Another indication bearing on the same ques tion is afforded by the Milky Way. This vast girdle, which consists of agglomerations of stars, has a unity of structure throughout its whole extent which justifies us in considering it as, in a certain sense, a single object, and it is not improbable that this represents the densest portion of our own limited universe of suns.
Another conception of the universe is gained by considering the thickness with which the stars are scattered through space; in other words, how many stars given volume of the celestial spaces, in the general average, may contain. Measures of parallax, and studies of proper motion, lead to the conclusion that the thickness of the stars, as thus defined, is a fairly definite quantity. To measure the volume of space we require a unit. The most convenient unit for our purpose is the volume of a sphere whose centre is in the solar system, and whose surface is at a distance represented by a parallax of half a second. This distance is, in round numbers, 400,000 times that of the sun from the earth, a space through which light would travel in about seven years. The volumes of spheres being as the cubes of their radii, it fol lows that a sphere whose surface is at twice the distance of the unit sphere, or 400,000 times the distance of the sun, will have a volume repre sented by the number 8, while 100 times the radius will have a volume of 1,000,000 units. Now, the indications are that the stars are strewn through space with such thickness that, in the general average, each unit of space con tains one or two of these bodies. The law ac cording to which the stars thin out, and in vestigations into the statistics of the stars gen erally, lead to the conclusion that the parallax of the most distant of these bodies is about 0"001. This distance is 500 times that which we have chosen as the radius of our unit sphere; the volume of space included within it is, by the law of cubes, 125,000,000 and, were the stars scattered with equal thickness throughout the whole space, the number contained would be between 125,000,000 and 250,000,000.
Although, in the general average, it is prob able that the thickness of the stars, in space does not vary greatly within the limits we have indicated, there are exceptions in special cases.
The most notable exception is that of the Milky Way, where the stars are undoubtedly much more thickly strewed in space than they are in the central regions of the system. We also find in many regions of space collections of hundreds or even thousands of stars evidently in close proximity to each other. But outside of these collections the scattering is probably nearly uniform as far out as the limit we have mentioned.
Altogether, we may say with some confi dence, that if we could fly through space to a distance over which light would require 10,000 years to travel, we should find ourselves ap proaching the boundary of the stellar system, if we were not actually outside of it. But all that has been said refers only to our own visible universe, that is, to the Milky Way cluster, or cloud, of stars of which our own sun is one. It is as impossible for us to conceive of a totally empty space extending without limit in all directions from our universe as it is for our minds to grasp the conception of other, far distant, universes which perhaps more or less resemble our own, and which succeed one another throughout a space which is also abso lutely endless. If such other universes exist, it can only be said that as yet we have no known means of discovering them.
Notwithstanding the darkness of the sky, it seems probable that we receive more light from it than could be supplied by all the stars, seed and unseen, which make up the known part of the universe. This conclusion is based on the fact that the amount of light received from a circle one degree in diameter on the darkest night-sky is about equal to that of a star of the 5th magnitude, while, when we carry out the progression in the light of the stark of dimin ishing orders of magnitude, we find that the total of their light could not well amount to so great a quantity as this. The source of this excess of light is yet to be investigated. Whether it is atmospheric, whether it is re flected from innumerable opaque bodies, or whether it is emitted by a nebulous mass of almost inconceivable tenuity, has not yet been determined.