Description

“DESCRIPTION DE LA LUNETTE ACHROMATIQUE, AVEC LE MICROMETRE OBJECTIF ACHROMATIQUE DIVISE, FAITS PAR PIERRE ET JEAN DOLLOND, OPTICIENS DE SA MAJESTE, DANS LE CIMITIERE DE ST. PAUL, A LONDRES,” English, c. 1775, being a rare example of the separately published 20 page booklet describing Peter and John (II) Dollond’s refractor with its micrometric split-objective, that being a triplet achromat of 3-1/2 feet focal length. The large fold-out plate shows the instrument with its fine stand, long control rods, extra eyepieces, and complex mount for the split objective. Condition is very fine, with a sympathetic 20th century binding measuring 5-1/4″ x 8-1/4″ (13 x 21 cm). Several typographic errors in the text show old hand corrections. The use of French here is not surprising. There was much cross-Channel activity at the time, and we find various of the Dollonds¹ booklets, instruction sheets, etc. offered in both languages.It was John Dollond senior who, in 1754, proposed his divided-glass micrometer to the Royal Society. His achromatic objective lens was now two half-lenses, mounted one above the other and with sideways offset adjusted by calibrated fine screw motion. One saw two offset images in the eyepiece, and could directly measure the diameters of planets, sun, or moon, just by bringing their opposite limbs into contact. (see King, 1955). A rare publication of which we find very few copies in libraries.