An ultramicroscope is a system of illumination for viewing tiny particles. When the diameter of a particle is below or near the wavelength of light (around 500 nanometers), the particle cannot be seen in a light microscope with the usual method of illumination. The ultramicroscope system is based on light scattering, not light reflection. In the system, the particles to be observed are dispersed in a liquid or gas colloid (or less often in a coarser suspension). The colloid is placed in a light-absorbing, dark enclosure, and illuminated with a convergent beam of intense light entering from one side. Light hitting the colloid particles will get scattered. In discussions about light scattering, the converging beam is called a "Tyndall cone". The scene is viewed through an ordinary microscope placed at right angles to the direction of the lightbeam. Under the microscope, the individual particles will appear as small fuzzy spots of light moving irregularly. The spots are inherently fuzzy because light scattering produces fuzzier images than light reflection. The particles are in Brownian motion in most kinds of liquid and gas colloids, which causes the movement of the spots. The ultramicroscope system can also be used to observe tiny nontransparent particles dispersed in a transparent solid or gel.
Ultramicroscopes have been used for general observation of aerosols and colloids, and in the study of Brownian motion, and in observing ionization tracks in cloud chambers.
The ultramicroscope was developed by Richard Adolf Zsigmondy (1865 - 1929), who was awarded a Nobel Prize in 1925 for his research on colloids and the ultramicroscope. The ultra in ultramicroscope refers to the ability to see objects whose size is shorter than the wavelength of visible light, on the model of the ultra in 'ultraviolet'.
For a different type of microscopy that also leverages light scattering against a dark background see dark field microscopy.