Hi David,
What are you trying to photograph and how much magnification are you attempting. As Dan indicates there are adapters directly for a microscope, preferably a trinocular microscope (but much more expensive). Other options if you are not requiring as much magnification are bellows and a bellows lens using an RMS thread lens such as a Leitz Photar, Zeiss Luminar, the equivalent Nikon, Canon Macrophoto lens. As an example, with the Leitz 12.5mm Photar, you can get as much as about 20X if you have enough bellows. Other options include other special macro lenses that can be used on the bellows including the older Schneider M Compon Lenses, Rodenstock Apo Rodagon D lenses, equivalent Nikon optics Dan can help here. In addition, with these lenses, because you are getting so close to the subject (almost no depth of field) you even have enough coverage for 4 X 5.
Additionally as noted there are different kinds of microscopes that can be used for different subjects. They include the more standard Biological for transmitted light. There are also Geological type with rotating stages (some without) using crossed Nickels (Crossed Polarizers) for transmitted light as well as Reflected light. There are also Stereo type microscopes requiring less magnification used for reflected and transmitted light.
Rich