Heliometer

Heliometer ('sun-measurer') is an instrument invented by Savery and Bouguer in 1743-48, by means of which the diameters of the heavenly bodies can be measured with great accuracy. As improved by Dollond, the object-lens of the instrument is in two halves, each of which will form a perfect image in the focus of the eyepiece; and the images may be made to diverge, coincide, or overlap each other, by varying the distance between the half-lenses. If the diameter of the sun is to be measured, the two lenses are adjusted so that the images may touch each other; then the distance between the centres of the two object-glasses measured in seconds gives the diameter of the sun. Fraunhofer made many remarkable improvements on the heliometer.