HELIOMETER

is an instrument used for measuring the sun’s diameter and for tmicrometrical work on the stars. It is based on the fact-that measures can be ‘made by double images. The heliometer consists of an achromatic telescope with its object gla divided (by a plane pa ing through the optic axis of the telescope) into two segments, one fixed in the telescope tube, the other mounted so that it can be moved in a plane perPendicular to the telescope’s optic axis. The movement is obtained by the revolution of a screw connected with an apparatus for counting the revolutions. In Very recent heliometers this reading is checked independently by a micrometer Which measures directly the separation of the segments. From two measureMents the actual diameter can be calculated. Recent modifications of the apparatus aim at increasing facility of movement and avoiding the effects of temp. changes.