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An ultramicroscope is a microscope with a system of illumination that allows viewing of tiny particles. When the diameter of a particle is below or near the wavelength of visible 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 be 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.

The "ultra" in "ultramicroscope" refers to the ability to see objects whose diameter is shorter than the wavelength of visible light, on the model of the "ultra" in ultraviolet.

Ultramicroscopes have been used for general observation of aerosols and colloids, in studying Brownian motion, in observing ionization tracks in cloud chambers, and in studying biological ultrastructure.

In 1902 the ultramicroscope was developed by Richard Adolf Zsigmondy (1865–1929) and Henry Siedentopf (1872–1940), working for Carl Zeiss AG. Applying bright sunlight for illumination they were able to determine the size of 4 nm small nanoparticles in cranberry glass. Zsigmondy further improved the ultramicroscope and presented the immersion ultramicroscope in 1912, allowing the observation of suspended nanoparticles in defined fluidic volumes. In 1925 he was awarded the Nobel Prize in Chemistry for his research on colloids and the ultramicroscope.

Later the development of electron microscopes provided additional ways to see objects too small for light microscopy.

For a different type of microscopy that also leverages light scattering against a dark background see dark field microscopy.

In the first decade of the 21st century, the illumination mode of the ultramicroscope has been extended to fluorescence measurement, as light sheet fluorescence microscopy.