312 M. Arago ow Nehulc&. 



buted around the centre of the figure with perfect regularity. 

 Accordingly, at equal distances from this centre, the lumino- 

 sity is absolutely equal in all directions. 



If we place at a very great distance a spherical nebula, in 

 which the stars are equally condensed in the centre, edges, 

 and throughout ; the eye will misrepresent this composition. 

 Let us bring the visual ray which traverses the sphere near 

 the margin. The space comprised between the point of en- 

 tering and issuing will be very short ; the ray will therefore 

 fall upon very few stars. In proportion as this visual ray ap- 

 proaches the centre, the part comprised in the sphere will 

 become longer, and the number of stars it encounters will go 

 on increasing. The maximum will be observed in the centre 

 itself. 



The gradual augmentation of intensity from the margin to 

 the centre presented by all nebuhne apparently circular, may 

 thus be considered as a manifest proof of the globular form, of 

 the spherical shape of the starry group. 



It is easy to push these considerations further. 



We have stated that the parts of the visual rays which are 

 comprised in a sphere, go on increasing in size from the mar- 

 gin to the centre. If the sphere is filled with stars equally 

 distant, the lengths of these parts of the visual rays will be 

 proportioned to the number of stars which the rays touch upon ; 

 they will give the measure of the luminous intensity of all the 

 regions of the nebula from the edge to the centre. Well, let 

 us bring nearly parallel lines across a sphere. Near the edge, 

 these lines will vary in length rapidly ; near the centre, on the 

 contrary, they will vary very little. The nebula ought, there- 

 fore, to vary in splendour very rapidly at the edges, and scarce- 

 ly at all in the centre. This is the reverse of what is wit- 

 nessed. There must be something inaccurate, therefore, in 

 the hypothesis with which we set out ; we must have been 

 wrong in supposing that stars exist in all the parts of the 



his powerful telescope, directed to 18h 49m 53 of right ascension and 

 53* 10' of polar distance, presented a nebula to his view of 3-^ diameter; 

 compo$ed entirehj of bluish stars. 



