HELIOMETER

(Yunanca köken — orijinale bakınız.) 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. 8nvery {Phil. Trans, vol. xlviii.), about the year 1743; but it was first applied by Bouguer in 1747, and has since been improved by Hollond nnd Fraunhofer. The principle on which the instrument is constructed is as follows: Two object-glasesofthe 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 nt the focus of the common eye-cla . Thus the circle A A A representing the field of view of the telescope,or the visible circle at the common focus of the two [s. 565]