To use a simple usb webcam as a microscope, only a few modifications are needed.
In short, a small hack to the optics of a standard webcam with an adjustable focus-lens, allows to create video data, with a magnification of around 100 to 400x (depending on the model) at a working distance of a few mm. The highest magnifications can be achieved by inverting (putting upside down) the lens. Addition of good lighting by the use of leds allows to create images using a bright-field method (shine through the stuff) or dark field method (look at the reflections and scattering).
Movie recorded with modified webcams: Hackteria in Berlin
Magnification and Field of View
See: fieldofview for more details.
after completing the hack with the lenses, the magnification can be increased upto a field of view of 0.7 mm.