an instrument equipped to pick up the reflections of light rays dispersed by ultramicroscopic objects lighted from the side and against a dark background, thus making them visible: used esp. in the study of colloidal particles
See ultramicroscope in American Heritage Dictionary 4
(ŭlˌtrə-mīˈkrə-skōpˌ)
noun
A microscope with high-intensity illumination used to study very minute objects, such as colloidal particles that scatter the light and appear as bright spots against a dark background.