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On the Nature of the Light of Comets, 



At the meeting of the Academy of Sciences, 26th October 

 1835, M. Arago continued his account of the physical changes 

 observed in Hal ley's comet. 



The weather has proved unfavourable ; a clouded sky, for se- 

 veral successive days, having completely concealed that body 

 from observation. It has therefore been impossible, from the 

 observations made at Paris, to determine the question, by what 

 kind of transformation the nebulous matter has passed from one 

 state of agglomeration to another. To settle this point, it will 

 be necessary to have recourse to the observations which, it is to 

 be hoped, have been made in other places. 



It was mentioned in a preceding account, that on Thursday 

 the 15th October, a luminous sector was visible in a certain part 

 of the head of Halley's comet ; that on the day following this 

 sector had disappeared, and that a more brilliant one, more open 

 and of greater longitudinal extent, was formed in another place ; 

 that this second sector was observed on the 17th, when it ap- 

 peared less bright, and on the 18th its weakness had decidedly 

 increased. Since that time the comet continued concealed till 

 the 21st. On that day, at 6| h. three distinct sectors were vi- 

 sible in the nebulosity. Of these, the weakest and least dis- 

 tinctly defined, was situated on the prolongation of the tail. 

 On the 23d October all traces of these sectors had disappeared. 

 The comet had then undergone such a change, — the nucleus, 

 which had hitherto been brilliant and well defined, having be- 

 come so large and diffuse, that the observer could scarcely be- 

 lieve in the reality of such a sudden and important alteration, 

 until he had satisfied himself that the appearance was not oc- 

 casioned by moisture on the glasses of his instrument. The nu- 

 cleus perhaps still occupied, as on former days, the centre of the 

 nebulosity ; but the eastern region of this nebulosity was cer- 

 tainly much less bright than the opposite part. 



From the first appearance of this comet, M. Arago had point- 

 ed out SL photometrical method, which, if properly applied, seem- 

 ed to afford certain means of determining whether these stars 

 borrow their light from the sun, or are luminous in themselves. 



