256 SPECIAL RESULTS IN THE URANOLOGICAL 



as such every darker portion of the heavens, even though it 

 may present no definite boundary ; (as between a Centauri 

 and ft and y Trianguli ( 452 ), between 77 and Argus; and 

 especially, in the northern celestial hemisphere, the vacant 

 space in the Milky Way between e, a, and y Cygni) ( 453 ) . 



The phenomenon of this class which has been longest 

 known, and which is most striking to the unassisted eye, 

 viz. the dark patch in the Southern Cross, is situated on 

 the eastern side of that constellation, and is pear-shaped, 

 with a length of 8 and a breadth of 5 degrees. There is 

 in this large space one star visible to the naked eye, (between 

 the 6th and the 7th magnitude), and a large number of 

 telescopic stars from the llth to the 13th magnitudes. A 

 small group of 40 stars occupies nearly the centre of the 

 space ( 454 ). Paucity of stars and contrast with the sur- 

 rounding brightness have been assigned as the causes of the 

 sensible blackness of the space in question ; and since the 

 time of Lacaille ( 455 ) this explanation has been generally 

 received. It has been more particularly supported by the 

 results of " star-gauges and sweeps" taken around the space 

 where the Milky Way appears as if covered by a black 

 cloud. With equal fields of view the sweeps gave withii> 

 the coal-sack from 7 to 9 telescopic stars, (never perfect 

 vacuity or blank fields), while around and beyond the 

 borders from 120 to 200 stars were counted. Whilst I 

 remained under the southern tropic, and under the influence 

 of the powerful impression made upon me by the aspect of 

 the celestial canopy towards which my attention was con- 

 tinually drawn, the above explanation, from the effect of 

 contrast, appeared to me, probably erroneously, to be an 



