PORTION OF THE COSMOS. NEBULAE. 257 



unsatisfactory one. Sir William Herschers considerations 

 on the quite starless spaces in Scorpio and Ophiucus, which 

 he terms " openings in the heavens/' led me to the idea that 

 perhaps in such regions the sidereal strata may be thinner 

 or may even be entirely interrupted ; that our optical in- 

 struments fail to reach the last strata, and that "we 

 look as through tubes into the remotest regions of 

 space." I have already alluded elsewhere to these " open- 

 ings" ( 456 ) ; and the effects of perspective on such inter- 

 ruptions in the sidereal strata have very recently formed the 

 subject of grave discussion. ( 457 ) 



The consideration of the outermost and remotest strata 

 of self-luminous worlds, the distances of nebulae, and all 

 the subjects which have been crowded into the last of 

 the seven sidereal or astrognostic sections of this work, 

 fill our imagination with images of time and space sur- 

 passing our powers of conception. Great and admirable as 

 have been the advances made in the improvement of optical 

 instruments within the last sixty years, we have at the same 

 time become sufficiently familiar with the difficulties of 

 their construction not to give ourselves up to such daring, 

 and, indeed, extravagant hopes, as those with which the in- 

 genious Hooke was seriously occupied between 1663 and 

 1665 ( 458 ). Here, also, we advance further and more se- 

 curely towards the goal by moderation in our anticipations. 

 Each of the successive generations of mankind is in its turn 

 enabled to rejoice in the greatest and highest results attain- 

 able by man's intellect freely exerted from the standing 

 place to which art may then have risen. Without enun- 

 ciating in determinate numbers the extent of space-pene- 



