21 6 JOURNAL OF SCIENCE. 



southern object, "the nebula about Eta Argus," it will be remem- 

 bered that last session I exhibited a photograph of it with three 

 hours' exposure, stating that it had not been exposed long enough. 

 On April 9, 1891, I obtained a clear night and an exposure of eight 

 hours ; again, with the short camera, which brings out a host of stars 

 and show T s the milky way with a brilliance it has never been seen to 

 have before, at the same time the nebula is more distinctly shown and 

 larger ; and in reference to this object — also after a series of trials — 

 I have succeeded in getting several fine photos with the star camera, 

 which make it 1 8 times larger than the one T used last year. I have 

 been unable to get a continuous exposure of eight hours with this 

 camera; still, in plate 77, taken March 18, 1891, with 5 hours 43 

 minutes exposure on a fine clear night, and in others taken about the 

 same time, we have a marvellous revelation of the striking details of 

 light and shade in this object, which have never been seen before in 

 any photograph or by any telescope. I fear to attempt a description 

 of what can be seen only in the photograph ; the general form is the 

 same as in drawings and in the photos exhibited last session, but 

 there are certain main features which may be indicated. In the first 

 place, there is evidence here that the nebula is much more extended, 

 and the indications of spiral structure are more decided, and are seen 

 to extend even to the details of the familiar branches. The nebula 

 covers a much larger area than that of Orion, and in passing I may 

 mention that it proves conclusively that a conspicuous part of the 

 nebula, which Herschel drew and described in 1838, has entirely 

 disappeared, as I pointed out in 1872 ; but as I then used a telescope 

 inferior in power to Herschel' s, its invisibility to me was not proof 

 that it was gone. Now the star camera is vastly more powerful than 

 Herschel's telescope, therefore how much may be judged from the 

 fact that in one small space, where he could see only one star, the 

 camera shows 10 ; and in another place examined by Herschel with 

 equal care, and said to contain four stars, the camera shows 20. 

 There can then, T think, be no doubt that in this case a bright nebu- 

 lous mass has entirely disappeared in 34 years, and it is significant 

 that the part of the nebula where it was is now replaced by a dark 

 round spot ; the decided folds of the nebula visible here in 1838 have 

 entirely disappeared. I have photographed the object many times 

 with both cameras, and the dark spot is always there. Can it be that 

 in the 34 years, 1838 to 1872, one of the supposed dark clouds of 

 space has drifted in between us and the nebula? It cannot be a solid 

 body because the stars are there ; but a slight misty body would hide 

 the nebula and not affect the stars very much. It would be tedious 

 to attempt to describe the details which the photograph shows, 

 especially to the central part of the nebula ; but I may say that while 

 the eye, aided by the best telescopes, sees the nebula of fairly uniform 

 brightness interrupted by certain well-known darker spaces, and 

 especially by that which Herschel calls the Lemniscate, much the 

 same, in fact, as the great nebula in Orion, and just as the photos of 

 that nebula reveal a sort of texture in the nebulous mass — as if it 

 were made up of a series of curved bands of nebulous light — so this 

 photo of the nebula about Eta Argus shows a most complex structure 

 of the same character, and with a greater variety of light and shade ; 



