1912] on Sir Williani Herschd. 455 



surface, and it becomes necessary to obtain a parabolic form. This 

 last staiie is done by further tests of the kind described, with a 

 diaphragm placed over the mirror which only permits the observer 

 to see tile light reflected from chosen zones of the mirror. The time 

 at my disposal will not allow me to describe this in further detail, or 

 to tell you how there is always found to be one definite diameter of 

 the glass along which its weight must be supported. I must pass by, 

 too, the system of counterpoised levels used for supporting the back 

 of the glass, and the method by which silver is chemically deposited 

 on its surface. Meagre although this sketch has been, it will have 

 served to show you how beautiful are the processes employed, and I 

 would ask you to realize that at first Herschel was a mere amateur, 

 and had to discover everything for himself. 



As I have said, Herschel had to do all his polishing by hand, and 

 he found W'hen once the linal stage had begun, it was necessary that 

 it should never stop even for a moment. Caroline relates how she 

 was kept busy in attending on her brother when polishing : " Since 

 by way of keeping him alive I was constantly obliged to feed him by 

 putting the victuals by bits into his mouth. This was once the case, 

 when in order to finish a 7-f t. mirror, he had not taken his hand from 

 it for sixteen liours together." 



The making of the mirror is, however, but a small part of the 

 difficulty of making a telescope, for it involves high engineering 

 skill to provide a solid stand, an observing platform, the graduated 

 circles in right ascension and declination for setting the telescope 

 and the clock, whereby it is made to follow the stars in their daily 

 motion. The great size of Herschel's mirrors and the weight of the 

 long tube introduced mechanical difficulties which were at that time 

 entirely new. 



A dozen years after his establishment at Bath, Herschel began to 

 be well known in the w^orld of science, and many of the most 

 illustrious astronomers came to see him. In 17^1 he was elected to 

 the Royal Society, and in the same year he discovered the planet 

 Uranus, and called it by the now almost forgotten name of G-eorgium 

 Sidus, in honour of Gleorge III. The magnitude of the discovery 

 may be estimated by the fact that only the five principal planets, 

 famiUar to all men for centuries, w^ere then known ; and the 

 asteroids or minor planets had not yet been discovered by Herschel 

 himself. His fame from this and his other discoveries led to a 

 command from the King to take his 7-f t. telescope to Windsor, and 

 there he was requested to act as celestial showman to the King, the 

 Queen, and the Princesses. The expedition put him to much expense, 

 and he was kept hanging about Windsor for mouths, but at length 

 the King offered him the post of Private Royal Astronomer, with the 

 modest salary of £200 a year. 



Herschel's friend. Sir William Watson, said that never had a 

 monarch bought honour so cheap, and Caroline pours scorn on the 



