First of all, a reference specimen showing a common structure has to be laid under the lens. After having set the correct film sensitivity at the exposure meter, the exposing time to a freely selected aperture value is measured at the ocular take. Please note the measured time/aperture combination.
Then, an exposure chart is made with different times.
The set illumination must be continuous at all times.
After having developed the film, please select the result with the correct exposure. The time with which this photo had been exposed corresponds to an aperture value for the measured exposure. This aperture value is the reference value for all future measurings. So the corresponding exposure time is measured to this fictitious aperture value. Nevertheless, it is clear to make aperture charts which is due to the different microscopic illumination procedures, specimen structures and required image effects.