298 THE POPULAR SCIENCE MONTHLY 



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 of 

 magnitude only, and may waive all minute details. 



