COMETS. 105 



zenith distance of the stars, does not affect the ray of light 

 passing through it ? In the passage of a comet over a star, a 

 more or less considerable diminution of light has often been 

 observed ; but this has been justly ascribed to the brightness 

 of the ground from which the star seems to stand forth during 

 the passage of the comet. 



The most important and decisive observations that we pos- 

 sess on the nature and the light of comets are due to Arago's 

 polarization experiments. His polariscope instructs us re- 

 garding the physical constitution of thfe Sun and comets, indi- 

 cating whether a ray that reaches us from a distance of many 

 millions of miles transmits light directly or by reflection ; and 

 if the former, whether the source of light is a sofld, a liquid, 

 or a gaseous body. His apparatus was used at the Paris Ob- 

 servatory in examining the hght of Capella and that of the 

 great comet of 1819. The latter shov/ed polarized, and there- 

 fore reflected hght, while the fixed star, as was to be expect- 

 ed, appeared to be a self-luminous sun.* The existence of 

 polarized cometary light announced itself not only by the in- 

 equality 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 complementary colors, de- 

 duced from the laws of chromatic polarization discovered by 

 Arago in 1811. These beautiful experiments 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 ui- 

 dependent light seems very probable. 



The variable intensity of light in comets can not always be 



* On the 3d of July, 1819, Arago made the first attempt to analyze 

 the light of comets by polarization, on the evening of the sudden ap 

 pearance of the great comet. I was present at the Paris Observatory, 

 and was fully convinced, as were also Matthieu and the late Bouvard, 

 of the dissimilarity in the intensity of the light seen in the polariscope, 

 when the instrument received cometary light. When it received light 

 from Capella, which was near the comet, and at an equal altitude, the 

 images were of equal intensity. On the reappearance of Halley's com- 

 et in 1835, the instrument was altered so as to give, according to Ara- 

 go's chromatic polarization, two images of complementary colors (gieen 

 and red). {Annates de 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 po- 

 larized, that is, coming from the sun. It can not be stated with abso- 

 lute 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." 



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