Nature of NiiliiUe. 309 



nous clouds, and of which Herschel effected the decomposition 

 into stars by means of telescopes of 10, 20, and 40 feet, led 

 this great astronomer to a rash generalization. He maintained 

 for many years, that all nebulsc are masses of stars ; that there 

 is no other essential difference between nebulae, the most dis- 

 similar in appearance, but a greater or less distance, or a 

 greater or less condensation, in the stars composing them. He 

 thus placed himself in direct opposition to Lacaille, who, on 

 his return from the Cape of Good Hope, wrote thus, in the Me- 

 moirs of the Academy of Sciences for 1755 : — " It is not cer- 

 tain that the whiteness of these parts (the clouds of Magellan 

 and the whitenesses of the Milky Way) is caused, as is commonly 

 supposed, by masses of small stars more thickly crowded to- 

 gether than in other parts of the heavens ; for with whatso- 

 ever attention I examined the best defined extremities, whether 

 of the Milky Way, or the Magellan clouds, I perceived nothing 

 with a glass of 14 feet, but a whiteness in the depth of the 

 sky, without observing more stars than in other places where 

 the sky was obscure." Minute and very delicate observations, 

 made in entire good faith, at last induced Herschel to modify 

 his first opinions. In a memoir of 1791, we find the fol- 

 lowing words : — " There are nebulosities (whitenesses) which 

 are not of a starry nature." Once Laving come to the opinion 

 that there exist in the celestial spaces numerous masses of 

 diffused and luminous matter, Herschel saw a field of research 

 open before him, almost entirely new, and which he explored, 

 in all its parts, with indefatigable zeal. The amount of ne- 

 bulae then surpassed the restricted limits which had been usu- 

 ally assigned to them; his object was no longer merely to 

 remove uncertainties and the mistakes of astronomical ob- 

 servers ; to prevent the wandering comet, even from the time 

 of its first appearance, from being ever confounded with an im- 

 moveable nebula, notwithstanding the apparent resemblance in 

 their physical constitution and their great similarity of form.. 

 It came to be well understood, from that period, tliat stars, 

 planets, satellites, and comets, were not the only objects to 

 which the investigations of astronomers ought to be directed. 

 The non-condensed celestial matter, — the celestial matter 

 nearest, if the expression may be allowed, to the elementary 



