ton faculty. Its 9/2 -inch objective was constructed on the Gaussian 

 curves; and the distance between the components could be adjusted to 

 give the best color correction for the work at hand, whether visual, spec- 

 troscopic, or photographic. 191 This lens combination never failed to show 

 Young objects usually considered tests for a 1 2-inch, 192 and was said to ex- 

 hibit less outstanding color than one of the ordinary form. The cost of this 

 equatorial refractor was $3,000. For use with this telescope the Clarks also 

 made a 3-cylinder chronograph and a spectroscope which could hold 

 either a single prism or a diffraction grating. 193 This spectroscope, 194 to- 

 gether with a diffraction grating ruled in February 1880 by D. C. Chap- 

 man on a Lewis M. Rutherfurd engine, has recently been donated to the 

 Smithsonian Institution (q.v.). 



The 23-inch equatorial refractor of the Halsted Observatory at Prince- 

 ton, installed in 1 882, was a more typical Clark instrument. The bi-convex 

 crown lens was separated from the bi-concave flint lens by over 7 inches, a 

 distance sufficient to prevent the formation of "ghost" images. This sepa- 

 ration also hastened the equalization of temperature between the lenses 

 and the external air, as well as permitting an effective aperture one-half 

 inch larger than otherwise possible with that flint component. The glass 

 discs, cast by Feil, were more transparent and freer from bubbles and striae 

 than were the large Chance discs of the U.S. Naval Observatory and Uni- 

 versity of Virginia instruments (qq.v. ). The mount, although similar to 

 that of the U.S. Naval Observatory 26-inch, was heavier and freer of vi- 

 brations. The tailpieces of the telescope, and of its 5-inch finder, were made 

 the same size as that of the gj/o-inch equatorial, so that the eyepieces and 

 various auxiliary apparatus could be used with all three instruments. 195 

 This telescope was remounted in 1935 and has recently been transferred 

 to the Naval Observatory for use in the Southwest. 



191 Edward S. Holden, "Astronomy," Annual Record of Science and Industry (1877)) 

 p. 47. 



192 Charles A. Young, "Measures of the Polar and Equatorial Diameters of Mars," 

 Observatory, vol. 3 (1879), p. 471. 



193 Edward S. Holden, "Astronomy," Annual Record of Science and Industry (1877), 

 p. 28. 



194 Charles A. Young, The Sun (New York, 1896), pp. 66-69, %■ l & (p- 69) • 



195 Charles A. Young, "The Twenty-three Inch Telescope of the Halsted Observa- 

 tory at Princeton," Proc, American Association for the Advancement of Science, vol. 31, 

 (1882), pp. 1 12-1 16. 



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