104 



SCIENCE 



[N. S. Vol. XXXVI. No. 917 



The only general test of the relative 

 nearness or farness of the stars is their 

 brightness, because the faint stars must, on 

 the average, be more distant than the bright 

 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 oiit 

 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 declination, and each zone 

 was examined throughout by the process 

 which he 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 300,000 of such fields 

 of view to cover the whole of the hemi- 

 sphere 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 de- 

 scribes the method of observation as fol- 

 lows: 



He lias 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 declina- 

 tion 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° 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 dis- 



tant 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 proc- 

 esses of evolution in the heavens. In pho- 

 tography 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 won- 

 derful 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 de- 

 grees. 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 nebulse. 

 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 nebuljE are 

 mostly fleecy clouds. On the other hand, 

 the spiral and planetary nebulae are more 

 frequent away from the Milky Way, and 

 these are presumably older than the cloudy 

 and flocculent nebulas. The shape of the 

 Milky Way seems to resemble a huge mill- 

 stone or disk of stars, and since it forms 

 a complete circuit in the heavens the sun 

 must lie somewhere towards its middle. It 

 is probable that we look much further out 

 into space along this tract than elsewhere, 

 although it happens that by far the near- 

 est of all the stars — namely, a Centauri — ■ 

 lies in the line of the Milky Way. 



This great congregation of stars is far 

 from uniform in density, for there are 

 places in it where there are but few stars 

 or none at all. Caroline Herschel, writing 



