During the past century, at least until light sources such as electric lamps and voltaic arcs were not available, sunlight was frequently used in optical experiments. The easiest way to do it was by means of device made essentially of a plane mirror placed outside a window and a drilled plate set in the window shutter. By means of knobs attached to the inside of the plate it was possible to set the mirror in desired position so that the reflected sunlight would assume the wanted direction inside the laboratory. With the heliostats the manual adjustment was substituted by a clock movement which not only avoided the trouble of frequent rectifing maneuvers but allowed a reflected beam of almost invariable directio to be used. The instrument presented here is, on the whole, in a good state of repair. Entirely of brass. Overall height 50 cm. Signed: "Gambey, Paris". Mid-l9th century.
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