NOTES ON THE EARLY EVOLUTION OF THE REFLECTOR. 73 



actually knows, but as a founder, familiar with bronze and bell metal, 

 it is not unlikely that he tried it. 



It was bell metal, by the way, which was the basis of Newton's 

 speculum metal. He merely whitened it a little with arsenic, as the 

 alchemists had done before him, thinking the alloy one stage in the 

 transmutation of copper to silver. Bell metal ranged in early times 

 from one part tin to four of copper, to one part tin to two of copper. 

 Newton's was very likely between the two, since he recommends one 

 of tin to three of copper, a material which works and polishes well, 

 but tarnishes with great rapidity, as Sir William Herschel found to his 

 cost many years later. 



Exit now the reflector, for more than about half a century. Newton 

 made one more try at it, working on a 4 inch glass speculum to be 

 silvered on the back after the plan early proposed by Gregory. The 

 instrument was apparently never completed. 



Another item often ascribed to Newton was the discovery of pitch 

 polishing. That he found the process to work well is undeniable, but 

 he did not disclose it for more than thirty years after his little telescope 

 had been laid on the shelf of the Royal Society, in fact not until several 

 years after there had been published, subsequent to Huygens' death, 

 the fact that he had been in the habit of polishing his true tools for 

 lens grinding in exactly this manner. 



It was not until Newton was a venerable invalid, a half century after 

 his telescope had been put away and forgotten, that the reflecting 

 instrument was finally put upon the stage by John Hadley, the in- 

 ventor of the reflecting octant. 



Hadley knew enough to make his own speculum metal and in his 

 struggles with it derived very little information from Newton's ex- 

 perimentation. He realized the importance of parabolizing his main 

 mirror and of giving a hyperbolic figure to his small mirror. He 

 polished them both not directly upon pitch, but upon pitch over-laid 

 with the finest silk fabric, which apparently helped in distributing and 

 holding the abrasive, and in 1722 presented to the Royal Society the 

 first veritable reflecting telescope. 



This instrument was of about 6 inches aperture and of about 5 feet 

 focal length and proved on test to be better than the telescope of 123 

 feet focus belonging to the Royal Society and made by Huygens. The 

 tests of the telescope show that it had a pretty good figure, and in fact 

 Hadley hit upon the method of testing for figure used for many years 

 thereafter, by examining the speculum at the centre of curvature and 

 judging of the parabolic figure by the default of the image from the 

 characteristics of a spherical mirror. 



