In response to Barry:

The coverglass thickness (or thinness if you are a dieter) that you use
should primarily be determined by your microscopy setup. Most objective
lenses are designed to be used with 0.17mm thick coverslips (#1 and a 1/2).

True, but rarely observed in practice.  Theory is correct; understanding is
incomplete; implementation is flawed.  Use No. 1.

Some lenses, especially 40X and above, have a correction collar which
adjusts the internal lens arrangement for optimal microscopy and for
different coverslip thicknesses

True again.  But most microscopes are not so equipped.  Furthermore,
adjusting same is time consuming and not a straightforward thing to do.

For oil immersion the thickness of the coverslip is somewhat immaterial (as
long as it is no so thick that focusing is impossible) as the oil, coverslip
and slide should all be approximately the same refractive index.

True 2/3 of the time.  Immersion oil and cover glass refractive indexes are
set by standard specifications.  However, mounting media can have refractive
indexes that range from 1.49 to 1.58.  All mountants begin life at a lower
refractive index when liquid than later, after dry, when the resin alone
remains (with perhaps some plasticizer and anti-oxidant).  No. 1 is still
best in all routine applications.  I have a collection of approximately 20
technical papers to substantiate claim.

And finally, remember to use the substage condenser aperture diaphragm as
your optical safety net.  If the combined thicknesses of the mounting medium
and cover glass are too thick, and introduce contrast degrading spherical
aberration, simply slightly close the diaphragm to reduce the working
numerical aperture of the high dry objective and decrease its sensitivity to
thickness.  This assumes, of course, the microscope is clean and adjusted to
produce Kohler illumination.

Gary W. Gill