Advanced features of compound microscopes Basic compound microscopes are most useful for examining stained specimens or those with natural color or varying opaqueness.
Problems occur, however, with specimens that are transparent, or nearly so.
Such specimens can be common with plankton or any living or recently living material. Specimens taken in a physician's office are often of this nature. Observation of transparent specimens depends on the difference between their refractive index and that of the surrounding water or differences within the specimens themselves. Or it may depend on the light scattered as it comes in to the sides of the specimen. The oldest solution to the problem of transparent specimens is dark-field illumination.