334 M. Arago on Nebuke. 



extensive in one direction than in another. The numbers 

 which 1 have given are those which the scrupulous observer 

 has himself made use of to give a section, and even a figure, 

 under three dimensions, of the vast nebula, in which our Sun 

 iigures as an insignificant star, and the Earth as an impercep- 

 tible grain of dust. 



Will the Milky JVay endure for ever hi the form in which we 

 now see it? Does it not begin to shew symptoms of dislocation 

 ami dissolution ? — Herschel has clearly established, by thou- 

 sands upon thousands of observations, that the whiteness of 

 the Milky Way proceeds, in the greater joart, from agglomera- 

 tions of stars, too small and too feeble to' be distinguished 

 separately. The diffused matter, mingled in certain propor- 

 tions with the stars, here plays a part as in many resolvable 

 nebulae ; but it is evidently a secondary part. 



Almost in every instance in which stars placed near each 

 other are presented to our view without the apparent limits 

 of the Milky Way, we have perceived that they tend to group 

 themselves around many centres ; that they seem to obey, like 

 the various bodies of our solar system, an attractive force ; that 

 this force, in fine, has already produced, in certain rounded 

 groups, very considerable effects and concentrations. Why 

 should the stars of this great nebula, of which we form a part, 

 escape this kind of action more than the others ? If formerly 

 they were uniformly distributed, this state must cease, and ap- 

 proach its termination, more and more every day. Facts con- 

 firm the results of reasoning. The stars, far from appearing 

 uniformly distributed over the whole extent of the Milky Way, 

 have presented to Herschel, armed with his telescopes, 157 

 distinct and circumscribed groups, which have taken their 

 place in the catalogue of nebulae, without reckoning eighteen 

 analogous groups situated on the edge of this same zone. 



Any one who examines with his eye, during a dark and very 

 clear night, the portion of the Milky Way comprised between 

 Sagittarius and Perseus, may remark in it eighteen regions 

 perfectly characterized by the particular brilliancy of their 

 light. 



I shall here mention a few of these : 



