322 M. Arago on Nehulm. 



Nothing being first established in principle, I hasten to re- 

 mark, that the condensation of the diffused matter does not 

 increase the luminous properties of each of the molecules. 

 But I set entirely aside this possibility of increase of splen- 

 dour, and reduce the question to very simple terms ; are the 

 feeble lights spread over all the points of such or such a 

 diffused nebula, equal m the sum, to the light of such or such 

 a star \ 



There are no practicable experimental means of convenient- 

 ly uniting in a single point, the light emanating from the 

 whole superficial extent of a great nebula. The inverse opera- 

 tion is, on the contrary, easy. If we gradually withdraw the 

 glass of a telescope from the place which it occupies when 

 the vision is distinct, we see the image of each star successive- 

 ly enlarge and lose its intensity. In displaying one of these 

 images in this manner, till we make it fill nearly the whole 

 field of vision, we make it at last not more brilliant than the 

 milky nebula. This once obtained, calculations into which 

 various elements enter, as well as various corrections of which 

 I cannot give a complete enumeration without exceeding 

 the limits imposed on me, lead to the results sought for : I 

 may say to the numerical approximations which exist between 

 the intensities of the total lights dispersed over a great extent 

 of milky nebula, and the concentrated light of stars. The 

 result of these experiments and calculations strengthens the 

 ideas of Tycho, Kepler, and Herschel, on the transformation 

 of nebulae into stars. 



Changes observed in certain Nebulce. — By comparing the 

 observations of the years 1780 and 1783, with those of 1811, 

 Herschel found that the nebula of Orion had sensibly changed 

 both in form and extent. This was, according to the expres- 

 sion of Fontinelle, to have caught Nature in the fact. 



Boulliaud, Kirch, and Le Gentil, believed, as early as 1667, 

 1678, and 1759, that the nebula of Andromeda underwent 

 great variations. Mairan says the same thing of the nebula 

 of Orion, and supports his statement by the authority of Go- 

 din and Fouchy ; astronomers, nevertheless, continued in un- 

 certainty. They remarked, not without reason, that, in or- 



