reflector construction which had been made during the previous thirty 

 years. He spoke, for instance, only of speculum metal mirrors, disregard- 

 ing silvered-glass mirrors because the problems of preserving and re- 

 placing the silver film were so formidable; Henry Draper and John A. 

 Brashear, men with whom he was well acquainted, had, however, shown 

 this type of mirror to be practicable. And just as Alvan Graham was 

 stressing the impossibility of mounting a mirror so as to prevent flexures 

 detrimental to good definition, George Willis Ritchey was solving that 

 problem. 



Until about 1880 the Clarks usually supplied the tubes, mounts, and 

 various accessories, as well as the optical parts of telescopes; after that 

 date small mounts for their lenses were frequently made by Fauth and 

 his successors of Washington, D.C., 82 and larger mounts were made by 

 Warner & Swasey of Cleveland, Ohio. 83 The reasons for this change 

 probably included the rise of these mechanical companies and George 

 Clark's declining health. 84 The Clark mounts were usually of the German 

 equatorial type and were furnished — money permitting — with a clock 

 drive and graduated circles. 



The Clarks were among the earliest telescope makers to use light metal 

 for telescope tubes. A good tube should be strong, rigid, and lightweight. 

 During the first half of the 19th century the common materials for tubes 

 were wood and brass: wood was used, for instance, by Henry Fitz in 

 America, and by Merz und Mahler for the Pulkowa and Harvard equa- 

 torials ; brass was used for spyglasses, and by the Repsolds for the Oxford 

 heliometer. Following suit, the Clarks used wood for such instruments as 

 the 18/2-inch Dearborn and the 9-inch Yale refractors. As early as 1853, 

 however, the Clarks were making some tubes of thin tin — or zinc for 

 objectives of about 7 inches or more in diameter — overlaid with paper 

 and paste until of a suitable thickness and until "tolerably straight," and 

 then sanded. Finally, according to Alvan Clark, they gave the tube 

 several coats of paint "of different tints, blue, red and yellow, all faint, 

 but of the same depth when dry; they are rubbed down together. The 

 effect is very pleasing to the eye, and I think, under copal varnish, this 



82 Sidereal Messenger, vol. 8 (1889), p. 192. 



83 The first Warner & Swasey telescope mount was for the g^-inch aperture 

 refractor for Beloit College. See below, catalog of Clark instruments. 



84 "George Bassett Clark," Proc, American Academy of Arts and Sciences, op. cit., 

 p. 362. 



29 



