Florida State University
Florida State University: Virtual Microscopy
These interactive tutorials allow you to adjust the magnification and focus of the object. Requires Java.
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Science Stock Photography
This site has numerous pictures of microscopic images. It also has a search engine so you can find specific objects. Images were captured with light microscopes, as well as electron microscopes.
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Bowling Green State University: Algal Image Archive
A host of marine and freshwater algal images. View many different types of algal through Bowling Green State University's Center for Algal Microscopy and image Digitization.