SOLAR LIGHT. 393 



A comparison between solar light and the two most intense 

 kinds of artificial light which man has hitherto been able to 

 produce, yields, according to the present imperfect condition 

 of photometry, the following numerical results : Fizeau and 

 Foucault found, by their ingenious experiments, that Drum- 

 mond's light (produced by the flame of the oxy-hydrogen 

 lamp directed against a surface of lime) was to the light of 

 the Sun's disc as 1 to 146. The luminous current, which in 

 Davy's experiment was generated between two charcoal 

 points, by means of a ISunsen's battery, having forty-six 

 small plates, was to the light of the Sun as 1 to 4'2 ; but 

 when very large plates were used, the ratio was as 1 to 2 '5, 

 and this light was, therefore, not quite three times less intense 

 than solar light. 26 When we consider the surprise still 

 experienced at the circumstance of Drummond's dazzling 

 light forming a black spot when projected on the Sun's disc, 

 we are doubly struck by the felicity with which Galileo, by 

 a series of conclusions as early as 1 6 1 2, 21 on the smallness of 

 the distance from the Sun at which the disc of Venus was 

 no longer visible to the naked eye, arrived at the result that 

 the blackest nucleus of the Sun's spots was more luminous 

 than the brightest portions of the full Moon. 



26 Fizeau and Foucault, Recherches sur Vintensite de la 

 Lmniere emisc par le Charbon dans I' experience de Davy, ill 

 the Comptes Rendus, torn, xviii. 1844, p. 753. '"The most 

 intensely ignited solid (ignited quicklime in Lieutenant Drum- 

 mond's oxy-hydrogen lamp), appear only as black spots on the 

 disc of the Sun when held between it and the eye." Outlines, 

 p. 3G (Cosmos, vol. ii. pp. 707-708.) 



27 Compare Arago's commentary on Galileo's letter to 

 Marcus Welser, as well as his optical explanation of the 

 influence of the diffuse reflected solar light of the atmospheric 

 strata which covers the object seen in the sky upon the field 

 of H telescope, as it were, with n luminous veil, in the 

 airedu Bureau des Long, pour 1842, pp. 4S2-437. 



