52 COSMOS. 



boring heavens, are assigned as the causes of the remarkable 

 blackness of this portion of the firmament. This opinion, 

 which has been generally maintained since Lacaille's time,* 

 has been especially confirmed by the " gauges" and " sweeps" 

 made round the region where the Milky Way appears as if 

 covered by a black cloud. The Coal-bag yielded from seven 

 to nine telescopic stars for every sweep, but never an entirely 

 blank field ; "while in a field of equal size the margins pre- 

 sented from 120 to 200 stars. This mode of explanation, 

 which ascribes the darkness to contrast alone, did not, al- 

 though perhaps incorrectly, appear quite satisfactory to me 

 while I was in a tropical region, and remained under the 

 vivid impression produced on my mind by the aspect of the 

 southern heavens. William Herschel's considerations on 

 w^holly starless regions in Scorpio and Serpentarius, and 

 which he has termed "openings in the heavens," led me to 

 the idea that the starry strata lying behind one another in 

 such regions may be less dense, or even Mdiolly 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,! and the effects of perspective on such interruptions 

 in the starry strata have again been lately made the subject 

 of earnest consideration. $ 



The extreme and most remote strata of self-luminous cos- 

 mical bodies — the distances of nebulae — 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. 



* " Cette apparence d'uii noir fonc§ daus la partie Orientale de la 

 Croix du Sud, qui frappe la vue de tous ceux qui regardent le cie 

 austral, est causee pai* la vivacite de la blancheur de la voie lactee qui 

 reuferme I'espace noir et I'entoure de tous cotes." " The appearance 

 of deep black in the eastern portion of the Southern Cross, which 

 strikes all who observe the heavens in those regions, is owing to the 

 intensity of the whiteness of the Milky Way surrounding the black 

 space on every side." — Lacaille, in the M6m de V Acad, des Sciences. 

 annee 1755 (Paris, 1761), p. 199. 



t Cosmos, vol. i., p. 152, and note. 



\ " When we see," says Sir John Herschel, 'Mn the Coal-sack (near 

 a Crucis) a sharply-defined oval space free from stars, it would seem 

 much less probable that a conical or Uib^darhoWow traverses the whole 

 of a starry stratum, continuously extended from the eye outward, than 

 that a distant mass of com{)aratively moderate thickness should be sim 

 ply perforated from side to &\^e.^''~ -Outlines, $ 792, p. 532. 



