I have a simple microscope, with only one lens (though two eyepieces). Is there any way to do this, without too much money and modification?
If you have a SLR, it's as simple as pulling one of the microscope eyepieces, removing the SLR lens, and pointing the camera lens-box at the eyepiece hole.
You generally need to hold the camera about 1-2" from where the eyepiece sits.
You lose contrast from light-leakage, but it works pretty well.
Scopetronix carries a whole line of eyepieces and adapters to allow afocal photography for telescopes and microscopes. I have used their stuff for my telescope assembly for planetary work where the subject is relatively small from the entire field.
Their website used to work better back when it wasn't modern, but now it's a mess with poorly designed css and no pictures.
Still, the basic premise is the same: get a camera as close to the eyepiece as you can and magnify as much as you can to avoid vignetting. Webcams work well for this kind of stuff, too.
The easiest way that I have tried is simply to put a point and shoot camera into macro mode, and hold the camera up to the lens of the microscope. The view isn't the greatest, but it works in a pinch.
If you are unfamiliar with Macro mode, usually it is the symbol of a flower or leaf on the camera, and it allows(or forces) the camera to focus on objects that are very near to the lens, such as a flower.