The sun is the great lamp that illuminates the world, while the atmosphere, which is filled with particles of various substances, becomes the shade of the lamp which diffuses and softens the light and gives it its color tones, whether of warmth or coldness. We could not well do without the reflected light of the sky. The poetry of life would be sadly marred. The beautiful effects of color and purity of tone would be wanting. We need to bathe in light as much as in water, and the character of the light is almost as important as the character of the water. Imagine a world with an atmosphere devoid of all substances that would in any way reflect light or give to it softness or color tone. Imagine a sun or a moon without visible rays--for without a reflecting atmosphere there would be none. Imagine a sky that was no sky at all, but only a dark void, with no protecting vault. Think of the shadows, so dark that you could see nothing in them. These would be some of the effects that would come from an atmosphere that had no sky substance in it. Imagine the world lighted by one great arc light. The reflex action upon the race living in such a light would be anything but desirable. The world would develop into an arc-light civilization--if one can imagine what that would be like; certainly one of intensely violent contrasts. Look on this picture and let us be thankful for the blue sky and golden sunsets. "But," you ask, "why is the sky blue?" In one of the chapters on the subject of light in Vol. II. the properties of soap bubbles are discussed. It is shown that when a film is stretched across the mouth of a tumbler held in a position so that the film is perpendicular, by the action of gravity (the moisture constantly falling to the lower part of the film) it will continually grow thinner, and horizontal bands of color will appear upon it,--first red, then followed by the other colors of the solar spectrum, ending with violet.