458 Sir George H. Darivin [April 26, 



stars is their brightness, because the faint stars must, on the average, 

 be more distant than the liright ones. Herschel then proposed to 

 penetrate into space by means of a celestial census of the distribution 

 and of the brightness of the stars. With this object he carried out 

 four complete reviews of the heavens, as far as they may be seen 

 from our latitude, passing successively to the fainter and fainter 

 objects by means of the increased size of his telescope. 



He divided the heavens into sweeps 2° 15' of breadth in declina- 

 tion, and each zone was examined throughout by the process which 

 be called star-gauging. His census was made with the 20-ft. reflector, 

 with which instrument the field of view was about one quarter of the 

 size of the full moon. It needs over 800,000 of such fields of view 

 to cover the whole of the hemisphere of space, and Herschel surveyed 

 the whole northern hemisphere, and as much of the southern one as 

 he could. 



Von Magellan in a letter to Bode describes the method of obser- 

 vation as follows : " He has his 20-ft. Newtonian telescope in the 

 open air. . . . It is moved by an assistant w^ho 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 'l"^ 15' 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." 



Herschel points out that by this survey he was not only looking 

 into the most distant space, but also into the remotest past, for the 

 light of many of the stars must have started on its journey towards 

 us thousands or even millions of years ago. The celestial museum 

 therefore exhibits to us the remotest past alongside with the present, 

 and we have in this way the means of reconstructing to some extent 

 the processes of evolution in the heavens. In photography the 

 modern astronomer possesses an enormous advantage, but Herschel 

 laid the foundation of this branch of astronomy without it. 



The most conspicuous and the most wonderful object in the 

 heavens is the Milky Way. It runs all round the skies in a great 

 band, with a conspicuous rent in it forming a streamer which runs 

 through many degrees. To the naked eye it shines with a milky 

 light, but Herschel was able to show that it consists of countless stars 

 in which there lie embedded many fleecy nebulaj. There is good 

 reason to believe that the Milky Way on the whole consists of stars 

 which are younger than those in the other parts of space, for the 

 stars in it are whiter and hotter, and the nebulae are mostly fleecy 



