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I've noticed the same thing.
Discarding the grumpy usual, on the basis of not having "people like 
that" using the confocals, the only thing I can think of is minute 
pinholes in the mounting cement allowing the oil to soak through - by 
gravity if on on inverted microscope, or capillary action if on an upright.

Phil

On 04/04/2016 05:48 , Gary Laevsky wrote:
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> To join, leave or search the confocal microscopy listserv, go to:
> http://lists.umn.edu/cgi-bin/wa?A0=confocalmicroscopy
> Post images on http://www.imgur.com and include the link in your posting.
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>
> Hi All,
>
> Trying not to be cynical here, but, can anybody please explain to me if
> there is a way for a drop of oil to end up on the center of the back focal
> plane of an objective (oil immersion objective) on an inverted scope.  No
> smear from the side.
>
> Thanks.
>

-- 
Philip Oshel	
Microscopy Facility Supervisor
Biology Department
024C Brooks Hall
Central Michigan University
Mt. Pleasant, MI 48859
(989) 774-3576