ALVAN GRAHAM CLARK. 523 



be mentioned one of 18| inches aperture for the Northwestern University, 

 now located at Evanston, 111. The next increase in size was marked by 

 the construction of the Princeton refractor of 23 inches aperture. After 

 this, a lens of 26 inches was made for the Naval Observatory at Wash- 

 ington, and one of about 26^ inches for the Leander McCormick Ob- 

 servatory of the University of Virginia. Then followed the 30-inch 

 refractor for the Imperial Observatory at Pulkowa, for which a gold 

 medal was awarded by the Russian government. Finally, as the last of 

 the joint productions of the Clarks, came the famous Lick refractor of 36 

 inches aperture. It may be added that Mr. Alvan G. Clark did, practically, 

 all the work on this lens, and brought it to its final perfection. 



After the death of his brother George, he became sole manager of the 

 firm. In this capacity he either made or superintended the construction 

 of several great lenses, besides many smaller ones. These include the 

 20-inch lens for the Denver Observatory, one of 24 inches for Mr. 

 Percival Lowell, the 24-inch Bruce Photographic telescope for the 

 Harvard Observatory station at Arequipa, Peru, and, finally, as a crown- 

 ing triumph, the great Yerkes lens of 40 inches in diameter. 



This last he accompanied to its final destination, and superintended its 

 mounting only a few days before his death. Beside his optical work, he 

 was a member of several governmental eclipse expeditions, and also the 

 discoverer of a number of close double stars. Nothing gave him more 

 pleasure than the discovery of these difficult doubles, especially as they 

 were all made with Clark glasses. One of his earliest and most im- 

 portant discoveries in this line was that of the companion of Sirius, in 

 1862, having been made in the yard adjoining the workshop with the 

 newly constructed 18|-inch lens before it was sent to Chicago. Turning 

 on Sirius one evening, he detected something unusual, and shortly re- 

 marked that the star had a companion. This discovery, splendid enough 

 in itself, was all the more so from the fact that, although irregularities 

 in the motion of Sirius had been noticed and theorized upon, he knew 

 nothing of them or of the predicted place of the companion. For this 

 discovery, he received the Lalande Prize of the French Academy of 

 Sciences. 



He married, January 2, 1865, Mary Willard, daughter of Joseph 

 Willard. She died on July 10, 1892. They had one son, Alvan, who 

 died when a young man, and three daughters, still living. 



In personal appearance and social intercourse Mr. Clark was unusually 

 attractive. With finely cut features, of sympathetic nature and serene 

 temper, he drew to himself a host of friends. 



