Figure 16. — Polishing lenses in the 

 Alvan Clark & Sons factory. 

 From cover of Scientific Ameri- 

 can, 24 September 1887. 



Henry Draper, although an outstanding instrument maker him- 

 self, purchased several pieces of apparatus from the Clarks. A Clark sider- 

 ostat on the roof, and secondary mirror, directed sunlight to all corners 

 of his New York City laboratory. 75 This laboratory was used primarily 

 for photographing and studying the spectra of the elements. Around 

 1879, to verify the existence of oxygen emission lines in the solar spec- 

 trum, Draper ordered from the Clarks a spectroscope which would "give 

 the dispersion of twenty heavy flint prisms and . . . bear high magni- 

 fying power". 70 



Draper used both reflecting and refracting telescopes at his observatory 

 at Hastings-on-Hudson. He made the reflectors himself, but relied on the 

 Clarks for the refractors. In 1875 he bought a Clark telescope of 12 

 inches aperture. Alvan Clark thought this objective one of the best he 



75 George F. Barker, "Memoir of Henry Draper," Biographical Memoirs, National 

 Academy of Sciences, vol. 3 (1895), pp. 1 13-1 14. 



76 Henry Draper, "On the Coincidence of the Bright Lines of the Oxygen Spectrum 

 with Bright Lines in the Solar Spectrum," American Journal of Science, vol. 18 (1879), 

 p. 268. 



57 



