POLARIZATION OF LIGHT. 45 



tia of the light reflected from the moon, from Venus, Mars, 

 and the clouds, we recognize, as might be anticipated, all the 

 peculiarities of the solar spectrum ; but, on the other hand, 

 the dark lines in the spectrum of Sirius differ from those of 

 Castor and the other fixed stars. Castor likewise exhibits dif- 

 ferent lines from Pollux and Procyon. Amici has confirmed 

 this difference, which was first indicated by Fraunhofer, and 

 has ingeniously called attention to the fact that in fixed stars, 

 which now have an equal and perfectly white light, the dark 

 lines are not the same. A wide and important field is thus 

 still open to future investigations,* for we have yet to distin- 

 guish between that which has been determined with certain- 

 ty and that which is merely accidental and depending on the 

 absorbing action of the atmospheric strata. 



We must here refer to another phenomenon, which is pow- 

 erfully influenced by the specific character of the source of 

 light. The light of incandescent solid bodies, and the light 

 of the electric spark, exhibit great diversity in the number 

 and position of Wollaston's dark lines. From Wheatstone's 

 remarkable experiments with revolving mirrors, it would ap- 

 pear that the light of factional electricity has a greater veloc- 

 ity than solar light in the ratio of 3 to 2 ; that is to say, a ve- 

 locity of 95,908 miles in one second. 



The stimulus infused into all departments of optical science 

 by the important discovery of polarization,! to which the in- 

 genious Malus was led in 1808 by a casual observation of the 

 light of the setting sun reflected from the windows of the Pa- 

 lais du Luxembourg, has afforded unexpected results to sci- 

 ence by the more thorough investigation of the phenomena of 

 double refraction, of ordinary (Huygens's) and of chromatic po- 

 larization, of interference, and of diffraction of light. Among 

 these results may be reckoned the means of distinguishing 

 Vttween direct and reflected light, $ the power of penetrating, 



* On the relation of the dark lines on the solar spectrum in the Da- 

 guerreotype, see Comptes Rendus des Stances de VAcadSmie des Sciences, 

 torn, xiv., 1842, p. 902-904, and torn, xvi., 1843, p. 402-407. 



t Cosmos, vol. ii., p. 332. 



t Arago's investigation of cometary light may here be adduced as an 

 instance of the important difference between proper and reflected light. 

 The formation of the complementary colors, red and green, showed by 

 the application of his discovery (in 1811) of chromatic polarization, that 

 the light of Halley's comet (1835) contained reflected solar light. I was 

 myself present at the earlier experiments for comparing, by means of 

 the equal and unequal intensity of the images of the polariscope, the 

 proper light of Capella with the splendid comet, as it suddenly emerged 

 From the rays of the sun at the beginning of July, 1819. (See Annuaire 



