spectra of these two stars, he found that whereas in the case of the orange star there are several strong dark lines in the blue part of the rainbow-tinted streak, in the case of the blue star there is quite a cloud of fine lines in the red and orange portions. Hence we learn that the two stars owe their colour to the nature of their vaporous envelopes. Each star glows in reality with a white light; but the white light has in one case to pass through vapours of a somewhat ruddy hue (because absorbing blue light), and therefore this star looks ruddy, while the light of the other star shines through bluish vapours, and therefore this star looks blue.

We do not yet know how it chances that the vaporous envelopes of these stars, and of other pairs of stars, differ in this way. Perhaps we shall never know. It is, however, an important gain to our knowledge to have ascertained that the colours of the double stars are not inherent, but that these stars are, as it were, celestial signal lamps, shining through coloured matter.

And now let us turn our thoughts for a brief space to the consideration of the state of the worlds circling around coloured pairs of suns. It is not quite clear what sort of arrangement would commonly prevail-whether such worlds would circle round the pair, travelling outside both, and having as the true centre of their motions the centre of gravity of their ruling pair of suns, or whether each sun of the pair would have its own family of dependent worlds. It may be that both arrangements would

sometimes prevail in one and the same system. To show how this might be, we may conceive the following modification of the solar system.

Suppose Jupiter somewhat larger than he is, and attended upon by a larger and more important scheme of worlds. Also suppose Jupiter to be a subordinate sunblue, green, or purple, as may best please the imagination. Then the other planets would be regarded as the sun's family, and of these, four-Mercury, Venus, the Earth, and Mars-would revolve as now around the sun, and within the path of Jupiter, his companion sun; the other three-Saturn, Uranus, and Neptune-would revolve outside both the pair of suns; and, lastly, there would be a scheme of worlds circling specially around the smaller of the two suns.

In some cases, however, very different arrangements would be requisite for the stability of the system. For in our illustrative case Jupiter travels nearly in a circle round the sun; but some of the double stars move on very eccentric paths. Now, when the smaller star of the pair, travelling along with a family of dependent worlds, came sweeping close round the leading star, it would not only be exceedingly probable that some of the family would be left behind owing to the superior attractive power of the chief sun, but it would certainly happen that all the creatures living on the worlds thus brought into unusual proximity to a sun much larger than their own would suffer seriously, unless the family of worlds belonging to

6

the smaller sun were close to their own ruler. To use Sir John Herschel's expressive words, unless closely nestled under the protecting wing of their immediate superior, the swoop of their other sun in its perihelion passage round their own might carry them off, or whirl them off into orbits utterly incompatible with the conditions necessary for the existence of their inhabitants.' We may well agree with what Herschel proceeds to say, that we have here a strangely wide and novel field for speculative excursions, and one which it is not easy to avoid luxuriating in.'

In whatever way the systems depending on double suns are arranged, this at any rate is certain, that the beings inhabiting any world in any one of these systems have two suns. There may be, and in many cases there must be, a great inequality between the apparent size and brightness of the two luminaries, but we cannot question that even the lesser (in appearance, as viewed from any particular part of a double sun system) must be a veritable

sun.

Taking the lesser suns of an unequal pair as seen from the earth, it must be remembered that that orb which looks so faint is in reality glowing with so great an intensity of heat and inherent lustre, that its light has passed to us after travelling over the tremendous abysses that separate us from the fixed stars. It is not an opaque orb shining by reflected light, but a mass of matter instinct with fire. We know this from its spectrum, which shows

that in its atmosphere are the vapours of elements which our fiercest furnaces can only liquefy. If, then, we could approach that self-luminous orb, we should find long before we reached the confines of its system that it is a true sun. And within its system-as seen, in fact, from a distance which, though enormous, is reduced to absolute nothingness when viewed from our enormous distanceit is certain that the star is a sun in this sense, that it is capable of dispelling night, that when it is above the horizon of any world having airs like ours there must be a glowing sky like that which, during our own day, hides the stars from our view.

Thus every one of the worlds, in systems belonging to a double star, has a quadruple alternation, in place of that double alternation which we call day and night. There is, first, double day,' when both suns are above the horizon; next, single day with one sun; then, single day with the other sun; and, lastly, true night when both suns are below the horizon.

In my next I shall consider some of the results which must follow from these singular vicissitudes as well as the peculiarities of scenery, &c., which must prevail in worlds circling around coloured double stars.

WORLDS LIT BY COLOURED SUNS.

I WILL consider, first, the case of a world circling as our earth does in her orbit, but around a sun of a rich orange colour, while a companion sun of a blue colour travels around the same sun on a path resembling that of the planet Jupiter. The blue sun would be a large and brilliant orb, as seen from the world whose condition I propose to describe; but the orange sun would necessarily be far more brilliant and look far larger, being in reality the larger sun, and also the nearer. We will assume that the world we are considering has a moon somewhat like our own, and we may reasonably imagine that several other planets travel around the orange sun, others around both suns (that is, outside the path of the blue sun), and that, again, the blue sun has several planets travelling in immediate dependence upon it.

Now, in the first place, let us take the case where the world is between the orange sun and the blue one, and let us suppose that the season corresponds to our spring. Then it is manifest that since one sun illumines one side

1 Speaking exactly, we should say that the two suns circle around their common centre of gravity; but here I deem it sufficient to use such expressions as accord best with ordinary modes of speaking.