17 Argils and surrounding Nebula. 15 



background almost completely black, and the contour of the 

 nearest bright nebula is pretty sharply marked. 



In the Cape drawing the curious lemniscate vacuity is a 

 conspicuous feature, and has its borders almost equally well 

 marked throughout ; in the Melbourne sketch the lemnis- 

 cate is still conspicuous, but the south end is not clearly 

 indicated, for the surrounding nebula in that direction is 

 extremely faint. 



Mr. Abbott of Hobartown in the year 1864, and Mr. 

 Powell of Madras, about the same time called attention to 

 the fact that the star rj was completely outside the bright 

 nebula, and that the south end of lemniscate had disappeared. 



The instruments used by these observers were, however, 

 so inferior in power to Sir. J. Hersch ell's 18 inch, that it 

 was not unreasonable to suppose the change merely apparent, 

 due regard being had to the fact that the difference in the 

 representation was precisely of a nature to be accounted for 

 by the difference of instrumental means employed. 



With the Melbourne telescope the conditions are reversed, 

 yet Sir J. Herschell saw bright nebula where with a much 

 more powerful instrument, either none at all is seen, or a very, 

 faint one barely suspected. 



The presence of a bright star has, however, a large dis- 

 turbing effect (increased by aperture) in apparently obliterat- 

 ing even bright nebula in its immediate neighbourhood ; 

 and although the effect could not possibly spread over such 

 a large space as that over which change appears to be indi- 

 cated, it was not unimportant to get some further proof of 

 the nature of the background on which -q is situated. The 

 spectroscope here comes to our aid ; the excessively faint 

 nebulosity over the region in question is incompetent to 

 show even a trace of bright lines, and when by shifting the 

 spectroscope the bright lines do appear, it is in positions 

 which indicate that the eye- view configuration represents the 

 actual facts. 



These proved facts, therefore, being that the star rj is on 

 a background almost completely dark ; that the south end 

 of the lemniscate opens out into a space almost as vacuous 

 as itself ; it is difficult to imagine any conditions instrumental 

 or atmospheric which could produce an appearance at all 

 approachiag to that seen by Sir J. Herschell at the Cape. 



We have, therefore, evidence entitled to much weight 

 that enormous changes have taken place in the nebuLi since 

 the year 1838. 



