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
Hacking the webcam
mounting the microscope
There are many ways to finally build the microscope itself. here are a few examples from the workshop hackteria & lab_hacking @ NK, Berlin.
Simple microscope setup
Fixed, massiv and stable setup
Silicone embedded setup
Tape and cardboard setup
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.
hacking a digital camera
more about microscopy
For more enlargement, you can not only invert the lense but also put some (iron) rings between the lense and the "webcam" and fix it with duct tape. The more rings you put between the more light you will need that you can see something. 5mm works without problems, the light from 1-2 LEDs will suffice to see the things.
For savers: A cheap solution is a standard logitech webcam, which can be bought for about 25.- SFr. --> videos(500x600), pics (3MPixel)