41. Divided object glass micrometer or heliometer by Dollond for 3-foot parallactic telescope
London, 1787
John Dollond Jr. (London ?-1804)
Peter Dollond (London 1730-1821)
[Inv. MdS-41]

The heliometer was an important accessory of the parallactic telescope with achromatic object lens bought from the London firm of Dollond [file 38].
The 1843 inventory described it as "A divided object glass Micrometer or heliometer with two handles to produce both circular motion and separation of the images". It was fitted with a counterweight to balance the telescope and an extension tube for the eye-piece.
This particular type of micrometer had been invented by Pierre Bouguer (1698-1758) in 1748 and improved on by Joseph Fraunhofer, who replaced the two original object-lenses - identical and mounted one parallel to the other on the same tube - with a single lens, cut in half along the optical axis. The two halves can be slid one on top of the other, each producing an image. When the centres of the two halves coincide the images of the object under examination are superimposed. Viceversa, it is possible to superimpose the images of two nearby stars, once the separating line of the two lenses has been oriented and the relative optical centres of the lenses moved by the amount desired. As with the micrometer, it is first necessary to compute how many arcseconds correspond to a millimetre shift of one centre compared to the other. It was with the second exemplar built by Fraunhofer that Bessel made the first measurements of stellar parallaxes (see Part I, par. 13).
Instead of an optical system to replace the object-lens, our exemplar is a complete additional system to be placed in front of the object lens, consisting of a diverging lens cut in half along the optical axis, capable of boosting by 10 per cent the magnified images provided.
As can be seen from the original bill of sale of the Rubini brothers to the Administrators of the Institute, dated 5 October 1788, found in the Archives of the Department of Astronomy (busta XXVIII) the heliometer cost 23, or 555 Bolognese lire.

E. Baiada, A. Braccesi (1983), p. 122.
R.C. Brooks (1991).
M. Daumas (1953).
J.A. Repsold (1908), p. 72, fig. 109a, 109b.
G. LE. Turner (1981).