$$ 254, ass] The Discovery of Uranus: Slough 327 



Datchet, then, after a few months (1785-6) spent at Clay 

 Hall in Old Windsor, at Slough in a house now known 

 as Observatory House and memorable in Arago's words as 

 " le lieu du monde oil il a e"te" fait le plus de decouvertes." 



255. Herschel's modest salary, though it would have 

 sufficed for his own and his sister's personal wants, was of 

 course insufficient to meet the various expenses involved in 

 making and mounting telescopes. The skill which he had 

 now acquired in the art was, however, such that his telescopes 

 were far superior to any others which were available, and, 

 as his methods were his own, there was a considerable 

 demand for instruments made by him. Even while at 

 Bath he had made and sold a number, and for years after 

 moving to the neighbourhood of Windsor he derived a 

 considerable income from this source, the royal family and 

 a number of distinguished British and foreign astronomers 

 being among his customers. 



The necessity for employing his valuable time in this 

 way fortunately came to an end in 1788, when he married 

 a lady with a considerable fortune; Caroline lived hence- 

 forward in lodgings close to her brother, but worked for 

 him with unabated zeal. 



By the end of 1783 Herschel had finished a telescope 

 20 feet in length with a great mirror 18 inches in diameter, 

 and with this instrument most of his best work was done ; 

 but he was not yet satisfied that he had reached the limit 

 of what was possible. During the last winter at Bath he 

 and his brother had spent a great deal of labour in an 

 unsuccessful attempt to construct a 3o-foot telescope ; the 

 discovery of Uranus and its consequences prevented the 

 renewal of the attempt for some time, but in 1785 he began 

 a 4o-foot telescope with a mirror four feet in diameter, the 

 expenses of which were defrayed by a special grant from 

 the King. While it was being made Herschel tried a new 

 form of construction of reflecting telescopes, suggested by 

 Lemaire in 1732 but never used, by which a considerable 

 gain of brilliancy was effected, but at the cost of some loss 

 of distinctness. This Herschelian or front-view construc- 

 tion, as it is called, was first tried with the 2o-foot, and led 

 to the discovery (January nth, 1787) of two satellites of 

 Uranus, Oberon and Titania ; it was henceforward regularly 



