This means taking two or three exposures at, above, and below the film's rated speed. An increase in a stop will double the exposure time. Bracketing your exposures will increase the probability that you will get at least one very good exposure of the particular image. Your time is much more valuable than a couple of rolls of film. Also, if you are doing fluorescence, it is very probable that your specimens will have faded and not be as good as the first time you photographed them.
Photographic films have a property which is undesirable for microscopists.
This is known as reciprocity failure. It usually occurs with the more commonly available photographic films for exposure times beyond an eighth of a second. Exposures taken with microscopes often extend way beyond this time. Reciprocity failure can in most cases be compensated by increasing the length of the exposure beyond what an exposure meter normally would indicate for a given exposure time. And, of course, reciprocity failure becomes even more important for fluorescence photomicrography because of the long exposures required.