were polished on pitch. . . . We have never used cloth polishers." 214 If 

 indeed this doublet was made in the early i85o's, it was the largest lens 

 the Clarks had then made, and, except for Whipple's lens (q.v.), it is 

 the only known photographic objective made by the Clarks at that time. 



Each of the eight American parties authorized by Congress to observe 

 the 1874 Transit of Venus was outfitted with photographic apparatus, 

 a 5-inch equatorial refractor, a chronograph — all made by the Clarks — 

 and a Stackpole transit instrument with Clark optics, and two chronom- 

 eters. 215 Appropriations for the equipment were not made until the 

 summer of 1872, just two years before the first parties were to leave 

 for their Pacific stations. George Clark apparently drove himself so hard 

 to finish these instruments in time that, when they were finally delivered, 

 he experienced a severe physical breakdown from which he never fully 

 recovered. 216 



The various national expeditions used different methods for photo- 

 graphing the transit. The American system was an adaptation of Win- 

 lock's Harvard photoheliograph (q.v. ) . A heliostat directed sunlight onto 

 a stationary photographic objective. The focal ratio of this objective — 

 5 inches aperture, and almost 39 feet focus — was so great that sufficiently 

 large pictures could be taken in the focal plane, with no additional en- 

 larging lens. The heliostat mirror was moved by a simple and inexpensive 

 clock drive which needed only occasional manual adjustment. The mirror 

 itself was of unsilvered glass, 7 inches in diameter, and slightly thicker on 

 one side so that light reflected from the rear surface would be thrown 

 away from the camera. The success of the photographs depended, to a 

 great degree, on the perfection of the plane mirrors. The front surface of 

 each mirror was to have a radius of curvature of not less than 4 miles; 

 measurements made after the transit showed the mirrors to have more 

 than twice the requisite flatness. 



The purpose of the photographs was to determine the distance, at 

 various times, that Venus had passed over the solar surface. To ensure 

 precision, three special features were included: the photographic tele- 



214 Alvan Clark & Sons to Elihu Thompson [sic], 25 November 1892 (letter in 

 Thomson Papers, American Philosophical Society). 



215 Simon Newcomb, ed., Observations of the Transit of Venus, December 8-g, 18J4, 

 (Washington, D.C., 1880), pp. 14-16, 25-31, 61-65. 



216 "George Bassett Clark," Proc, American Academy of Arts and Sciences, vol. 27 

 (1891-1892), p. 362. 



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