HELIOMETER

(Gr. *jA/*f, and /aetjov, measure.) A kind of micrometer for measuring the diameters of the sun, moon, and planets, or any small apparent distance between celestial objects. The instrument best known by this name appears to have been proposed or suggested by a Mr. Savery {Phil. Trans, vol. xlviii.), about the year 1743; but it was first applied by Bouguer in 1717, and has since been improved by Dollond and Fraunhofer. The principle on which the instrument is constructed is as follows: — Two object-gla es of the same focal distance, or rather the two halves of a divided object-gla , are placed side by side in the same tube with an apparatus so contrived that the distance between the centres can be increased or diminished at pleasure. In this manner two images of the sun are formed at the focus of the common eye-gla . Thus the circle AAA representing the field of view of the telescope, or the visible circle at the common focus of the two object-gla es and the eye-gla , and the two small circles representing the two images of the sun formed by the two object-gla es, when the observer proposes to measure the diameter of the sun, the two object-gla es are brought by means of a tangent screw to such a distance from each other that the two images touch in a point T, and the distance between the centres of the two object-gla es, estimated in seconds, gives the distance between B and C, the centres of the images; that is, the diameter of the sun. Formerly the heliometer was formed by two complete lenses; but as it was. found very difficult to give them precisely the same focal length, and as the size of the tube was required to be inconveniently large, they are now made of the same lens by dividing it acro its centre. The centres of the two semi lenses are separated by means of the screws S S, which act on the plates in which they are mounted; and the distance of the centres A and B is measured by a scale and vernier. This apparatus is otherwise called the divided object-gla micrometer. The principle has also been applied to the microscope. See Micrometer. [s. 560]