82 COSMOS. 



envelope must be of a thickness d.fierent fiom 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 cii- 

 cumstance ofDrummond'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 

 a.3 1612,1 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 

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

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

 3leu& would yet possess 2000 times more light than the full 



* Fizeau and Foucault, Recherckes sur VIntensiti de la Lumiere Smise 

 par le Charbondans V Experience de Davy,\n the Comptcs Rendus,\.om. 

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

 jitne in Lieatenant Drummond's oxyhydrogen lamp) appear only at 

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

 — Outlines, p. 3G {Cosmos, \o\. ii., p. 325-326). 



t Compare Arago's connnentary on Galileo's letter to Marcus Wolser, 

 as well as his optical explanation of the influence of the diffuse reflected 

 solar light of the atmospheric strata which covers the ohject seen in the 

 skv upon the field of a telescope, as it were, with a luminous vail, in the 

 jKTiuaire du Bureuu des Lo7ig. kr- 1842, p. 482-487. 



