24 COSMOS. 



trapezium is a mass of stars, the rest of the nebulae als' 

 abounding with stars, and exhibiting the characteristics of re- 

 solvabiHty strongly marked." At a subsequent period (1848) 

 Lord Rosse had not announced that his expectations had as 

 yet been fulfilled, although he cherished the hope of being 

 able to resolve the remaining portion of the nebula into stars. 



When we separate the results of actual observation from 

 those of mere inductive conclusions in this much-disputed 

 question of the existence or non-existence of a self-luminous, 

 vaporous matter in the universe, we find that although the 

 increasing improvements in telescopic vision may indeed con- 

 siderably diminish the number of nebulsB, they caniiot by any 

 means wholly exhaust them. By the application of increas- 

 ing powers, each new instrument may resolve what the pre- 

 ceding ones had left unresolved, but it must, at the same time, 

 in consequence of its greater powers of penetrating space, re- 

 place (at least partially) the resolved nebulse by others not 

 previously reached. =^ A resolution of the older, and the dis 

 covery of new nebulse, would therefore follow one another in 

 endless succession, as the fruit of increased optical power. 

 For if we suppose a different result, we must either, accord- 

 ing to my view, assume the occupied regions of space to be 

 limited, or that the world-islands, to one of which our system 

 belongs, are so remote from each other that no telescopic in- 

 strument can ever be invented of sufficient power to penetrate 

 to the confines of any other of these worlds, and that our last 

 or extremest nebulae may resolve themselves into clusters of 

 stars, which, like the stars in the Milky Way, " are projected 

 on a black ground entirely free from vapor."t But can we 

 believe in the probability of a condition of the universe, and 

 of a degree of perfection in optical instruments, in which the 

 entire firmament will no longer exhibit any unresolved neb- 

 ulous spots ? 



The hypothetical assumption of a self-luminous fluid, ap- 

 pearing, when sharply defined, in round or oval nebulous spots, 

 must not be confounded with the equally hypothetical as- 

 sumption of a non-luminous ether pervading the universe, and 

 generating by its unduJatory motion the phenomena of light, 

 radiant heat, and electro-magnetism.$ The emanations from 

 cometary nuclei, which, in the form of tails, frequently extend 

 over enormous tracts of space, disperse the substance of which 

 they are composed — and with which we are unacquainted — 



* Compare Edinburgh Review, vol. Ixxivii., 1848, p. 1 86. 



t Cosmos, vol. iii., p. 144, and note. X Ibid., p. 34. 



