328 A Short History of Astronomy [Ca. xn. 



employed. After several mishaps the 4o-foot telescope 

 (fig. 82) was successfully constructed. On the first evening 

 on which it was employed (August 28th, 1789) a sixth satellite 

 of Saturn (Enceladus) was detected, and on September i yth a 

 much fainter seventh satellite (Mimas). Both satellites were 

 found to be nearer to the planet than any of the five hitherto 

 discovered, Mimas being the nearer of the two (cf. fig. 91). 



Although for the detection of extremely faint objects such 

 as these satellites the great telescope was unequalled, for 

 many kinds of work and for all but the very clearest 

 evenings a smaller instrument was as good, and being less 

 unwieldy was much more used. The mirror of the great 

 telescope deteriorated to some extent, and after 1811, 

 HerscheFs hand being then no longer equal to the delicate 

 task of repolishing it, the telescope ceased to be used 

 though it was left standing till 1839, when it was dismounted 

 and closed up. 



256. From the time of his establishment at Slough till 

 he began to lose his powers through old age the story of 

 Herschel's life is little but a record of the work he did. It 

 was his practice to employ in observing the whole of 

 every suitable night ; his daylight hours were devoted to 

 interpreting his observations and to writing the papers in 

 which he embodied his results. His sister was nearly 

 always present as his assistant when he was observing, and 

 also did a good deal of cataloguing, indexing, and similar 

 work for him. After leaving Bath she also did some 

 observing on her own account, though only when her 

 brother was away or for some other reason did not require 

 her services ; she specialised on comets, and succeeded from 

 first to last in discovering no less than eight. To form any 

 adequate idea of the discomfort and even danger attending 

 the nights spent in observing, it is necessary to realise that 

 the great telescopes used were erected in the open air, 

 that for both the Newtonian and Herschelian forms of 

 reflectors the observer has to be near the upper end of the 

 telescope, and therefore at a considerable height above 

 the ground. In the 40-foot, for example, ladders 50 feet 

 in length were used to reach the platform on which the 

 observer was stationed. Moreover from the nature of 

 the case satisfactory observations could not be taken in the 



