COMETS. 



91 



The existence of polarized cometary light announced itself not 

 only by the inequality of the images, but was proved with 

 greater certainty on the reappearance of Halley's comet, in 

 the year 1835, by the more striking contrast of the comple- 

 mentary colours, deduced from the laws of chromatic polar- 

 ization discovered by Arago in 1811. These beautiful ex- 

 periments still leave it undecided, whether, in addition to 

 this reflected solar light, comets may not have light of their 

 own. Even in the case of the planets, as, for instance, in 

 Venus, an evolution of independent light seems very probable. 



The variable intensity of light in comets cannot always be 

 explained by the position of their orbits, and their distance 

 from the Sun. It would seem to indicate, in some individuals, 

 the existence of an inherent process of condensation, and an 

 increased or diminished capacity of reflecting borrowed light. 

 In the comet of 1618, and in that which has a period of three 

 years, it was observed first by Hevelius, that the nucleus of 

 the comet diminished at its perihelion, and enlarged at its 

 aphelion, a fact which, after remaining long unheeded, was 

 again noticed by the talented astronomer, Valz, at Nismes. 

 The regularity of the change of volume, according to the 

 different degrees of distance from the Sun, appears very 

 striking. The physical explanation of the phenomenon can- 

 not, however, be sought in the condensed layers of cosmical 

 vapour occurring in the vicinity of the Sun, since it is difficult 

 to imagine the nebulous envelope of the nucleus of the comet 

 to be vesicular and impervious to the ether.* 



The dissimilar eccentricity of the orbits of comets has, in 

 recent times (1819), in the most brilliant manner enriched 

 our knowledge of the solar system. Encke has discovered 

 the existence of a comet of so short a period of revolution, 



chromatic polarization, two images of complementary colours (green 

 and red). (Annalesde Chimie, t.xiii. p. 108 ; Annuaire, 1832, p. 216.) 

 " We must conclude from these observations," says Arago, " that the 

 cometary light was not entirely composed of rays having the properties 

 of direct light ; there being light which was reflected specularly or polar- 

 ized, that is, coming from the sun. It cannot be stated with absolute 

 certainty, that comets shine only with borrowed light, for bodies, in 

 becoming self-luminous, do not on that account lose the power of 

 reflecting foreign light." 



* Arago, in the Annuaire t 1832, pp. 217-220. Sir John Herschel, 

 Astron., 488. 



