Ultramicroscope

The ultramicroscope is a system of illumination for extremely small objects such as colloidal particles, fog droplets, or smoke particles. The objects are held in liquid or gaseous suspension in an enclosure with an intensely black background (usually a black body) and illuminated with a convergent pencil of very bright light entering from one side and coming to focus in the field of view — the "Tyndall cone" familiar in experiements on scattering. With this arrangement, objects too small to form visible images in the microscope produce small diffraction ring patterns that appear as bright specks on a dark field. Ultramicroscopes are used in the study of Brownian motion, in the Millikan droplet experiment for measuring the electric charge of the electron, and in observing ionization tracks in cloud chambers.