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> The Rayleigh criterion is applicable to microscopes and 
> Rayleigh did indeed consider this case.  Since he considered 
> the sample as a collection of self-luminous points (which 
> accurately describes a fluorescence image) the condenser does 
> not come into it at all.  How well the mercury lamp fills the 
> pupil of the objective makes no difference to your 
> resolution.  This is called 'incoherent' imaging because 
> light from any one point of the sample cannot interfere with 
> light from any other point.

Is this also the reason why there usually is no condenser diaphragm in the
epi-illumination path of the fluorescence microscope? Closing down a
condenser diaphragm would cause parallel and more coherent illumination, but
since fluorescence emission is incoherent it would not change anything to
the resolution or depth-of-field?

Kevin