20 On the Nebula around Eta Argus. 



1854 on Eta enabled Professor Loomis to correct the period of 

 its variation, noted change in some of the stars in 1868, and, in a 

 recent communication to the E. A. S., notes particularly the 

 changes I have remarked in the largest stars, neither of us being 

 at the time aware of the other's results. 



In 1834-8 No. 71 was 6 magnitude, No. 72, 7 magnitude ; now 

 No. 72 is fully half a magnitude larger than No. 71. So that at 

 least one must have changed ; and it is remarkable also that the 

 nebula has almost faded from these two stars, as from Eta. No. 

 105 was 6 magnitude, and No. 101, 7 magnitude ; now both are 7 

 magnitude, and equal to No. 71. =Tet all these stars are much 

 brighter than the other 7 magnitude stars of the Cape list, viz., 

 Nos. 1, 2, 291, 300 : magnitude of 51 R = 408, 59 E = 844, and 

 1215. I would not, however, lay too much stress upon this until 

 it is known whether all the 7th magnitudes in the Cape list were 

 equal, for, if not, it may be that only Nos. 71 and 105 have 

 changed. 



As to the colours of the stars near Eta, thej^ are, in my estima- 

 tion, pale indeed compared with K Crucis ; and if in 1865 bright 

 enough to merit Mr. Abbott's remark, " that although Sir John 

 Herschel has not overdrawn the beauty of K Crux, the object 

 Eta Argus is much more superb," they must have faded wonder- 

 fully since, for I only remarked colour in two beside Eta, and 

 both were red. Dr. AVright, who has examined both objects with 

 an 8i-inch Browning reflector, failed to detect anything striking 

 in those near Eta. I have measured several of the double stars, 

 and have not yet found any evidence of angular motion . During 

 my first evening's observation I carefully measured the diflferences 

 in E.A. and declination of 54 stars fi*om Eta, with a fine parallel 

 wire micrometer ; a sheet of paper four times the size of the Cape 

 drawing was then taken, and lines drawn on it to the same angular 

 scale, but twice the distance apart; upon this, the 54 measured 

 stars were carefully laid down, and the drawing of the nebula 

 then proceeded with. The sheet was kept on a desk near the 

 telescope, and as each outline was traced with the telescope 

 amongst the measured stars, it was laid down on the sheet. 

 When, in order to define the positions of particular parts, other 

 stars were required, these were measured, and the drawing pro- 

 ceeded with. The bright wires on a dark field of the micrometer 

 were also found very useful in guiding the eye, and in three 

 places used to measure the distance of the definite parts from 

 Eta. 



The drawing was then reduced to its present size with propor- 

 tional compasses, and afterwards compared with the object under 

 diiferent states of the atmosphere, with moon and without moon, 



