A compound microscope is one of the standard lab tools for biology. In compound microscopes light is passed through a thin or transparent specimen, and then through two or more lenses which magnify the image thus created.
Compound microscopes can be used to observe living cells, although care must be taken not to overheat the specimen with the light. Because they depend on light passing through the specimen, compound microscopes can be focused on more than one depth plane, so that rather than just looking at the top of a specimen, an observer can actually focus on and see the insides of the cells. This gives the viewer a good sense of the three dimensional nature of the specimen.
Compound microscopes vary in magnification, but commonly range from 40X to 1000X. Some compound microscopes are fitted with a measuring device within the optics so that the size of a cell or other specimen can be determined.