96 COSMOS. 



two hundred stars of the sixth magnitude. Since it is very 

 probable, from analogy with the experiments already made, 

 that all cosmical bodies are subject to variations both in their 

 movements through space and in the intensity of their light, 

 although such variations may occur at very long and unde- 

 termined periods, it is obvious, considering the dependence 

 of all organic life on the sun's temperature and on the intens- 

 ity of its light, that the perfection of photometry constitutes 

 a great and important subject for scientific inquiry. Such 

 an improved condition of our knowledge can render it alone 

 possible to transmit to future generations numerical determ- 

 inations of the photometric condition of the firmament. By 

 these means we shall be enabled to explain numerous geog- 

 nostic phenomena relating to the thermal history of our at- 

 mosphere, and to the earlier distribution of plants and ani- 

 mals. Such considerations did not escape the inquiring mind 

 of William Herschel, who, more than half a century ago, be- 

 fore the close connection between electricity and magnetism 

 had been discovered, compared the ever-luminous cloud-en- 

 velopes of the sun's body with the polar light of our own ter- 

 restrial planet.* 



Arago has ascertained that the most certain method for 

 the direct measurement of the intensity of light consists in 

 observing the complementary condition of the colored rings 

 seen by transmission and reflection. I subjoin in a note,t in 



* William Herschel, On the Nature of the Sun and Fixed Stars, in 

 the Philos. Transact, for 1795, p. 62 ; and On the Changes that happen 

 to the Fixed Stars, in the Philos. Transact, for 1796, p. 186. Gomparo 

 also Sir John Herschel, Obsero. at the Cape, p. 350-352. 



t Extract of a Letter from M. Arago to M. de Humboldt, May, 1850. 



(a.) Mesurcs Photom6triqucs. 



" II n'existe pas de photometre proprement dit, c'est-a-dire d'instru- 

 ment donuant 1'intensite d'une lumiere isol^e ; le photometre de Les- 

 lie, a 1'aide duquel il avail eu 1'audace de vouloir comparer la lumiere 

 de la lune a la lumiere du soleil, par des actions calonfiques, est com- 

 pletement defectueux. .T'ai prouve, en effet, que ce preteudu photo- 

 metre monte quand on 1'expose a la lumiere du soleil, qu'il descend 

 sous 1'action de la lumiere du feu ordinaire, et qu'il reste complete- 

 ment stationnaire lorsqu'il re9oit la lumiere d'une lampe d'Argand'. 

 Tout ce qu'on a pu faire jusqu'ici, c'est de comparer entr'elles deux lu- 

 mieres en presence, et cette comparaison n'est meme a 1'abri de toute 

 objection que lorsqu'on ramene ces deux lumieres a I'egalit^ par un 

 amublissement graduel de la lumiere la plus forte. C'est comme crite- 

 rium de cette egalit6 que j'ai employ^ les anneaux colores. Si on place 

 I'une sur 1'autre deux lentilles d'un long foyer, il se forme autour de 

 leur point de contact des anneaux colored tant par voie de reflexion que 

 par voi 5 de transmission. Les anueaux reflechia sout conipleineutaires 



