tube must be very secure." 85 The Clarks doubtless learned from the 

 example of the 25-inch refracting telescope Thomas Cooke had made for 

 R. S. Newall in England, and in 187 1 they began making tubes of steel 

 plates riveted together. These metal tubes weighed less, yet were stiffer 

 than wooden ones. 86 



The Clarks — particularly Alvan and Alvan Graham — were known as 

 practical astronomers as well as astronomical instrument makers. Besides 

 drawing the above-mentioned map of the Orion nebula, they searched 

 for double stars, participated in solar eclipse expeditions, made photo- 

 metrical comparisons of the brighter celestial objects, and measured the 

 lines in the auroral spectrum. These observations, although sporadic, 

 were respected and often quoted in contemporary literature. 



Before the development of the Hartmann test, the standard way of 

 describing the perfection of a lens was in terms of its actual defining 

 power. The Clarks tested each completed lens by searching for new and 

 difficult double stars as well as by separating known binaries. Alvan 

 Clark began by sweeping the southern skies — where he found some curi- 

 ous binaries — because he naively assumed that the great Struves, with 

 the 9.6-inch Dorpat refractor, and later with the 15-inch at Pulkowa, 

 must have discovered every double star in the northern hemisphere that 

 was visible with smaller telescopes. When he eventually turned his tele- 

 scope northward, Clark found several binaries the Struves had missed. 87 

 Without the benefit of astronomical background or library, Alvan Clark 

 was ignorant of the novelty of some of his earlv observations. He wrote 

 repeatedly to the Bonds, seeking confirmation of his discoveries; although 

 helpful, the Bonds were hardly enthusiastic. 



On 31 March 1851 the Boston Daily Evening Transcript published a 

 letter from Alvan Clark reporting his recent discovery of an eighth- 

 magnitude double star in Canis Minor, not listed in Smyth's Celestial 

 Cycle. According to Clark, "This discovery (to which is attached a strong 

 negative evidence of originality,) was the result of a systematic and 

 laborious search, for unknown double stars: a search hitherto only 



85 Alvan Clark to William R. Dawes, quoted in Frederick Brodie, "Notes on the 

 Manufacture of Tubes for Refracting Telescopes," Monthly Notices, Royal Astro- 

 nomical Society, vol. 17 (1856-1857), pp. 35-36. 



86 Boston Journal of Chemistry, vol. 6 (August 1871), p. 16. 



87 William R. Dawes, "New Double Stars Discovered by Mr. Alvan Clark, 

 Boston, U.S.; with Appended Remarks," Monthly Notices, Royal Astronomical Society, 

 vol. 17 (1857), pp. 257-259. 



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