126 COSMOS. 



proved by an ingenious elaboration of the materials 70 probably 

 deserves the preference over any other approximative method 

 practicable in the present imperfect condition of photometrical 

 instruments, however much the exactness of the estimates 

 must be endangered by the varying powers of individual ob- 

 servers the serenity of the atmosphere the different altitudes 

 of widely distant stars, which can only be compared by means 

 of numerous intermediate stellar bodies and above all by the 

 unequal colour of the light. Very brilliant stars of the 

 1st magnitude, such as Sirius and Canopus, a Centauri and 

 Achernar, Deneb and Vega, on account of their white light, 

 admit far less readily of comparison by the naked eye 

 than fainter stars below the 6th and 7th magnitudes. Such 

 a comparison is even more difficult when we attempt to 

 contrast yellow stars of intense light, like Procyon, Capella, 

 or Atair, with red ones, like Aldebaran, Arcturus, and Betel- 

 geux. 71 



Sir John Herschel has endeavoured to determine the rela- 

 tion between the intensity of solar light, and that of a star of 

 the 1st magnitude by a photometric comparison of the moon 

 with the double-star a Centauri of the southern hemisphere, 

 which is the third in brightness of all the stars. He thus 

 fulfilled (as had been already done by Wollaston) a wish 

 expressed by John Michell 72 as early as 1767. Sir John 

 Herschel found from the mean of eleven measurements con- 

 ducted with a prismatic apparatus ; that the full moon was 

 27408 times brighter than a Centauri. According to Wol- 

 laston the light of the sun is 801072 times brighter than 



70 Argelander, Durchmusterung des riordL Himmels zwi- 

 s?hen 45 und 80 Decl. 1846, s. xxiv. xxvi.; Sir John 

 Herschel, Astr. Observ. at the Cape of Good Hope, pp. 327, 

 340, 365. 



71 Op. cit., p. 304, and Outl., p. 522. 



72 Philos. Transact., vol. Ivii. for the year 1767, p. 234. 



