scope workshop." Maria Mitchell, to whom lens grinding seemed a 

 tedious operation, told her students that "into every superior work the 

 martyrdom must come." 67 



There was nothing unusual, unless perhaps their great carefulness, 

 in the Clark technique for rough grinding and polishing a lens. cs The 

 blank disc was first polished so it could be tested for purity and evenness — 

 a piece of glass too heavily striated would be rejected. The grinding and 

 preliminary polishing were done on a rudimentary machine which con- 

 sisted simply of a horizontal turntable rotated by steam power. The table 

 held the tool, a cast iron lap of the same curvature as the lens, but re- 

 versed. The lens was held on the rotating lap and slowly moved about. 

 Small lenses could be worked by one man; a larger lens was fitted with 

 four wooden handles by which two workmen, walking around the table, 

 could give the lens the proper movement. Alternately, the lens was sup- 

 ported from a horizontal beam which mechanically imparted a recipro- 

 cating motion. 69 The early Clark lenses were ground with emery, but by 

 1887 the Clarks were using cast iron sand as an abrasive because it had 

 a lesser tendency to break down. 70 When the rough grinding was finished 

 the metal lap was exchanged for one of pitch, and the lens was polished. 

 The pitch was grooved to prevent it from sticking, and fed with water 

 and rouge. In a letter written in 1892 the Clarks noted that a certain 

 lens had indeed been polished with pitch and that they had "never used 

 cloth polishers." 71 Small lenses were polished in the same way they were 

 ground. Larger lenses were placed on the original metal lap but separated 

 from it by a piece of Brussels carpet, and the pitch lap, which rested on 

 the rotating lens, was moved back and forth. 



Then came the delicate process of perfecting the lens by local correc- 

 tion — a method the Rumford Committee found both important and 



66 Garth Galbraith, "The American Telescope Makers," op. cit. 



67 Maria Mitchell, Alvan Clark and Telescope Making, loc. cit. 



68 The fullest discussion of the Clarks' methods of figuring a lens is in "The Alvan 

 Clark Establishment," Scientific American, vol. 57 (1887), pp. 198-199. 



69 John F. Sullivan, "A Visit to Alvan Clark, Jr.," Popular Astronomy, vol. 35 

 (1927), p. 389. 



70 Boston Journal of Chemistry, vol. 6 (August 1871), p. 16. See also "The Alvan 

 Clark Establishment," op. cit., p. 198. 



71 Alvan Clark & Sons to Elihu Thompson (sic), 25 November 1892 (letter in 

 Thomson papers, American Philosophical Society). 



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