
From time to time, somebody asks if there's an easy way to visualize what a microscope's condenser aperture does to the light beam.
For not much money, one can buy an elegant device that is specifically designed to do this job.
Or, for a quick demo using stuff that you might already have lying around, you can use a very dilute solution of milk in a small plastic container.
The colloidal fat does an excellent job of scattering just enough light to see what's going on. Just don't use too much -- a couple of drops in a half-cup of water is a good place to start.
Be sure also to use a flat-bottomed container.
The illustration shown here is using the NA 1.2 condenser from an aus Jena Laboval 2 microscope.
The top row is with the plastic container "oiled" to the condenser using water. The bottom row is using the condenser dry, at maximum aperture, and demonstrates the narrower maximum cone imposed by using the condenser dry.
--Rik