The second spectroscope, built in 1863, was the largest and most 

 powerful yet made. The spectrum was produced by a train of nine carbon 

 bisulphide prisms; and an alternate set of solid glass prisms — not figured 

 by the Clarks — was also provided. The instrument incorporated a new 

 method, devised by Cooke and executed by George Clark, of adjusting 

 the prisms for the angle of minimum deviation for any ray. At the center 

 of the spectroscope was a truncated iron cone, against which the backs 

 of the prisms rested: by raising or lowering the cone the prisms were 

 equally spread or drawn toward the center. For measuring the relative 

 positions of the spectral lines, the rim of the spectroscope was made of 

 silver and graduated to 10 seconds of arc. 1220 



In 1902 Charles S. Hastings, the theoretical as well as practical 

 optician, advertised to sell cheaply a 4-inch refracting telescope "made 

 by Alvan Clark/' 123 



The Fitz equatorial refractor at Haverford College was reworked 

 by the Clarks in 1880. They corrected and repolished the 8^4 -inch 

 objective, made a new micrometer, and added a rotary pendulum regu- 

 lator to the driving clock. Even with these improvements, however, the 

 telescope was not capable of very accurate work, and a new instrument 

 was ordered from the Clarks. The 10-inch refractor, erected in 1884, 

 had an equatorial mount equipped with clock drive and graduated 

 circles. 124 This instrument was reconditioned after fifty years' service and 

 is still in use. 



Hebron Academy in Hebron, Maine, owns a 6.1 -inch aperture Clark 

 refractor dated 1859. r "" ne tuDe > originally mounted on a tripod, is of 

 brass coated with nickel. The telescope has recently been remounted and 

 reactivated. 125 



Andrew P. Henkel, a physician in Cincinnati, Ohio, owned an 

 8*4 -inch aperture refracting telescope made by the Clarks in *88o. The 

 equatorial mount was of the usual Clark form; the polar axis was sup- 



122c Josiah P. Cooke, Jr., "On the Construction of a Spectroscope with a Number 

 of Prisms, by Which the Angle of Minimum Deviation for Any Ray May Be Ac- 

 curately Measured and Its Position in the Solar Spectrum Determined," American 

 Journal of Science, vol. 40 (1865), pp. 305-313. 



123 Advertisement in Popular Astronomy, vol. 10 (1902). 



124 Haverford College Observatory (1884) (pamphlet in Haverford College Library). 



125 p r i va te correspondence with Jerald R. Twitchell, chairman of Hebron Academy 

 science department. 



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