8^IB M. Arago en Ncbuloi. 



may seem possible in such or such particular point. It could 

 not be the same either for the whole of the circumscribed ra- 

 diating places presented by the whole firmament, nor even 

 for two, three, or four of these places which are remarked in 

 a single nebula. It must, therefore, be admitted, that it is the 

 produce of a condensation, an increase of density in certain 

 points of the nebulous spaces, the vast extent of which we 

 have already computed. 



Is this condensation the eifect of an attractive force, ana- 

 logous to that which predominates over and regulates all the 

 motions of our solar system ? Such is the magnificent pro- 

 blem which we must now endeavour to solve. 



In after times, it will be sufficient to throw a double glance, 

 one on the nebulas of the period, and another on the drawings, 

 so admirable for their delicacy and fidelity, which astronomers 

 of the present day have given of them, to enable the question 

 to be decided, whether time sensibly alters the dimensions and 

 forms of these mysterious groups ; but antiquity having left 

 no term of comparison in this respect, we are reduced to the 

 necessity of encountering the problem by indirect means. 

 However, I have every reason to hope that the solution of it 

 will not appear much the less evident. 



The phenomena, which the existence of diverse centres of 

 attraction, spread over the whole extent of a single and vast 

 nebula, ought to produce, would develope themselves in this 

 order : — 



Here and there, the disappearance of the phosphorescent 

 light ; the commencement of breaks in the continuity, or rents 

 in the primitive luminous curtain, the necessary result of the 

 motion of the matter towards the attractive centres ; 



The increase of the rents, that is to say, the transformation 

 of a single nebula into many distinct nebulas, but little distant 

 from each other, and sometimes connected by very delicate 

 fillets of nebulosity ; 



The roxmdlng of the exterior contour of the separate nebu- 

 lae ; an augmentation more or less rapid of their intensity from 

 the circumference to the centre ; 



The formation at this centre of a nucleus, very apparent 

 either by its dimensions or its splendour ; 



