330 M. Arago on Nebulce. 



something gradual. Thus he adds : *' The regions not com- 

 prised in the whitish track of the Milky Way, are the richer in 

 stars the nearer they approach the centre of that track ; the 

 greater part of the 2000 stars discernible in the firmament by 

 the naked eye, is included in a zone not very broad, of which 

 the Milky Way occupies the centre." 



Kant condensed his ideas in the fewest words possible, 

 when he called the Milky Way " t/ie 9vorld of worlds'' 



We likewise find an explanation of the Milky Way, injthe 

 Cosmological Letters published at Leipsic in 1761. From the 

 contemplation of the heavens, Lambert came to the following 

 conclusions : The system of the stars is not spherical : the stars, 

 on the contrary, are arranged nearly in a uniform manner be- 

 tween two planes extending in every direction, and compara- 

 tively near each other ; our sun occupies a region but little 

 remote from the immense stratum of stars. This is almost 

 exactly the whole of the hypotheses adopted by Kant in his 

 History of the Heavens. How has it happened that six years 

 after the publication of this work, Lambert has made no men- 

 tion of the views developed in it % And how is it that, 29 

 years later, Herschel. when addressing himself to the same 

 problems, never allowed the name of the philosopher of Koenigs- 

 berg, or of the geometrician of Mulhouse, to drop from his 

 pen ? These are two questions which I cannot answer. 



HerschbVs labours on the Milky Way. — I hasten to take up 

 the minute analysis which Herschel substituted for the imper- 

 fect sketches of his predecessors. 



We have perceived that the brilliant zone, the physical 

 cause of which the great observer wished to discover, may 

 have nothing real in it. It has been shewn that it is very 

 possible that it may be only a deceptive appearance, a simple 

 effect of projection. It was not enough, therefore, to enume- 

 i-ate the stars in the regions alone where they appear most 

 condensed ; it was necessary to enquire if, in gradually re- 

 tiring from these regions, their number diminished with re- 

 gularity or without rule. Such a labour seemed to demand 

 the united efforts of many generations of astronomers. Her- 

 schel, however, executed it alone, and in a few years, at least 

 as far as the question of the Milky Way required. The 



