In due time the new order came and with astounding suddenness. Just at the end of 1722 James Bradley (1692-1762) measured the diameter of Venus with an objective of 212 ft. 3 in. focal length; about three months later John Hadley (1682-1744) presented to the Royal Society the first reflecting telescope worthy the name, and the old order practically ended.
John Hadley should in fact be regarded as the real inventor of the reflector in quite the same sense that Mr. Edison has been held, de jure and de facto, the inventor of the incandescent electric lamp. Actually Hadley’s case is the stronger of the two, for the only things which could have been cited against him were abandoned experiments fifty years old. Moreover he took successfully the essential step at which Gregory and Newton had stumbled or turned back—parabolizing his speculum.
The instrument he presented was of approximately 6 inches aperture and 62⅝ inches focal length, which he had made and tested some three years previously; on a substantial alt-azimuth mount with slow motions. He used the Newtonian oblique mirror and the instrument was provided with both convex and concave eye lenses, with magnifications up to about 230.
The whole arrangement is shown in Fig. 16 which is for the most part self explanatory. It is worth noting that the speculum is positioned in the wooden tube by pressing it forward against three equidistant studs by three corresponding screws at the rear, that a slider moved by a traversing screw in a wide groove carries the small mirror and the ocular, that there is a convenient door for access to the mirror, and also a suitable finder. The motion in altitude is obtained by a key winding its cord against gravity. That in azimuth is by a roller support along a horizontal runway carried by an upright, and is obtained by the key with a cord pull off in one direction, and in the other, by springs within the main upright, turning a post of which the head carries cheek pieces on which rest the trunnions of the tube.
A few months later this telescope was carefully tested, by Bradley and the Rev. J. Pound, against the Huygens objective of 123 feet focus possessed by the Royal Society, and with altogether satisfactory results. Hadley’s reflector would show everything which could be seen by the long instrument, bearing as much power and with equal definition, though somewhat lessened light. In particular they saw all five satellites of Saturn, Cassini’s division, which the inventor himself had seen the previous year even in the northern edge of the ring beyond the planet, and the shadow of the ring upon the ball.
The casting of the large speculum was far from perfect, with many spots that failed to take polish, but the figure must have been rather good. A spherical mirror of these dimensions would give an aberration blur something like twenty times the width of Cassini’s division, and the chance of seeing all five satellites with it would be negligibly small.
Further, Hadley presently disclosed to others not only the method he used in polishing and parabolizing specula, but his method of testing for true figure by the aberrations disclosed as he worked the figure away from the sphere—a scheme frequently used even to this day.
The effect of Hadley’s work was profound. Under his guidance others began to produce well figured mirrors, in particular Molyneux and Hawksbee; reflecting telescopes became fairly common; and in the beginning of the next decade James Short, (1710-1768), possessed of craftsmanship that approached wizardry, not only fully mastered the art of figuring the paraboloid, but at once took up the Gregorian construction with its ellipsoidal small mirror, with much success.
His specula were of great relative aperture, F/4 to F/6, and from the excellent quality of his metal some of them have retained their fine polish and definition after more than a century. He is said to have gone even up to 12 inches in diameter. His exact methods of working died with him. Even his tools he ordered to be destroyed before his death.