270 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK in. 



The following are some interesting details borrowed from the 

 excellent notice published by Arago on the labours of the great 

 astronomer at Slough : 



" Before finding direct and certain methods of giving the neces- 

 sary images to the mirror, it was requisite that Sir W. Herschel 

 should win his way by degrees like his predecessors. His trials were 

 directed in such a way that he did not take a step backwards. In his 



FKI. '205.- Gregory's telescope. 



method of work, according to an old adage, ' Le miewx rietait jamais 

 Vennemi du bien.' When Herschel undertook the construction of a 

 telescope, he cast l and worked several mirrors at a time ten, for 

 instance. The one of these mirrors with which observations made 



1 The metal of which the mirrors of telescopes are made is generally composed 

 of 67 parts of copper and 33 of tin. This alloy is of a yellowish tint, and is sus- 

 ceptible of a beautiful polish. Sometimes small proportions of brass, silver, arsenic, 

 and also platinum are added. 



