Martin Wessendorf queried how to prepare fluorescent beads. Molecular
Probes are very concentrated, and I have generally used a 1% dilution
in distilled water, ringing the slide with nail varnish.  I find that
a substantial proportion of the beads adhere to the coverslip, so that
the optics aren't too bad even though they're in water.  Also, they stay
put.  Doing a thru-focal series for PSF determination one gets the
occasional stray streak as a 'loose' one wanders through by
Brownian motion.  The main disadvantage is that they fade rather too
easily.
Just today I tried mixing 1 microlitre of beads into a substantial
drop of Citifluor on a slide.  This works but is if anyhing too
concentrated.  I haven't used it seriously yet, so I don't know if
it will prevent fading, but it should be better optically (glycerol
+ solutes should be reasonably high refractive index).
                        Guy Cox
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