a very high reputation such as the future rival German firm 

 was never able to achieve for the manufacture of large discs 

 of optical glass of the finest quality for astronomical work. 

 A few instances may be mentioned of the successful pro- 

 duction of large telescope discs, that most severe test of the 

 skill of the optical glass manufacturer. 



At the Great Exhibition of 1851 the firm showed a 20-inch 

 disc of light flint "for daguerreotype apparatus" and a 2Q-inch 

 disc of dense flint, weighing 200 Ibs., for which a Council Medal 

 was awarded. 



In 1855 a companion crown disc was shown at Paris, and 

 this Foucault pronounced to be the finest piece of glass he 

 had ever seen. Sir David Brewster states that he did his 

 best to persuade the British Government to buy the pair and 

 " construct with them the greatest achromatic telescope that 

 was ever contemplated by the most sanguine astronomer," 

 but he could arouse no interest. 



Discs of 24 inches and 29 inches were produced in 1856, 

 and a pair of i8-inch discs sold to Messrs. Alvan Clark & Sons 

 in 1860 resulted, during their testing, in the discovery of the 

 companion to Sirius. A pair of 26-inch discs, produced in 

 1862, were worked by Messrs. Cooke & Son, York, for Mr. R. S. 

 Newall, into the largest refractor then in existence, and this 

 was later given to Cambridge University. 



In 1871 and 1874 Messrs. Clark worked pairs of 26^-inch 

 discs, the former being used for the Washington refractor with 

 which the satellites of Mars were discovered. The 28-inch 

 objective for Greenwich was finished by Sir Howard Grubb in 

 1887. The flint disc was completed at once, but the crown 

 disc took five years to obtain. 



For a number of years from about 1880, during which 

 period the discs for the great Lick and Yerkes telescopes 

 were manufactured, Britain took a place second to that of 

 France. A few years before the war, however, a number 

 of orders for big discs were again placed in England, the 

 production of which has been interrupted by war requirements. 



MANUFACTURE. 



As the manufacture of optical glass is an industry with 

 which most people, even those who are greatly interested in 

 its use, are somewhat unfamiliar owing, partly, to the 

 reticence and secrecy observed respecting many of the opera- 

 tions it may be of interest to give some account of the 

 different processes involved. 



In following the course of manufacture from the com- 



B 



