MILKY WAY. It is desirable, in describing astronomical objects, to keep as close as possible to the words of those who are accustomed to the sight and description of such things. Two passages in Sir John Herschel's Astronomy ' will describe the Milky Way, and the theory of it, by Sir William Herschel, with excellent brevity and distinctness.
" There are not wanting natural districts in the heavens, which offer great peculiarities of character; and strike every observer : such is the Milky Way, that great luminous band which stretches, every evening, all .across the sky, from horizon to horizon, and which, when traced with diligence, and mapped down, is found to form a zone, completely encircling the whole sphere, almost in a great circle, which is neither an hour circle nor coincident with any other of our astronomical granzmata. It is divided in one part of its course, sending off a kind of branch, which unites again with the main body after remaining distinct for about 150°. . This remarkable belt has maintained, from the earliest ages, the same relative situation among the stars ; and when examined through powerful telescopes, is found (wonderful to relate !) to consist entirely of stars scattered by millions, like glittering dust on the black ground of the general heavens.
" If the comparison of the apparent magnitudes of the stars with their numbers leads to no general conclusion, it is otherwise when we view them in connection with their local distribution over the heavens. If indeed we confine ourselves to the three or four brightest classes, we shall find them distributed with tolerable impartiality over the sphere ; but if we take in the whole amount visible to the naked eye, we shall perceive a great and rapid increase of number as we approach the borders of the Milky Way. And when we come to telescopic magnitudes," or stars of so small a magnitude as to be invisible except through a telescope, " we find them crowded, beyond imagination, along the extent of that circle, and of the branch which it sends off from it ; so that in fact its whole light is composed of nothing but stars, whose average magnitude may be stated at about the tenth or eleventh.
"These phenomena agree with the supposition that the stars of our firmament, instead of being scattered in all directions indefinitely through space, form a stratum, of which the thickness is small, in com parison with its length and breadth ; and in which the earth occupies a place somewhere about the middle of its thickness, and near the point where it subdivides into two principal laminar, inclined at a small angle to each other. For it is certain that to an eye so situated, the apparent density of the stars, supposing them pretty equally scattered through the space they occupy, would be least in a direction of the visual ray (as S A) perpendicular to the lamina, and greatest in that of its breadth, as S TS, s C, s D ; increasing rapidly in passing from one to the other direction, just as we see a slight haze in the atmosphere thickening into a decided fog-bank near the horizon, by the rapid increase of the mere length of the visual ray. Accordingly, such is the view of the construction of the starry firmament taken by Sir William Herschel, whew powerful telescopes have effected a complete analysis of this wonderful same, and demonetrated the fact of its consisting entirely of stars. So crowded are they in some parts of it, that by counting the stars in a single field of his telescope, ho was let to conclude that 50,000 had passed under his review in a zone two degrees in breadth, during a ,single hour's observation. The immense distance at which the remoter regions must be situated, will sufficiently account for the east predominance of small magnitudes which are observed in it.." (licesciitt, Sin Wx., and HERSCHEL, Sin Joux, in Btoo. Div.; r.ave.e ; and STARS.] Bit to the above it must be added, that the fifty thousand stars thus mentioned as contained in a zone of 15° by 2' include only those which could be steadily seen and distinctly numbered; besides which, twice as many more were suspected, of which only occasional glimpses could be got for want of sufficient light.