POftTION OF THE COSMOS. NEBULAE. 227 



is sufficient to show that by the increasing perfection of 

 telescopic vision the number of unresolved nebulae may, 

 indeed, be considerably diminished, but that it is very im- 

 probable that the diminution should ever proceed to actual 

 exhaustion. . By the successive employment of telescopes of 

 increasing power, each in its turn may be expected to resolve 

 nebulae which its predecessor had left unresolved ; but it 

 will at the same time,( 385 ) by its increased space- penetrating 

 power, replace, at least in part, the resolved nebulas by new 

 ones previously inaccessible to our view. Thus, by in- 

 creasing optical power, resolution of old, and discovery of 

 new, would follow each other in an endless succession. 

 Should this not be so, we must, it appears to me, either 

 imagine occupied space to have a limit, or else suppose that 

 the world-islands, to one of which we belong, are so distant 

 from each other that no telescope which may hereafter be 

 invented can ever suffice to reach the opposite shore, and that 

 our last (extremest, or outermost) nebulae will be resolved 

 into clusters of stars, projected, like the stars in the Milky 

 Way, upon a black ground wholly without nebulosity ( 386 ). 

 But it may be fairly asked, whether we can with probability 

 assume both such a state of the Universe, and such a degree 

 of improvement in optical instruments, that in the whole 

 firmament there shall not remain one unresolved nebula ? 



The hypothetical assumption of a self-luminous fluid pre- 

 senting itself in well defined nebulae, round or oval, must 

 not be confounded with the similarly hypothetical assump- 

 tion of a non-luminous ether pervading universal space, and 

 producing by its undulations light, radiant heat, and electro- 

 magnetism ( 387 ) . The emanations from the nuclei of comets, 

 which as comet-tails often occupy enormous portions of 



