FIVE EARLY GREGORIAN TELESCOPES OF UNKNOWN ORIGIN

PETER ABRAHAMS UNAFFILIATEDFive surviving Gregorian telescopes, of similar design but unknown origin, appear to date from the early 1700s, and could possibly represent the first useable Gregorians. Examples are found at:

--Oxford Museum of the History of Science

--Harvard Collection of Historical Scientific Instruments

--The Science Museum, London.

--Collection of Peter Louwman

--Collection of Garry Tous. (Unmounted tube.)

These telescopes were made with unusual stylistic metal working, the coarse spiral edge to the tube cover and the rear plate on which the eyepiece mounts. This might be found in another instrument. Identifying the maker of these telescopes is the hope of this presentation.

British instrument makers who could have fabricated these telescopes include the following. ('w' indicates working dates; 'd' is year of death):

John Hadley (w1717, d1744, optical instrument maker, sold telescopes). The mount used with these telescopes is credited to Hadley in Smith’s Opticks, 1738, where, in 'Remarks', p24, is written that the Gregorian was first brought to perfection soon after 1719 by Hadley. In Desaguliers' appendix to D. Gregory 1735, p250, is 'An account of the Gregorian reflecting telescope, as perfected, by John Hadley... in the year 1726.'

Francis Hauksbee the younger (w1712, d1763). Associated with John Hadley in improving telescopes, and sold telescopes.

George Hearne (a freeman in 1705, w1741, sold telescopes)

Matthew Loft (apprenticed 1711, w1724, d1747, sold telescope) An extant Loft Gregorian does not resemble these examples.

John Rowley (w1697, d1728, mathematical instrument maker, sold telescope) Rowley is known to have been constructing a Newtonian reflector as early as 1710. The patron of Rowley was Charles Boyle, 4th Earl of Orrery, and the Orrery Collection of instruments is now part of the Oxford Museum.

Edward Scarlett I (w 1705, d1743, sold telescope) J.T. Desaguiliers wrote in 1735 that Hadley had perfected the Gregorian and then taught the skills to Edward Scarlett & his son.

A European instrument maker is also possible.

Claude Siméon Passemant made Gregorians by the mid 1730s, with a spiral ornament on the rear plate in a less-coarse style, and with a somewhat similar ball & socket mounting system. The plate to his 1741 book shows similar mechanisms.

Claude Paris made Gregorian telescopes by the mid 1730s.

Collection of Garry Tous. Three inch aperture. A Ronchi test of the primary mirror indicated a parabolic surface.

(Left) Push-pull screws on rear cell adjust (tilt) primary speculum and eyepiece.(Right) Three screws surrounding field lens of eyepiece allow alignment of field stop at the focal point. This feature is also illustrated in Passemant, Construction d'un telescope par reflexion, 1741. James Gregory designed a field stop to place at the focus of the eye glass.

Four leaf springs retain the primary mirror in its cell. A letter 'J' is stamped on one of the springs. These springs are possibly a later modification; noting the three drilled holes, unthreaded, adjacent to the large opening.

Harvard Collection of Historical Scientific Instruments. Owned by Edward Holyoke. Black leather cover added during conservation in 2005. With solar filter.

Oxford Museum of the History of Science. From the Orrery Collection. Originally covered with fish skin. Two inch aperture.

Collection of Peter Louwman(Lower middle) The primary speculum, the mirror cell back plate, and a dished triangular spring to hold the mirror in place. This triangle was possibly present in the the Tous example, when original; and replaced with the four sheet metal springs.(Lower left ) Disassembled mount illustrating the post with a wood screw at the lower end, to use in attaching the telescope to a tree or a wood post, the screw also fitting into a socket in the case, as shown in the Harvard instrument.

(Left, center) Collection of The Science Museum, London.(Right) "A little machine for supporting and managing Mr. James Gregory's reflecting telescope, brought into use by Mr. Hadley... " --Robert Smith. A Compleat System of Opticks. 1738. p366, 367.