82 COSMOS. 



envelope must be of a thickness difierent from that of the 

 polar, density for density, so that a difierent obstacle must 

 be thereby opposed to the escape of heat from the equatorial 

 and the polar regions of the Sun." Arago is engaged at the 

 present moment in a series of experiments, by which he pur- 

 poses to test not only his own views, but also to reduce the 

 results of observation to accurate numerical relations. 



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 oxyhydrogen lamp 

 directed against a surface of chalk) was to the light of the 

 Sun's disk as 1 to 146. The luminous current, which in Da- 

 vy's experiment was generated between two charcoal points 

 by means of a Bunsen'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.* 

 When we consider the surprise still experienced at the cir- 

 cumstance of Drummond's dazzling light forming a black spot 

 when projected on the Sun's disk, we are doubly struck by the 

 felicity with which Galileo, by a series of conclusions as early 

 as 1612,t on the smallness of the distance from the Sun at 

 which the disk 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. 



William Herschel, assuming the intensity of the whole 

 light of the Sun at 1000, estimated the average light of the 

 penumbrae at 469, and the black nuclei at 7. According to 

 this estimate, which is certainly very conjectural, a black nu- 

 cleus would yet possess 2000 times more light than the full 



* Fizeau and Foucault, RecJierch.es sur VIntensiti de la Lumiere imise 

 par le Charbon dans V Experience de Davy, in the Comptes Rendus, torn. 

 xviii., 1844, p. 753. " The most intensely ignited solid (ignited quick- 

 lime in Lieutenant Drummond's oxyhydrogen lamp) appear only as 

 black spots on the disk of the Sun when held between it and the eye." 

 —Outlines, p. 36 (Cosmos, vol. ii., p. 325-326). 



t 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 a telescope, as it were, with a luminous vail,m the 

 Annuaire du Bureau des Long, for 1842, p. 482-487. 



