321: M. Arago on Nebalcc. 



natural. In order to explain why the lustre of nebulous 

 planetary discs is not much stronger in the centre than to- 

 wards the edges, it is necessary to admit that the light does 

 not come from the whole depth of the nebula (otherwise its 

 intensity would increase with the number of material and ra- 

 diating particles contained in the direction of each visual ray) ; 

 it is necessary to reduce the radiation to the state of being 

 purely superficial ; we must grant, in other words, that when 

 it attains a certain density, the diffused milky matter, as one 

 would call it, ceases to be diaphanous. 



1 do not know, but it seems to me, that all these suppositions 

 may be avoided by admitting that these planetary nebulae are 

 nebulous starSy so remote from the earth that the central star 

 no longer predominates by its splendour over the diffused lu- 

 minosity with which it is surrounded. It would be superfluous 

 to repeat here what I have already said in another part of this 

 essay. 



I add a single word on the danger that would arise from 

 drawing too absolute consequences from the evolutions of the 

 diffused matter, and the various forms it may assume when 

 agglomerating. Has it not been alleged but lately, that, in the 

 nebula of Orion, the milky substance is not in immediate con- 

 tact with the stars of the celebrated trapezium so well known 

 to all astronomers ? Has it not been said that these stars are, as 

 it were, isolated in the midst of the nebulosity, and that a dark 

 space surrounds them ? Astronomers, I admit, have not yet de- 

 monstrated that we ought to see, in the phenomenon of which 

 I have spoken, any thing else than a simple effect of contrast ; 

 nothing proves that it is any thing else than a very feeble 

 light becoming effaced by the contact of a more brilliant one. 

 To remove all doubts, it is necessary to throw, by means of 

 the reflection of a flat diaphanous mirror with parallel faces, 

 placed before the object-glass or the aperture of a telescope, the 

 image of some star on the image of the nebula, and observe if 

 the image of the star thus reflected shall seem likewise sur- 

 rounded with a dark space. In the mean time, every thing 

 authorizes us to suppose that the milky molecules are sub- 

 jected, in the vast regions of space, to forces of which we 

 have no idea. The observers who have followed the pro- 

 digious, and often almost instantaneous, changes of Halley*s 



