What, then, becomes of the missing blue rays? They are caught by the myriads of little floating particles in the air, and reflected away from us. That is why we do not see them; their course is turned back, just as waves breaking against a stone sea-wall are turned back or reflected. A person situated _behind_ such a wall will not see the waves which break against it; but suppose a _very_ big wave came: it would come right over, and then we should soon become aware of its presence. So it is with the little waves of light: some are stopped and turned back as they break against the myriads of little dust particles and the still more numerous particles of mist always floating in the air; while others, which are larger, break over them and travel on undisturbed until they reach our eyes. Now, the larger waves of light are the red waves, while the smaller ones are the blue waves; hence there is no difficulty in understanding why the red waves (or vibrations) are seen at sunset and sunrise, to the exclusion of the blue waves. But it must be borne in mind that light-waves are of infinitesimal smallness, thousands and thousands of them going to make up an inch. Sound also travels in waves, and the phenomena of sound serve to illustrate those of light; but sound-waves are very much larger. The reason why the sky overhead appears blue is that we see the blue rays reflected down to the earth from myriads of tiny dust and water particles, while the red rays pass on over our heads, which is just the reverse of what happens at sunset. On the southern slopes of the Alps the blues of the sky are generally very different from those on the northern side; and this is probably due to the greater quantity of water-vapour in the air, for the moist winds come from the south. Sunrises in the Alps are quite as glorious to behold as sunsets; but comparatively few people rise early enough to see them. Speaking generally, it may be said that in Alpine sunrises the prevailing colours are orange and gold, in sunsets crimson or violet-pink. After a cool night the atmospheric conditions will obviously be different from those which exist after a warm day, and more water-vapour will have been condensed into mist or cloud. Hence we should expect a somewhat different effect.