Heliometer

[1] The basic concept is to introduce a split element into a telescope's optical path so as to produce a double image.

The Syrian Arab astronomer Mu'ayyad al-Din al-Urdi, in his book, described a device called "the instrument with the two holes," which he used to measure and observe the apparent diameters of the Sun and the Moon.

[2] The first application of the divided object-glass and the employment of double images in astronomical measures is due to Servington Savery of Shilstone in 1743.

Pierre Bouguer, in 1748, originated the true conception of measurement by double image without the auxiliary aid of a filar micrometer, that is by changing the distance between two object-glasses of equal focus.

[1] John Dollond, in 1754, combined Savary's idea of the divided object-glass with Bouguer's method of measurement, resulting in the construction of the first really practical heliometers.