Some cheap computer video cameras have adjustable focus lenses that can focus almost right up to the lens. This allows you to aim the camera straight up, and put a microscope slide on the lens ring, and focus on the slide.
Shown below is a Logitech QuickCam camera, facing up, with the microscope slide on top of the lens ring. The video shows the procedure, with a voice-over from the author (me! Hi there!).