In 1722, John Hadley, an English mathematician, completed a Newtonian form of reflector (Fig. 11), in which the mirror evidently was suitably figured. It was about 5I/2" in diameter, and 62%" in focal length. With a mirror of
these dimensions, practically perfect definition could be realized if the surface was given a spherical figure. (See the discussion on surface tolerance in Chapter VI.) This instrument attracted considerable attention, and presently other makers were turning out Newtonian reflectors, following Hadley's technique, which consisted of removing the spherical aberration as it was revealed by the extra-focal diffraction rings of a star image.
Hadley then turned his attention to Gregory's design, and in 1726 he produced an instrument slightly over 2" in diameter and 12" in focal length. This proved so successful that construction was undertaken by other opticians, or artists, as instrument makers and craftsmen appear to have been then known. Notable among these was James Short, who made both Newtonians and Gregorians in great numbers, from about 1732 to the time of his death in 1768.
Observatories purchased his larger instruments, a tribute to his skill, and the smaller ones were marketed chiefly among the aristocracy and those amateur astronomers of the day who could afford them.
Fig. 11. The first practical reflecting telescope, made in 1722
by John Hadley.
The principal attraction of the Gregorian design was the erect image it gave, which made it suitable for terrestrial use. This circumstance influenced its preference over the Newtonian, notwithstanding the fact that its images must have been pretty dull. Well into the 19th century, however, the Gregorian rode a wave of popularity that no type of telescope has known, until overwhelmed
in comparatively recent years by the flood of amateurs who have Hocked to Newton's design.
From the time of the invention of the telescope, and the startling discoveries of Jupiter's moons and the rings of Saturn, interest in astronomy had become something infectious. Each new discovery was accorded the widest publicity, stimulating a desire among those of learning to gain at first hand a glimpse of these celestial wonders. It was not practicable as yet for the average individual to make his own speculum, but many contrived to fit spectacle lenses into tubes, much as Galileo had done some 150 years earlier. Those whose means permitted bought telescopes, and envied was the gentleman who possessed one of three or four inches aperture, by an "exclusive" artist. But, judged by present-day standards, many of those reflectors were tiny. There is one (maker unknown) in the Fugger Collection at Augsburg, barely 1" in diameter and 6" in focal length, that was concealed in a walking stick! Eyepiece lenses of 1/6" or less in focal length were quite common.
The metal used in those early mirrors was an alloy of copper and tin, the usual proportion about 75 to 25, which could be given a beautiful polish. But the metal was extremely hard to work, and a prodigious amount of labor was involved in grinding and polishing the curve. To facilitate the work, the comparatively thin disks were cast to the approximate curve, the backs also being curved to give uniform thickness and equalization of temperature effects.
Grinding was done on convex iron tools of similar radius, using emery, and sometimes sand. Polishing was done on a pitch lap, with rouge. Manufacturers usually devised their own machines to do the work of grinding and polishing. Except where the utmost perfection was imperative, figuring seems to have consisted for the most part of a final brief variation of the stroke, in an unguided
attempt to concentrate the polishing at the center. Critical testing, undoubtedly seldom indulged in on account of its laboriousness, could as yet only be performed on a star.
In reflective ability, speculum was only about 60 per cent efficient, and the surface tarnished rapidly, effecting a further serious light loss. This meant
frequent repolishing, and repolishing meant refiguring. It is interesting to inquire into the prices that were asked for telescopes in that period, the latter half of the 18th century. Listed below are prices and sizes of a few of the Gregorians made by Short, selected from his catalogue. Newtonians in similar sizes were priced only slightly lower.
Diameter Focal length Magnification Price
(inches) (inches) (guineas)*
1.1 3 18 3
1.9 7 40 6
4.5 24 90-300 35
6.3 36 100-400 75
18 144 300-1,200 800
*An English gold coin, issued until 1813, equivalent to 21 shillings, or about five dollars.
By the beginning of the 19th century, amateurs were able to procure specula for the primary and secondary mirrors, for both Newtonian and Gregorian designs. These could be had finished, ready for mounting, or as rough blanks to be ground and polished, not at the amateur's discretion, but to the curve of the iron tools furnished by the maker. Of course, the amateur had no means of correcting the figures of his mirrors, or even of knowing what they might be, the one reliable method of testing not being common knowledge.
William Herschel's Contribution
