light are nearly parallel and in consequence they can go a long distance without failing in brightness. Let us make the lantern send us a parallel beam of light like that of a searchlight: you can see that it does not alter much in cross-section from the lens right away to the wall; and if the rays were strictly parallel, it could go across a million or a billion miles without being diminished in brightness. But the light of a star does not behave like this: it spreads out in all directions continually, like the light from this other beam (see Fig. 88), getting, therefore, all the time fainter. On leaving the lantern it spreads out continually as wide as we like, if only we go far enough. That is how the light of a star behaves, and accordingly it gets fainter and fainter the farther it goes. You can see this very easily: we let both the beams fall on the same screen, and when we put the screen near the two lanterns we can make the two patches of about equal brightness. Now. they are about of equal brightness. But now if we move the screen away, the patch from the parallel beam remains about the same, while the other gets fainter and fainter. Let us stop here a moment; and now we will put a dark glass in front of the parallel beam lantern, so that its patch is made as faint as the other once more. Again move the screen further off, and the spreading beam again gets fainter. If we bring the screen back towards the lanterns, the spreading beam recovers its brightness, and finally it ends by being brighter than the other. So in spreading out in that way the light of a star is always losing itself; and it is wonderful that it should get to us across those vast distances without being so faint that we cannot really see it at all. The
