THE COAL- SACKS. 319 



heavens," led me to the idea that the sta'rry strata lying 

 behind one another in such regions may be less dense, or 

 even wholly interrupted, and that our instruments being 

 insufficient to penetrate to these last strata, " we look into the 

 remote regions of space, as through tubes." I have already 

 elsewhere noticed these openings, 19 and the effects of perspec- 

 tive on such interruptions in the starry strata have again been 

 lately made the subject of earnest consideration. 1 * 



The extreme and most remote strata of self-luminous 

 cosmical bodies, the distances of nebula?, all that has 

 been considered in the last seven sidereal or astrognostic 

 portions of this work, fill the imagination and the speculative 

 mind of man with images of time and space surpassing his 

 powers of comprehension. 



However wonderful are the improvements made in optical 

 instruments within scarcely sixty years, we are at the same 

 time too well acquainted with the difficulties of their con- 

 struction to indulge in the bold and even unlicensed antici- 

 pations so ardently cherished by the intellectual Hooke from 

 1663 to 1665. l01 Moderation in the expectations entertained 

 will be the most likely to lead to their fulfilment. Each 

 succeeding generation has reaped the noblest and most 

 exalted results from the triumphs of free intellect in the 



* Cosmos, vol. i. p. 1 43 and note. 



10 "When we see," says Sir John Herschel, " in the 

 Coal-sack (near a Crucis) a sharply defined oval space free 

 from stars, it would seem much less probable that a conical or 

 tubular hollow traverses the whole of a starry stratum, con- 

 tinuously extended from the eye outwards, than that a distant 

 mass of comparatively moderate thickness should be simply 

 perforated from side to side." Outlines, 792, p. 532. 



101 Lettre de Mr. Hooke a M. Auzout, in the Mem. de 

 V Academic, 1666-1699, toin. vii. partie 2, pp. 30, 73. 



