326 M. Arago on Nebulce. 



and us, and thus it happened that the phenomenon so visible 

 in 1774 no longer existed 36 years after.* 



Such is Herschel's theory, if I understand it aright. I shall 

 not here consider whether it might not have been more simple 

 to assimilate the circular nebulosities of the three stars of 

 Orion to the luminous atmospheres of ordinary nebulous stars, 

 than to attribute the weakened light of the largest, and the 

 disappearance of the two others, to a motion of the atmo- 

 spheres towards the centre of each star. I see nothing in the 

 observation which, at first sight, would oppose this mode of 

 explanation ; but the strictest caution is a duty whenever we 

 differ from the opinions professed by the illustrious astronomer 

 of Slough. 



MILKY WAY. 



Opinions of the Ancients on the Milky Way, — Such is the 

 name applied to the luminous whitish zone which every one 

 has remarked in the starry sphere. Every one also knows 

 that this zone goes round the whole firmament ; that it very 

 nearly traces one of its great circles, not, however, without 

 undergoing a sharp bifurcation from which results a secon- 

 dary bow, which, after continuing separated from the princi- 

 pal arc for the extent of about 120'', again becomes con- 

 founded with it.* 



The Milky Way excited the eager attention of the ancient 

 philosophers. Manilius describes at length, in his poem, 



* The authenticated disappearance of a starry nebulosity would be a very 

 extraordinary phenomenon and very fruitful in results. I have, therefore, 

 thought it requisite to inquire whether the annals of science offer any fact 

 analogous to the two cited by Herschel. My search has not been, in my 

 opinion, unfruitful. Lacaille, during his residence at the Cape, saw in the 

 constellation Argo (310 Bode) five small stars in the centre of a nebulosity, 

 of which Mr Dunlop, with much better instruments, could perceive no 

 traces in 1025. 



t The breadth of the Milky Way seems very unequal- In some places it 

 does not exceeds"; in others the breadth is 10* and even 1C.° Its two 

 branches, between Serpentarius and Antinous, retire more than 22" of the 

 sphere. 



