56 COSMOS. 



has shown that this phenomenon is owing to different altitudes 

 of the sun, and to the different absorption of the rays of light 

 in their passage through the atmosphere. In the spectra of 

 the light reflected from the moon, from Venus, Mars, and the 

 clouds, we recognize, as might be anticipated, all the pecu- 

 liarities 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 

 different lines from Pollux and Procyon. Amici has con- 

 firmed 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, 7 for we have yet to 

 distinguish between that which has been determined with 

 certainty, 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 

 powerfully 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 Wheat- 

 stone's remarkable experiments with revolving mirrors it 

 would appear that the light of frictional electricity has a 

 greater velocity than solar light, in the ratio of 3 to 2 ; that 

 is to say, a velocity of 95908 miles in one second. 



The stimulus infused into all departments of optical science 

 by the important discovery of polarisation, 8 to which the in- 

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



7 On the relation of the dark lines of the solar spectrum 

 in the Daguerreotvpe, see Comptes rendus des seances de V Aca- 

 demic des Sciences, torn. xiv. 1842, pp. 902-904, and torn. xvi. 

 1843, pp. 402-407. 



8 Cosmos, vol. ii. p. 715. 



