"Well, there is another queer thing about Sirius. He's got a big companion-star fussing round him, which gives such a dim light it can only be seen by the very biggest of astronomical telescopes. There is no reason why there should not be many invisible as well as visible stars in the firmament. As Bessel said, 'No reason exists for considering luminosity an essential property of stars.' "Just imagine, then, that it is quite possible that the heavens are not only full of those bright globes we see night after night, but besides them a multitude of dim, ghostlike stars, unseen by us, but there all the same, are pacing along their allotted paths like the rest." "Are these invisible, unlighted stars allowed by scientists, Jack?" asked Curly, in a subdued voice of awe. "Hinted at, hinted at," returned the rover carelessly. "But I'll tell you something more wonderful to think of than that--the systems of double, treble, and quadruple suns. Now, our solar system is at the bottom rung of the social ladder in the heavenly world. We just have a plain white sun, which we revolve round with regular seasons and fixed day and night; but take a system that revolves round a double star, and thus has two suns, and say these suns differ in colour,--as is often the case, for every star has a colour of its own--Sirius is a pale green, Aldebaran rose-red, Betelgeuse orange-red, Rigel a blue-white, Capella a pearly white, and so on. "Suppose, then, one of these suns is red and the other blue. Imagine, if you can, the combinations of colour that ensue, not to mention the variations in day and night and in the seasons. "At one time both suns will be high in the heavens at once, one shedding rays of red, the other rays of blue; and as they set in different corners of the horizon, two gorgeously coloured sunsets simply overwhelm the sky with beautiful colour-effects. "At another time, perhaps, one sun will be above the horizon for half the round of the clock, the other taking the other half--no night during that period, simply so many hours of red and so many hours of blue light, and perhaps the sunrise of the one coincides with the sunset of the other. Ye gods! What a prospect! But how much further does a quadruple system carry us--four suns glaring down upon one; no nights at all now (the people in those systems no doubt have reached a stage in the evolution of the body--if, indeed, they have bodies at all--when sleep is no longer required), just days of every hue, of every grade of colour--pale blue days, brilliant red days, gorgeous yellow days, violet days, green days----"