IMPORTANT DESIGN IN A LARGE ACHROMATIC TELESCOPE, English, c. 1770, signed on the objective cell “Pyefinch, London, No. 194.” The telescope has a tapered mahogany barrel with substantial brass fittings, constructed in four screw-together sections and one brass drawtube. Overall length varies from 6’3″ to 7’4″ (1.90 – 2.24 m). The slightly grey-green objective is 2″ in diameter, with two elements in notched mounts to assure they are kept in the optimum rotary alignment. The elements have adjustable separation; at position marked “7” they are in near contact, at “6” they have a 3/8″ air gap between them. The drawtube contains a four-element eyepiece system with dust slide, and is scored “7” and “6” for two different focal positions 3-7/16″ apart, “7” being the longer focal length. The combination gives good erect images of high magnification. Condition is generally fine, the wood with some wear and hairline cracks, the brass with minor dents.

The maker was Henry Pyefinch, apprenticed to Francis Watkins in 1753, made free in the Spectaclemakers Company in 1763, working until his death in 1790. He was a fine craftsman and innovator, noting the lovely telescope in Tesseract Catalogue 79 (Item 3), as well as his co-patenting of a novel instrument, the “Aerostathmion.”

The present instrument is an extremely rare example of an achromatic telescope with variable separation of the objective elements. Various 18th century makers had utilized crown / flint glass combinations to minimize chromatic aberrations in the image, but it was John Dollond who, in 1758, and with the assistance and financial support of his co-partner in this effort, the maker Francis Watkins, obtained letters patent for the achromatic lens. John died three years later, his son Peter becoming proprietor of the business. It is impossible to well summarize here the fascinating history of court challenges to the patent, but a recent book gives the remarkable story in depth (B. Gee, Francis Watkins and the Dollond Telescope Patent Controversy, 2014). Despite rulings upholding Dollond’s monopoly, achromatic telescopes were being produced by various London makers.

By 1764, Pyefinch left Watkins’ employ, and actually formed a trading agreement with Dollond. But within four years the two were exchanging actions and counter-actions. The innovative Pyefinch found an escape from Dollond’s monopoly, and in 1770 received his own patent for “refracting telescopes with object glasses therein, composed of two or more different glasses or mediums separate and distinct from each other, and so placed in the tube as that, by altering the distance of the one from the other, the glasses will have greater or lesser magnifying powers without destroying the effect arising from the different refractive qualitys of the several mediums….” 

Only one other example is known of Pyefinch’s freedom from the grip of the Dollond patent (see http://dioptrice.com/telescopes/258).