such as Gauss, J. Herschel, Hastings, or Taylor. They made only one 

 large lens- — the g-inch for Princeton — according to the Gaussian curves. 

 They found these components, both of which are meniscus, more difficult 

 to make, and disagreed that they gave a more complete achromatism 

 and better definition than the more conventional ones. 79 



The Clarks, who almost always corrected their object glasses for the 

 visual rays, had two methods of adapting these instruments for photog- 

 raphy. The Lick telescope is provided with a third lens which can be 

 added to the visual double achromat. The third lens was, of course, ex- 

 tremely expensive. In 1887, therefore, with the help of Edward C. Pick- 

 ering, then director of the Harvard Observatory, the Clarks devised a 

 new combination of two lenses which could be used for both photo- 

 graphic and visual observations. In this design the crown glass component 

 is more convex on one side than the other. For visual work the flatter 

 side is in contact with the flint lens; for photography the crown lens is re- 

 versed and moved farther forward. 80 



The Clarks were lens grinders par excellence, perhaps the most skillful 

 the world has ever known. Quite understandably, therefore, Alvan 

 Graham erred completely when he forecast the direction of 20th-century 

 telescope developments. At the first meeting of the Congress of Astronomy 

 and Astrophysics, held in Chicago during the Columbian Exposition of 

 1893, he read a paper on "Great Telescopes of the Future" in which he 

 emphatically pronounced himself in favor of large refractors. 81 Glass- 

 makers, he was sure, could cast optical discs greater than 40-inches in 

 diameter. Using local correction, an optician — an artist in light and 

 shade — could not only figure the large lenses but could also compensate 

 for slight imperfections in the glass. The extent of the absorption of 

 light through large lenses had been greatly exaggerated, as a recent ex- 

 periment in his manufactory had shown. Finally, deformation of lenses 

 under their own weight always occurred in a nearly compensatory man- 

 ner. Alvan Graham showed a lack of awareness of the advances in 



79 Letter from Alvan Clark & Sons of 10 March 1879, quoted in Thomas Nolan, 

 The Telescope (New York, 1881), p. 60. 



80 "Harvard Observatory and the Henry Draper Memorial," Scientific American, 

 vol. 57 (1887), p. 278. 



81 Alvan Graham Clark, "Great Telescopes of the Future," Astronomy and Astro- 

 physics, vol. 12 (1893), pp. 673-678. See also Alvan Graham Clark, "Possibilities of 

 the Telescope," North American Review, vol. 156 (1893), pp. 48-53. 



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