Among Dr. Roberts' photographs of spiral nebulæ (and the Andromeda nebula is undoubtedly a spiral) there are some which are apparently seen nearly edgeways, and show that these nebulæ are very thin in proportion to their diameter. From a consideration of these photographs we may, I think, assume a thickness of about one-hundredth of the diameter. This would give a thickness for the Andromeda nebulæ of about 500 times the sun's distance from the earth. This great thickness will give some idea of the vast proportions of the object we are dealing with. The size of the whole solar system--large as it is--is small in comparison. The diameter and thickness found above can easily be converted into miles, and from these dimensions the actual volume of the nebula can be compared with that of the sun. It is merely a question of simple mensuration, and no problem of "high mathematics" is involved. Making the necessary calculations, I find that the volume of the Andromeda nebula would be about 2·32 trillion times (2·32 × 10{18}) the sun's volume! Now, assuming that the nebulous matter fills only one-half of the apparent volume of the nebula (allowing for spaces between the spiral branches), we have the volume = 1·16 × 10{18}. If the nebula had the same density as the sun, this would be its mass in terms of the sun's mass taken as unity, a mass probably exceeding the combined mass of all the _stars_ visible in the largest telescopes! But this assumption is, of course, inadmissible, as the sun is evidently quite opaque, whereas the nebula is, partially at least, more or less transparent. Let us suppose that the nebula has a _mean_ density equal to that of atmospheric air. As water is about 773 times heavier than air, and the sun's density is 1·4 (water = 1) we have the mass of the nebula equal to 1·16 × 10{18} divided by 773 × 1·4, or about 10{15} times the sun's mass, which is still much greater than the probable combined mass of all the _visible_ stars. As it seems unreasonable to suppose that the mass of an individual member of our sidereal system should exceed the combined mass of the remainder of the system, we seem compelled to further reduce the density of the Andromeda nebula. Let us assume a mean density of, say, a millionth of hydrogen gas (a sufficiently low estimate) which is about 14·44 times lighter than air, and we obtain a mass of about 8 × 10{7} or 80 million times the mass of the sun, which is still an enormous mass. As possibly I may have assumed too great a thickness for the nebula, let us take a thickness of one-tenth of that used above, or one thousandth of the length of the nebula. This gives a mass of 8 million times the sun's mass. This seems a more probable mass if the nebula is--as Bohlin's parallax implies--a member of our sidereal system.