Heliometer, or ASTROMETER, is an instrument invented by Bougeur, for measuring with exactness the diameter of the sun, moon, and planets. This instrument is a telescope, having two object-glasses of equal focal distance, placed side by side, so that the same 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.) Hence, two distinct images are formed in the focus of the eyeglass, the distance of which depending upon that of the two object-glasses from one another, may be measured with the greatest accuracy.