original. 72 The Clarks had been using local correction for several years 

 when Foucault in Paris announced his invention of this method in 1859. 

 The idea for local correction might have been suggested to the Clarks 

 by Henry Fitz of New York, from whom they had bought some pieces 

 of optical glass. 73 Fitz retouched only one surface of his compound lenses, 

 however, while the Clarks regularly retouched all four surfaces of each 

 achromatic lens combination. 



Local correction was used to remove the errors of figure remaining 

 after the rough grinding and polishing were done. Because even the best 

 glass available during the 1 9th century was irregular — as their tests with 

 a polariscope showed — the Clarks used local correction to obtain the 

 sharpest possible focus rather than mathematically true curves. 74 This 

 method of figuring, therefore, lessened their dependence on absolutely 

 homogeneous glass discs. 



To locate the figure errors the Clarks developed a test similar and prior 

 to Foucault's knife-edge test for mirrors. Their test was made either on 

 an actual star or, more conveniently and more frequently, on an artificial 

 star in the horizontal tunnel which stretched out 230 feet from the cellar 

 of their workshop. The image of a point source of light was examined at 

 the focus of the lens: a perfectly figured lens would appear uniformly 

 illuminated while an imperfect lens would not. 75 The Clarks later de- 

 vised and used a test twice as sensitive as this original one. Light from a 

 point source was focused by the lens, and then reflected by an optically 

 plane mirror back through the lens to an eyepiece close to the light source. 

 Since light passed twice through the lens, the effect of any irregularities 

 was doubled. 



Tests of photographic lenses had to be made photographically. One 

 method used by the Clarks involved photographing a star several times, 

 with the plate at both sides of, and at varying distances from, the focus : 

 a perfect lens would form evenly illuminated images. The Clarks also 

 tested their photographic lenses by taking a spectrograph: a correctly 



72 "Remarks by Alvan Clark on Receipt of Rumford Medal," op. cit., p. 245. 



73 Henry Fitz's Account Book, 1 851 -1855 (in Smithsonian Institution, Division 

 of Physical Sciences, Cat. 317,026). 



74 "Alvan Graham Clark," Proc, American Academy of Arts and Sciences, vol. 33 

 (1897-1898), p. 522. 



75 Simon Newcomb, "New Refracting Telescope of the National Observatory, 

 Washington, D.C.," Science Record, (1874), p. 331. 



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