7 o THE POPULAR SCIENCE MONTHLY. 



to the arrested motion of the particles coming together, and the 

 accompanying light are produced, we mnst expect that that light 

 will at first be very dim, and will require very considerable optical 

 power to render it visible. 



We may now consider some early results obtained in connec- 

 tion with this matter. Sir William Herschel, although not the 

 first to examine into it, was the first to bring before us an idea of 

 the magnificent spectacle which the heavens present to mankind, 

 and he, without any difficulty, with his large instruments, began 

 by dividing these dim bodies into nebulosities and nebulae ; the 

 nebulosities extending over large spaces of the heavens, and being 

 of very, very feeble luminosity. 



When we pass from these we become acquainted with bodies 

 which may be truly termed nebulae, as opposed to nebulosities, 

 and the most magnificent of these is that in Orion, which has 

 recently been so grandly photographed by Mr. Common and Mr. 

 Roberts^ the latter using the intensifying action of four hours' 

 exposure of the photographic plate, hereby revealing details that 

 no human eye will ever see, thus demonstrating how true it is 

 that these changes may go on for aeons and eeons, though the eye 

 may never become acquainted with them. 



Thare is a magnificent arrangement in the human eye which, 

 though it invalidates it for some astronomical purposes, is con- 

 venient, because it enables us to go on using our eyes all our 

 lives, whereas a prepared photographic plate can only be used 

 once. By this arrangement, however long we look at an object, 

 it does not appear brighter, but in the case of the photographic 

 plate all the action upon it is totaled, so to speak, so that if the 

 plate be exposed, say for two hours or sixty hours, we shall go on 

 getting impressed upon it more and more of the unseen. Thus 

 the nebula of Orion, as seen, is almost insignificant compared 

 with the glorious object which the photographic plate portrays if 

 the integrating power be allowed to go on for hours. 



It seemed pretty obvious, since the light of such bodies is so 

 dim that a large portion of it beats upon the earth and upon our 

 eyes without having any effect upon either, that the temperature 

 was low ; and it seemed also that to test the idea that this lumi- 

 nosity might be produced, as I have suggested, by collisions of 

 meteoric dust, the way was open for laboratory work. 



gives of the Creation, alleging that Light could not be created without the Sun. But in the 

 following Instances the contrary is manifest ; for some of these bright Spots discover no 

 sign of a Star in the middle of them ; and the irregular form of those that have, shews 

 them not to proceed from the Illumination of a Central Body, since they have no Annual 

 Parallax, they cannot fail to occupy Spaces immensely great, and perhaps not less than our 

 whole Solar System. In all these so vast Spaces it should seem that there is a perpetual 

 uninterrupted Day, which may furnish Matter of Speculation, as well to the curious Natural- 

 ist as to the Astronomer." Edmund Hallet, Philosophical Transactions, vol. xxix, p. 392. 



