all of the galaxies; but these effects will be local and independent of the distance separating us from the galaxy in question.
If the intergalactic absorption is non-selective, and therefore not to be attributed to diffraction from particles comparable in size with the wave-lengths of light, nor to selective scattering produced by gaseous molecules, to what shall we refer it? We believe, on what seems to be good scientific evidence, that meteoric stones and meteoritic dust particles are strewn through the celestial spaces. Can they produce the depletion of the nebular light ?
In part, no doubt, the light is absorbed by meteoritic material; but there is a fatal objection to the supposition that all, or even a large part, of the absorption can be produced in this way. If there were enough meteoritic dust to reduce the light from the most distant nebulae to a small fraction, only this fraction could escape absorption. The rest of the radiant energy from the stars would be absorbed and reradiated from particle to particle, but without being able to escape, and the entire mass of meteoritic material accumulated in the untold depths of space must eventually glow. Long before this, the skies would become a scorching envelope. The universe would be a prison house. There would be no escape from its brazen walls.
Is there any other solution of the problem ? I think that there is ; but first let us get an approximate conception of the dimensions of this universe of galaxies. By combining the rate at which the nebulosity around Nova Persei expanded, with established principles from known physical laws, and noting further that the nova, like all of its kin, was a galactic object — a member of the condensed swarm of stars which constitutes our Milky Way — also that it was on the more distant branch of that mighty ring, I have deduced a first approximation to the dimen- sions of the more condensed portion of our Galaxy.
Next, I have passed from the Milky Way to the Great Nebula in Andromeda by asking how much farther the nebula must be in order that a new star which appeared almost at its very center in August, 1885, should have been comparable in brightness with a nova of moderate size in our own Galaxy. The answer is that approximately 1 andromede = 1600 light-years, or 15,000,000,000,000,000 kilometers. 12 An entirely independent computation, on somewhat different lines, by Mr. J. Ellard Gore, leads to a result of the same order. Mr. Gore is not quite as explicit as I have been ; but the general agreement between our results makes me feel confident that we are not far from the truth.
No other of the white nebulas compares with the Andromeda nebula
12 In Knowledge for September, 1912, I conclude that Lord Kelvin's esti- mate of the diameter of the Galaxy, which was five times as great as mine, is probably the better of the two, whence it follows that 1 andromede = 8,000 light- years. But we are concerned at present with rough estimates of an order ofmagnitude only, and may waive all minute details.