22 On the I^ebula around JEta Argus. 



to the urgent necessity for carefully recorded observations of all 

 such objects, and if this square degree is to be taken as a sample, 

 promising more discoveries than have ever before been made in so 

 small a space. At the spot + 40 seconds + 600 seconds in the Cape 

 drawing is a mass of nebula forming one of its marked features, 

 and particularly described by Sir John Herschel as so situated 

 with regard to certain stars, that the least change would be 

 apparent. Of this not a vestige is now to be seen ; yet it was 

 about as dense as that near Eta. Again, about star No. 100 

 (1103 H) there was a decided condensation of nebula, now it is 

 not to be seen ; and of the well-marked streams from stars Nos. 

 71 and 72 now nothing can be seen but a faint undefined haze, 

 much fainter than that now about Eta. The nebulous branch, 

 also, which terminated 150 sec. before Eta in the same parallel, 

 now extends to over 200 sec. Some changes have, I think, 

 taken place also at -^ 40 sec. + 1200 seconds ; but all is there so 

 faint that I am doubtful about it. The oval shewn in the north 

 extreme of the Cape drawing is still the same, limited by the 

 stars as it was then. In it I saw three minute stars ; Sir John 

 Herschel records four seen with his reflector. 



Taken as a whole, this object must have increased in brightness 

 very much, for it can now be seen in full moonlight ; and in 

 1834-8 it was at all times invisible to the naked eye. This fact, 

 and the great similarity in outline between the Sydney and the 

 Cape drawing, have inclined me to think that, if the same 

 reflector could again be turned to this object, the lemniscate 

 would be found very little altered, and the apparent difterence 

 quite as much, perhaps more, in the increase of light at two 

 points before indicated ; as in the loss of light in other parts. 



Still a great change has taken place, and since Eta has only a 

 very small proper motion, which, with one exception, appears to 

 be common to the stars and nebula near it, and that the latter 

 presents no signs of resolvability, even in the large Melbourne 

 reflector ; but, as far as the spectroscope has yet been applied, is 

 gas, like the great nebula in Orion ; I am inclined to think that 

 the mass surrounding Eta, and the lemniscate, is in reality two 

 or more nebulas in visual superposition ; and that the force, 

 whatever it may be, which renders them luminous, is decreasing 

 in that in which Eta is seen, and increasing in that part north of 

 Eta whose peculiarity in form was before remarked ; for it seems 

 more reasonable than to suppose that large nebula have had the 

 necessary angular motion to bring them visually over other parts. 

 A motion, be it remembered, so large that even astronomy pre- 

 sents us with no parallel ; while, on the other hand, it is known 

 that several nebula have faded in a few years so much that they 

 could not be seen without the aid of very large telescopes. Of 



