to the arrested motion of the particles coming together, and the accompanying light are produced, we must 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 connection 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 nebulæ; 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 nebulæ, 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. Robertsthe 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 æons and æons, though the eye may never become acquainted with them.
There is a magnificent arrangement in the human eye which, though it invalidates it for some astronomical purposes, is convenient, 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 luminosity 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 Naturalist as to the Astronomer."—Edmund Halley, Philosophical Transactions, vol. xxix, p. 392.