86 COSMOri. 



On comparing the velocities of solar, stellar, and terres- 

 trial light, which are all equally refracted in the prism, 

 with the velocity of the light of frictional electricity, we are 

 disposed, in accordance with Wheatstone's ingeniously con- 

 ducted experiments, to regard the lowest ratio in which the 

 latter exceeds the former as 3 : 2. According to the lowest 

 results of Wheatstone's optical rotatory apparatus, electric 

 light traverses 288,000 miles in a second. =^ If we reckon 

 189,938 miles for stellar light, according to Struve's observ- 

 itions on aberration, we obtain the difference of 95,776 miles 

 as the greater velocity of electricity in one second. 



These results are ap2)o.rently opposed to the views ad- 

 vanced by Sir William Herschel, according to which solar 

 and stellar light are regarded as the effects of an electro- 

 magnetic process — a perpetual northern light. I say ap- 

 farenily, for no one will contest the possibility that there 

 may be several very different magneto-electrical processes in 

 the luminous cosmical bodies, in which light — the product 

 of the process — may possess a different velocity of propaga- 

 tion. To this conjecture may be added the uncertainty of 

 the numerical result yielded by the experiments of Wheat- 

 stone, who has himself admitted that they are not sufficient- 

 ly estabhshed, but need further confirmation before they can 



associated with a slight degi'ee of refraction, while a smaller amount of 

 velocity involves a slighter degree of refraction. Thus every visible 

 red ray is accompanied by dark rays of the same nature, of which some 

 are more, and others less, refracted than the former; there are conse- 

 quently rays in the black lines of the red portion of the spectrum ; and 

 the same must be admitted in reference to the lines situated in the yel- 

 low, green, blue, and violet portions." — Arago, iu the Comptes Rendus 

 de V Acad, des Sciences, X. xvi., 1843, p. 404. Compare also t. viii., 1839, 

 p. 326, and Poisson, Traili de Mecaniqne, ed. ii., 1833, t. i., ^ 168. Ac- 

 cording to the undulatory theory, the stars emit waves of extremely 

 various transverse velocities of oscillations. 



* Wheatstone, in the Philos. Transact, of the Royal Soc.for 1834, p. 

 589, 591. From the expei-iments described in this paper, it would ap 

 pear that the human eye is capable of perceiving phenomena of light, 

 w^hose duration is limited to the millionth part of a second (p. 591). 

 On the hypothesis referred to in the text, of the supposed analogy be- 

 tween the light of the sun and polar light, see Sir John Herschel's Re- 

 sults of Astron. Observ. at the Cape of Good Hope, 1847, p. 351. Arago, 

 in the Comptes Rendus pour 1838, t. vii., p. 956, has referred to the in- 

 genious application of Breguet's improved Wheatstone's rotatory ap- 

 paratus for determining between the theories of emission and undula- 

 tion, since, according to the former, light moves more rapidly through 

 water than through air, while, according to the latter, it moves more 

 rapidly through air than through water. (Compai-e also Comptes Ren- 

 diis pour 1850, t. xxx., p. 489-495, 556.) 



