28 M. Arago on the Light of Comets. 



Observation has shewn us that this star becomes more and 

 more faint, even in the alternative of a theory, based upon the 

 hypothesis that an unchanging constitution would indicate a 

 constant light. The real increase of intensity which would have 

 resulted from the supposed condensation of the nebulous matter, 

 was calculated, therefore, to exhibit more strikingly the disagree- 

 ment between the calculation and the experiment. It ought to 

 add strength to the conclusion to which this disparagement has 

 conducted us ; and so, in our argument, we might legitimate- 

 ly have abstracted the alleged condensation which the nebulosity 

 of the comet was supposed to undergo. Now-a-days, however, 

 on the contrary, it is proved that, instead of contracting, the 

 nebulosity dilates, as it withdraws from the sun. I would not 

 venture now, as I did formerly in my public lectures, to con- 

 clude without more ado, that because the light of comets became 

 gradually more faint, therefore their light is a reflected light. 

 It is necessary, in addition, to take into account the scattering 

 which the nebulous matter undergoes. It is necessary to de- 

 monstrate, that the real diminution of intensity which thence re- 

 sults is not sufficient to explain how, sooner or later, the most 

 brilliant comets disappear. But this appears neither difficult 

 nor complicated, as the reader shall judge. 



Up to the present time, the most brilliant comets have ceased 

 to be visible from the earth, as soon as, in their progress round 

 the sun, they have removed themselves from this luminary to a 

 distance equal to a radius of the orbit of Jupiter ; that is to say, 

 to five times the radius of the almost circular curve which the 

 earth annually describes. Let us now then consider a comet 

 which, like that of 1680, had its perihelion within the orbit of 

 Venus. According to the calculations of M. Valz, the real dia- 

 meter of its nebulosity would augment with its distances from 

 the sun, according to the following progression : — At the dis- 

 tance of Venus, 10; the Earth, 29; Mars, 76; Ceres, 173; 

 Jupiter, 278. 



This progression of diameters differs but little from the fol- 

 lowing :— 1 ; 3 ; 8 ; 17 ; 28. 



The quantity of nebulous matter which, at the distance of 

 Venus, occupies a spherical volume of a diameter equal to 1, 



