ASTRONOMY OF EIGHTEENTH CENTURY. 259 



ceeded in producing instruments far superior to any that had yet been 

 made. In the year 1774 he finished a reflecting telescope made after 

 Newton's plan (page 217), and this achievement was but the first of 

 a long series of brilliant successes. The focal length of his first tele- 

 scope was 5 feet, and then succeeded others of 7, 10, 20, and at length 

 his great telescope of 40 feet focal length was erected at Slough, near 

 Windsor. When Herschel had entered upon his career of practical 

 optician and astronomical observer, he became so engrossed with his 

 scientific pursuits that, regardless of the loss of income, he began to 

 limit his musical engagements and the number of his pupils. In 

 forming the mirrors for the successively larger and larger telescopes, 

 his perseverance and labour were almost incredible He soon found 

 that it was quite impossible to give the true figure to the mirrors by 

 any mechanical means when their focal length was considerable. The 

 exact form for a mirror of upwards of 6 feet focal length can only be 

 finally given by delicate touches with the hand after repeated trials. 

 With all the care and skill given by practice, the chances are very great 

 against the correct form being given at any one operation. It was 

 Herschel's practice to cast three metallic mirrors for a telescope, and, 

 having worked at all three, to place the best one of them in the tele- 

 scope, and use it for observations while he continued to work at the 

 others successively, again leaving the best in the telescope, and so on. 

 Thus for each telescope very many mirrors were examined before the 

 finally perfect one could be arrived at. In giving the final polish to 

 a mirror, a single rub wrongly applied spoils the shape, and the diffe- 

 rence is distinctly perceptible to trained eyes when certain objects are 

 viewed in the telescope. Herschel would work at his mirrors con- 

 tinuously for twelve or fourteen hours without quitting his occupation 

 for a moment. So much was he convinced that " when his hand was 

 in " any interruption would spoil his labour, that he would not quit it 

 to help himself to food, and the little he ate on these occasions was 

 put into his mouth by his sister. It is said that Herschel himself 

 ground 200 mirrors of 7 feet focal distance, 150 of 10 feet, and So of 

 20 feet. 



Herschel was still residing at Bath, and dependent for the means 

 of subsistence on his profession as a musician, when he completed his 

 7 -feet reflector. He had brought his mirror (made of an alloy of tin 

 with copper) to a degree of perfection hitherto supposed impossible, 

 and carried the magnifying power of his eye-pieces to a range which 

 had been assumed to be altogether beyond the faculty of the eye to 

 bear. The ingenious fabricator found means of making this and 

 his subsequently still larger instruments so convenient in use, that 

 he had no more difficulty in observing with them than an ordinary 

 person finds in employing a spying-glass two or three feet long. His 

 eagerness to observe the heavens was so great, that for several years 

 he was never in bed while the stars were visible. Winter and summer 



17 2 



