In reply to Eulerís suggestion in 1747 that achromatic object lenses could be made using different types of glass pastes, John Dollond (1706-1761) pointed to Newtonís demonstrations that such lenses were not possible, and began corresponding with Euler on this score. In 1755, however, a professor from Uppsala, Klingenstierna, sent Dollond geometrical proof that Newtonís findings were wrong. This, together with the fact that Dollond had in the meantime found out about the first achromatic lenses made and sold by George Bass, encouraged him to produce his first achromatic lens. Made with a concave flint-glass lens and a convex crown-glass lens, it had a focal length of 5 feet (c. 1.6 m) and provided brighter images than a normal 15-metre focus lens. It was presented to the Royal Society by James Short on 8 June 1758. Dollond, who became member of the Royal Society - receiving its highest scientific accolade of the time, the Copley medal - copyrighted his process, having found out that other lens-makers had made achromatic lenses with flint glass, according to George Bassí model. Three years after his death, his son Peter successfully brought charges against other craftsmen who, convinced of the priority of George Bassí work and the idea of Chester Moor Hall, denied the validity of the copyright. Success in this case would halve the price of achromatic lenses.
Fitted with a top quality achromatic object lens, this telescope is part of the instruments bought by Petronio Matteucci - director of the Specola after the death of Zanotti in 1782 - to renew the equipment which was still basically the English instruments by Sisson of the first half of the century [files 38-41]. The original bill of sale of the Rubini brothers, dated 5 October 1788, to the Administrators of the Institute for the instruments built by Dollondís workshop in London is held in the Archives of the Department of Astronomy (busta XXVIII). From this bill of sale the cost of the achromatic telescope - which bears the words Dollond. London - can be put at £26, equal to 628 Bolognese lire at that time; the eye-piece micrometer had cost £18, equal to 445 lire.
A full description can be found in Ceschiís 1843 inventory:
"Telescope by Dollond with mahogany tube 8-foot long and divisable in two parts, which are screwed together, fitted with an excellent achromatic object lens of 3 and ĺ-inch aperture. This telescope is mounted on a metal frame, two handles on which allow the small vertical and horizontal movements of the telescope to be made. This frame is attached to the end of a long triangular wooden rod which fits into the big foot of the apparatus running on three metal runners. By appropriate mechanism the rod, on which the Telescope rests, can be raised to use the instrument in the most elevated regions of the sky. The length of the telescope however, and its disproportion with the size of the frame, make use of this instrument somewhat uncertain on account of the strong oscillation it is subject to.One of the astronomical eye-pieces is missing, and just the end for the eye is left of the other, with one of the two lenses and the glass for the Sun. It is not clear from examining the mount how the fine movements described in the inventory could be effectuated. The wooden case that contains it has also been found [Inv. MdS-187].
This telescope is fitted with
- Two astronomical eye-pieces of different magnification each provided with colored glass.
- A terrestrial eye-piece. [Inv. MdS-64]
Protective metal cover for object lens. Two sturdy metal screw covers that are used to close the two parts of the tube (not found). Eyepiece micrometer[Inv. MdS-42]...[file 40]"
E. Baiada, A. Braccesi (1983), p. 123.
M. Daumas (1953), p. 315.
G. LíE. Turner (1981)