The colloid is placed in a light-absorbing, dark enclosure, and illuminated with a convergent beam of intense light entering from one side.
The ultramicroscope system can also be used to observe tiny nontransparent particles dispersed in a transparent solid or gel.
In 1902, the ultramicroscope was developed by Richard Adolf Zsigmondy (1865–1929) and Henry Siedentopf (1872–1940), working for Carl Zeiss AG.
[1] Applying bright sunlight for illumination they were able to determine the size of 4 nm small nanoparticles in cranberry glass.
Later the development of electron microscopes provided additional ways to see objects too small for light microscopy.