izing photometer to reduce the apparent brightness of the objects under 

 consideration, the Clarks used a lens of very short focal length to enlarge 

 the image. This lens was placed below the vertical shaft at the open end 

 of their long underground testing chamber, and received sunlight from 

 mirrors and totally-reflecting prisms. The solar image formed by the lens 

 was viewed through a small hole in a brass strip, at such a distance that it 

 appeared as bright as did another object at a specified distance. The 

 Clarks explained that the sun, removed to ten times its present distance, 

 would appear as bright as the solar image magnified ten diameters, and 

 that either of these maneuvers would cause a hundred-fold decrease in 

 brightness. They expressed their results for different objects in terms of 

 the number of diameter reductions needed for comparable brightness. In 

 other words they found, for instance, that the midday sun is 23.89 magni- 

 tudes brighter than Sirius and 13.01 magnitudes brighter than the moon; 

 they saw only 1 . 1 1 magnitudes of difference between Sirius and Procyon 

 and only 0.14 magnitudes difference between Castor and Pollux. Edward 

 C. Pickering later took account of this work when he defined an average 

 magnitude for the sun. 106 



The Clarks' only known recorded spectroscopic observation was made 

 by Alvan Graham during a period of intensive auroral activity. On 24 

 October 1870, using a single-prism chemical spectroscope, he observed 

 the spectrum of the aurora borealis and marked the position of four 

 bright lines. Edward C. Pickering reduced his measurements to wave 

 lengths and reported them to the editors of Nature. 101 For several years, 

 while astronomers were seeking a consensus regarding the positions of 

 auroral lines, Alvan Graham's measurements were repeatedly cited; his 

 measures, however, differ considerably from the modern consensus. 



As they got older, and as their business expanded, the Clarks hired 

 other men to do at least the preliminary work on the instruments. The 

 most valuable of their assistants was Carl Axel Robert Lundin, 108 a Swed- 

 ish optician and mechanician who joined the Clarks as their chief instru- 

 ment maker in 1 874 at the age of twenty-three. He remained with them — 



106 Edward C. Pickering, "Dimensions of the Fixed Stars," Proc, American Academy 

 of Arts and Sciences, vol. 16 (1 880-1 881), p. 2. 



107 Edward C. Pickering, "The Spectrum of the Aurora," Nature, vol. 3 (1870), 

 pp. 104-105. 



108 "Carl Axel Robert Lundin," Dictionary of American Biography (New York, 1933), 

 vol. 11, pp. 505-506. 



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