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On Mon, 18 Mar 2013 10:13:51 -0700, Julio Vazquez <[log in to unmask]> 
wrote:

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Dear Julio,
There is no adjustable collar on the objectives - the link below shows the list 
of dipping objectives I have used and what a ROI plot looks like.
https://dl.dropbox.com/u/24087924/Convallaria%20slide.jpg
The collar adjustment would be of little use since I need to record changes 
during solution switches. My cells are definitely always staying wet. The effect 
is nothing to do with the effect of propionate on the cells since I see the 
distortions when I run the solution over a fixed glass slide with a preparation 
beneath a coverslip (embeded and sealed).
I agree that the refractive index change is small...
There are no air bubbles beneath the objective. I think it must be a Schlieren 
type of effect. 
Greetings,
Christof