HELIOMETER

, or Astrometer; an instrument for measuring, with particular exactness, the diameters of the sun, moon, and stars.

This instrument was invented by M. Bouguer in 1747, and is a kind of telescope, consisting of two object glasses of equal focal distance, placed by the side of each other, so that the same eye-glass serves for both. The tube of this instrument is of a conical form, larger at the upper end, which receives the two object-glasses, than at the lower, which is furnished with an eye-glass and micrometer. By the construction of this instrument, two distinct images of an object are formed in the focus of the eye-glass, whose distance, depending on that of the two object-glasses from one another, may be measured with great accuracy. Mem. Acad. Sci. 1748.

Mr. Servington Savery discovered a similar method of improving the micrometer, which was communicated to the Royal Society in 1743.