The
ultramicroscope is not an instrument for magnifying images, as in a
microscope or other such device. Instead, it 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[?].