128 COSMOS. 



mates the intensity of tlie light of Sirius to be equal to the 

 light of nearly two hundred stars of the 6th 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 undetermined periods, it is obvious, considering the de- 

 pendence of all organic life on the sun's temperature and 

 on the intensity of its light, that the perfection of photo- 

 metry 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 determinations of the photometric condition of the 

 firmament. By these means we shall be enabled to explain 

 numerous geognostic phenomena relating to the thermal history 

 of our atmosphere, and to the earlier distribution of plants 

 and animals. Such considerations did not escape the in- 

 quiring mind of William Herschel, who, more than half a 

 century ago, before the close connection between electricity 

 and magnetism had been discovered, compared the ever 

 luminotfs cloud-envelopes of the sun's body with the polar 

 light of our own terrestrial planet. 75 



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 coloured rings seen by trans- 

 mission and reflection. I subjoin in a note, 76 in his own words, 



75 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. Transct.for 1796, 

 p. 186. Compare also Sir John Herschel, Observ. at the 

 Cape, pp. 350-352. 



76 Extract of a Letter from M. Arago to M. de Humooldt, 

 May, 1850. 



Mesures photometriques. 

 " Iln'existe pas de Photometre proprement dit, c'est-a-dire 



