iv PURSUIT OF KNOWLEDGE 63 



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 ft. mirror, he had not taken his hand from it 

 for sixteen hours together." 



Herschel undertook the stupendous task of surveying 

 with his telescope the whole of the heavens visible from 

 his observatory. With his instrument the field of view 

 visible at one setting was about one-quarter the apparent 

 size of the full moon, and Herschel had to observe more 

 than 300,000 of such fields in order to make his census 

 of the stars in a hemisphere of space. The method of 

 observation is described by Von Magellen in a letter 

 to Bode, from which the following are extracts : 



He has his 20 ft. Newtonian telescope in the open air. It is 

 moved by an assistant who stands below it ; near the instrument 

 is a clock. In the room near it sits Herschel' s sister, and she has 

 Flamsteed's Atlas open before her. As he gives her the word, 

 she writes down the declination and right ascension. In this 

 way Herschel examines the whole sky ; he is sure that after four 

 or five years (from 1788) he will have passed in review every 

 object above our horizon. Each sweep covers 2 deg. 15 min. in 

 declination, and he lets each star pass at least three times through 

 the field of the telescope, so that it is impossible that anything 

 can escape him. Herschel observes the whole night through; 

 for some years he has observed every hour when the weather 

 is clear, and this always in the open air. 



Another astronomer whose diligence in observation 

 was astonishing was S. H. Schwabe, a native of Dessau, 

 who began to observe the sun in 1826, with the idea that 

 the labour " might be rewarded by the discovery of a 

 planet interior to Mercury," and was led to inquire into 

 the rotation of the sun as indicated by the spots which 

 appear from time to time. Each spot was noted and 

 numbered in the order of its appearance, and when this 

 system of registration had been carried on to the end 

 of 1843, Schwabe modestly remarked that the number 



