on the other celestial bodies (moon and planets excepted) would be of the slightest possible description.  All the stars of heaven would continue to shine as before.  Not a point in one of the constellations wrould be altered, not a variation in the brightness, not a change in the hue of any star could be noticed.  The thousands of nebulae and clusters would be absolutely unaltered; in fact, the total extinction of the sun would be hardly remarked in the newspapers published in the Pleiades or in Orion.  There might possibly be a little line somewhere in an odd corner to the effect “Mr. So-and-So, our well-known astronomer, has noticed that a tiny star, inconspicuous to the eye, and absolutely of no importance whatever, has now become invisible.”

If, therefore, it be not the sun which lights up this nebula, where else can be the source of its illumination?  There can be no other star in the neighborhood adequate to the purpose, for, of course, such an object would be brilliant to us if it were large enough and bright enough to impart sufficient illumination to the nebula.  It would be absurd to say that you could see a man’s face by the light of a candle while the candle itself was too faint or too distant to be visible.  The actual facts are, of course, the other way; the candle might be visible, when it was impossible to discern the face which it lighted.

Hence we learn that the ring nebula must shine by some light of its own, and now we have to consider how it can be possible for such material to be self-luminous.  The light of a nebula does not seem to be like flame; it can, perhaps, be better represented by the pretty electrical experiment with Geissler’s tubes.  These are glass vessels of various shapes, and they are all very nearly empty, as you will understand when I tell you the way in which they have been prepared.  A little gas was allowed into each tube, and then almost all the gas was taken out again, so that only a mere trace was left.  I pass a current of electricity through these tubes, and now you see they are glowing with beautiful colors.  The different gases give out lights of different hues, and the optician has exerted his skill so as to make the effect as beautiful as possible.  The electricity, in passing through these tubes, heats the gas which they contain, and makes it glow; and just as this gas can, when heated sufficiently, give out light, so does the great nebula, which is a mass of gas poised in space, become visible in virtue of the heat which it contains.

We are not left quite in doubt as to the constitution of these gaseous nebulae, for we can submit their light to the prism in the way I explained when we were speaking of the stars.  Distant though that ring in Lyra may be, it is interesting to learn that the ingredients from which it is made are not entirely different from substances we know on our earth.  The water in this glass, and every drop of water, is formed by the union of two gases, of which one is