figured lens imaged a spectrum as an even line, while an imperfect lens 

 caused the width of the spectrum to vary. 76 



Once found, the location of the irregularities would be marked with 

 a red powder. The lens was then laid flat and either Alvan Clark or Alvan 

 Graham would retouch the offending areas. Each lens was, of course, 

 tested and refigured innumerable times. Alvan Clark's sense of touch was 

 said to be so acute that when a lens appeared perfect to the eye his fingers 

 could still detect slight irregularities. For the final rub, Alvan Clark could 

 find no sufficiently soft cloth, and so he used his bare thumbs! This also 

 insured immediate detection of any particles of grit which might have 

 gotten into the fine polishing powder. George Davidson noted that, when 

 he last saw Alvan Clark, in 1885, the optician's thumbs had actually burst 

 open from this punishing technique. 77 



Most of the Clark objectives were on the pattern of Fraunhofer lenses 

 and were figured as simply as possible. The outer, crown glass lens was 

 equiconvex, while the flint lens was biconcave, with the side toward the 

 eyepiece nearly flat. The inner surfaces of the early objectives were 

 ground with equal, but opposite, curves. Since this arrangement was 

 found to produce an objectionable "object-glass ghost," in later instru- 

 ments these curves were given slightly different radii. The inner surfaces 

 of the two lenses were separated by a distance, depending on the aperture, 

 which might be as much as several inches. With vents in the side of the 

 tube, this separation allowed a free circulation of air between the com- 

 ponents, so as to equalize more quickly the temperatures of the glass and 

 the external air. Furthermore, it permitted easy cleaning of the lenses. 

 This last convenience was in Alvan Clark's mind as early as 1851, when 

 he patented an "Improvement in Telescopes" which consisted of "a sim- 

 ple and substantial eye-piece wherein ready access may be easily had to 

 the glasses or lenses in order either to cleanse or repair them, as the case 

 may require." 78 (See fig. 18, p. 61.) 



The Clarks did not join in the contemporary mathematical search for 

 more perfect lens configurations. In fact, there is little indication that they 

 concerned themselves at all with the new forms described by theoreticians 



76 "The Alvan Clark Establishment," op. cit., p. 199. 



77 Obituary of Alvan Clark, San Francisco Daily Examiner, 20 August 1887, p. 1; 

 this includes notes by Edward S. Holden and Ferdinand Clark. The latter's notes 

 are quoted extensively in English Mechanic, vol. 46 (1887- 1888), p. 83. 



78 U.S. Patent 8509 (11 November 1851). 



284-281 O—68 3 27 



