THE PHOTOGRAPHIC CHART OF THE HEAVENS. 69 



Star-Camera is vastly more powerful than Herschel's telescope, 

 how much may be judged from the fact, that in one small 

 space where he could see only one star, the Camera shows ten, and 

 in another place examined by Herschel with equal care, and said 

 to contain four stars, the Camera shews twenty. There can 

 then be no doubt, that in this case a bright nebulous mass, has 

 entirely disappeared in thirty-four years, and it is significant that 

 the part where this nebula was, is now replaced by a dark round 

 spot. 1 have photographed the object many times with both 

 Cameras, and the dark spot is always there, can it be that in the 

 thirty-four 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 all that the photo- 

 graph reveals, especially in 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 

 called the Lemniscate, this photograph shews a most complex 

 structure, with a great variety of light and shade ; and just as in 

 the case of the Theta Orionis, the nebula with its vast folds is 

 shewn to extend farther from the centre, with each increase in 

 the time of exposure, so I find with that about Eta Argus. The 

 southern nebula is however very much more difficult to photo- 

 graph, and I think it must have some tinge of colour in it, prob- 

 ably yellow, for a photograph of Orion with one hour's exposure is 

 more dense than one of Eta Argus with six hours exposure. 



Taken as a whole, the nebula about Eta Argus covers a much 

 larger space than that about Orion, even in these photographs 

 in which the southern nebula, although longer exposed, is com- 

 paratively under exposed : while that of Orion is much over 

 exposed. 



I have also brought two photographs of the Moon to show you, 

 they were taken on 1 9th and 28th of May last. As you are all aware, 



