by the Clarks were a 12-inch visual refractor and a 10-inch refractor 

 equipped with a photographic lens. 



The last major instrument the Clarks made for Harvard was the 24- 

 inch Bruce photographic telescope. Like the 8-inch Bache camera, this 

 had a doublet lens of short focus and an objective prism. It was compact 

 and fast and covered a wide field. For half a century this telescope was 

 used to photograph the southern skies; among its triumphs were the plates 

 of the Magellanic Clouds on which the luminosity pattern of the Cepheid 

 variables was discovered. 



Since the Clarks made apparatus for physical as well as astronomical 

 researches, it is not surprising to find Clark apparatus in departments of 

 physics and chemistry. By 1878 the Rumford Cabinet at Harvard Uni- 

 versity had a Clark objective lens of 6j/o centimeters aperture and 1.6 

 meters focus, which was used as a collimator for a spectroscope. 122 



Josiah Parsons Cooke, Jr., Professor of Chemistry and Mineralogy at 

 Harvard University, used at least two spectroscopes made by the Clarks 

 during the early i86o's. The first, probably a simple chemical spectro- 

 scope, was in use by 1862. Since large and perfect pieces of optical glass 

 were difficult to obtain, opticians tried various other methods of pro- 

 ducing a prism. For the small spectroscope the Clarks made a prism 

 about 4 inches on each face, built up of several pieces of glass cemented 

 together with Canada balsam. 122a The Clarks also made for Cooke some 

 prisms of hollow glass filled with carbon bisulphide. Although this liquid 

 smelled foul, and its index of refraction changed greatly with even slight 

 temperature changes, it gave a large dispersion. The prism frames were 

 composed of pieces of plate glass cemented together with a mixture of 

 glue and honey. To the outside surfaces of these were held, with castor 

 oil, carefully figured and polished plates of glass. The largest prisms of 

 this sort had faces about 5 inches long by 3 inches high and held nearly 

 a pint of liquid apiece. 122b 



122 "List of Apparatus Relating to Heat, Light, Electricity, Magnetism, and Sound, 

 Available for Scientific Researches Involving Accurate Measurements." Annual 

 Report . . . Smithsonian Institution . . . i8j8, p. 431. 



122a Letter to the editor from Josiah P. Cooke, Jr., American Journal of Science, 

 vol. 34 (1862), pp. 299-300. 



122b Note by Benjamin Silliman, Jr., in American Journal of Science, vol. 35 (1863), 

 p. 408. See also, Josiah P. Cooke, Jr., "An Improved Spectroscope," American 

 Journal of Science, vol. 36 (1863), pp. 256-257. 



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