M. Arago on the Comet of 1835. 33 



We do not yet know with certainty if comets are of them- 

 selves luminous, or if they borrow from the sun all the light with 

 which they shine. The investigation of their phases seemed to 

 be the only means of resolving the question, but hitherto it has 

 entirely failed. Comparative measures of the intensity of light, 

 photometric measures, may lead us to the result in a manner not 

 less incontestible. This kind of observation will doubtless ex- 

 cite the attention of astronomers, during the actual appearance 

 of Halley's comet. The second edition of the Annuaire of 1832 

 proves to amateurs of the science, that they even, with very 

 feeble instruments, may usefully assist in this curious research. 



In the year 1305, Halley's comet had extraordinary bright- 

 ness ; in 1456 it dragged after it a tail which embraced two 

 thirds of the space between the horizon and the zenith ; in 1682, 

 though notably less brilliant than in 1305 or 1456, it was classed 

 among the brilliant comets, and its tail was still 30' ; in 1759, its 

 appearance would certainly have occupied the attention of none 

 but astronomers, if it had not been the first comet announced a 

 long time previously* These facts seem to prove that comets 

 are diminishing in brightness, and we might be tempted to search 

 for the physical cause in the matter which near the perihelion 

 separates itself from the nebulosity to form the tail, and which 

 the comet seems to disseminate in space. M. Olbers, certainly 

 one of the most competent judges in a subject of this kind, does 

 not regard the gradual enfeebling of comets as proved ; he be- 

 lieves that the diminution observed in that of Halley from 1305 

 to 1456, from 1456 to 1682, and from 1682 to 1759, is only ap- 

 parent ; that it can be explained by theVery peculiar relative po- 

 sitions of the sun, the comet, and the earth ; and finally, he cites 

 in support of his opinion, the appearance of 1607, intermediate 

 between those whose dates I have mentioned, adducing which 

 for the positions analogous to those of 1759, the comet, accord- 

 ing to Kepler, presented nothing remarkable in its intensity. In 

 this state of the problem every one must perceive the interest 

 which must be attached to the circumstances attending the ap- 

 pearance of the comet next November. 



It will be on August 20. 1835, near | of Taurus; August 

 28, between Gemini; September 21, in the Coachman; Octc- 



VOL. XIX. NO. XXXVII.— JULY 1835, c 



