"On a New Method of Measuring Celestial Arcs," 51 which the extremely 

 practical Scientific American hailed as one of the most valuable papers 

 presented that year. 



The new double eyepiece micrometer described in the paper was 

 designed to measure celestial distances too great to be brought within 

 the field of view of a single eyepiece powerful enough to see the objects. 

 The two eyepieces moved independently so that the crosshairs of each 

 could be aligned with separate objects. The distance between the eye- 

 pieces was then measured, not with a fixed rule or screw, but with 

 calibrated parallel lines ruled on a rectangular glass plate. This plate 

 was placed under the eyepieces of the micrometer so that the ruled lines 

 were aligned with the crosshairs and the celestial objects. If the lines were 

 not the proper distance apart, another plate was ruled by an assistant 

 using a dividing machine made for this purpose. The micrometer was 

 attached to an equatorial refractor of 6 inches aperture, which was made, 

 of course, by the Clarks. To keep the stellar images on the crosshairs a 

 reliable driving clock was necessary; for this Clark used a Bond spring 

 governor. Shortly after George Phillips Bond, in 1851 , had described 

 the spring governor, 52 which had been devised for imparting equable 

 motion to a chronograph, the Clarks followed his suggestion and applied 

 the new regulator to telescope drives. At the AAAS meeting of 1856 

 Benjamin Peirce testified to the regularity of this drive mechanism, which 

 he found "vastly superior in convenience and value" to that of the 

 Harvard telescope. 53 Using this instrument the Clarks measured distances 

 up to one hundred minutes of arc with acceptable accuracy. 



Although his sons made frequent trips abroad to obtain glass and 

 to examine telescopes, Alvan Clark went to Europe only once. In 1859 

 he spent a month visiting his close friend William Dawes. By this time 

 Clark had sold Dawes five telescopes, and their correspondence had been 

 more extensive and their dealings more lucrative than any Clark had 

 had. 54 In company with Dawes, Clark attended a visitation at the Green- 



51 Alvan Clark, "On a New Method of Measuring Celestial Arcs," Proc, American 

 Association for the Advancement of Science, vol. 10 (1856), pp. 108-1 1 1. 



52 George P. Bond, "Description of the Apparatus for Observing Transits by 

 Means of a Galvanic Current, Now Used at the Observatory of Cambridge. U.S.," 

 Monthly Notices, Royal Astronomical Society, vol. 11 (1 851), pp. 163-165. 



53 Quoted in Scientific American, vol. 12 (1856), p. 3. 

 - 4 Alvan Clark autobiography, op. cit., p. 114. 



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