332 M. Arago on Nehuh^. 



The general aspect of the Milky Way, its form, and starry 

 composition, deduced from telescopic observations, are ex- 

 plained very simply, by supposing, with Herschel, that mil- 

 lions of stars, nearly at equal distances from each other, form 

 a layer or stratum, comprised between two even surfaces, 

 parallel to, and near each other, but prolonged to immense 

 distances ; that the stratum is thus very thin, compared with 

 the immense distances to which the two even surfaces which 

 contain it extend in every direction ; that our sun, — that the 

 star around which the earth revolves, and from which it does 

 not far recede, — is one of the stars composing this stratum ; 

 that we occupy very nearly the centre of it, both relatively 

 to its thickness and to all its other dimensions. These sup- 

 positions once admitted, it will be easily understood, that a 

 visual ray, turned in the direction of the immense dimensions 

 of the stratum, will there encounter everywhere a multitude 

 of stars, or, at least, that it will pass so near them that they 

 will seem to touch each other ; that, in the direction of its 

 thickness, on the contrary, the number of visible stars will be 

 comparatively smaller, and precisely in the relation of half 

 the thickness to the other dimensions of the stratum ; that, in 

 the passage of the visual lines coincident with the extended 

 dimensions, to the transverse directions, there will be, in this 

 respect, a sudden change ; that the greatest dimensions of 

 the stratum will thus be found indicated, or, as it were, deli- 

 neated on the firmament by an apparent condensation of stars, 

 by a maximum of manifest light, and a milky aspect ; finally, 

 that the maximum of light will appear to be a great circle of 

 the celestial sphere, since the earth may be considered as the 

 centre of this sphere, — since the stratum is one of its diame- 

 tral planes, — and that every diametral plane of a sphere, every 

 plane passing by its centre, necessarily divides it into two 

 equal parts, or, what is the same thing, cuts it according to 

 one of its great circles. The secondary arc, detached from 

 the principal arc of the Milky Way, towards Cepheus and Cas- 

 siopea, and rejoining it between Scorpio and Sagittarius, dis- 

 closes the existence of a stratum of stars forming a small 

 angle with the principal stratum, and again meeting it near 

 the region which the earth occupies, and not extending beyond. 



