Thank you for the answer. Well, I am using the confocal
microscope. That s why I have problems with understanding. I'm
certainly no expert in optics, but it is easier to understand it with
phase contrast.

However, it is nice to have a name on the phenomenon, I'm very
grateful for that!

Best regards,
Gisela