Edward C. Pickering, who succeeded Winlock as director of the Har- 

 vard Observatory in 1875, was especially interested in the new sciences 

 of stellar photometry and spectroscopy. As very few instruments had yet 

 been developed for these aspects of astronomy, the Clarks were called 

 upon to help design as well as construct much of the apparatus for 

 Pickering's studies. For the photometry the most important of the new 

 instruments were the two meridian photometers, with which the light 

 of any star near the zenith could be compared with that of a reference 

 star near the north pole. 115 Each photometer employed two similar and 

 horizontal objectives with a mirror — or prism — to reflect the starlight 

 through them; the two images were analyzed by means of a Nicol prism. 

 The smaller photometer, with 4-cm. objectives, was completed by 1879; 

 while the larger, with 10.5-cm. objectives, was in use three years later. 



Harvard at this time acquired several smaller, special purpose 

 photometers as well. In their Annual Report for 1878 they noted that 

 great credit was due to George Clark, "without whose ingenuity and 

 skill their construction would have been attended with great difficulty." 

 Photometer M, typical of these new instruments, was a double-image 

 micrometer which was used for measuring the positions and distances, 

 as well as equalizing the brightness of the components of double stars. 116 



All of the large instruments used in Harvard's Henry Draper Memorial 

 Study of stellar spectra were made by the Clarks, under the special 

 supervision of George Clark. In 1885, with a grant from the Bache fund 

 of the National Academy of Sciences, Pickering obtained a telescope 

 specially adapted for stellar photography. He bought a Voightlander 

 portrait lens of 8 inches aperture and 45 inches focus; this doublet, or 

 combination of two sets of crown and flint components, has a low 

 f/number, and thus is photographically fast. This lens system was then 

 given to the Clarks for correction and mounting. The correction involved 

 lengthening the focus by about 10 centimeters so that the scale of the 

 photographs would be the same as that of various other star charts. The 

 telescope was held in an equatorial fork mount equipped with a Bond 

 spring-governor drive. 117 



With an 8-inch prism over the objective this telescope was used for 



115 Annals, Harvard College Observatory, vol. 14, pt. 1 (1884), and vol. 23 (1890). 



116 Annual Report, Harvard College Observatory, vol. 33 (1878), pp. 5-6. 



117 Edward C. Pickering, "Stellar Photography," Memoirs, American Academy of 

 Arts and Sciences, vol. 11 (1888), p. 184. 



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