We may remember that God set his “bow” in the cloud, as a token that the world should no more be destroyed by a flood of waters. Where else may we see a variety of colors? In the soap-bubble. Why are these colors constantly changing? Because the walls of the bubble are constantly growing thinner, and different colors are, therefore, reflected from them. Why do the walls of the soap-bubble grow thinner? Because the water runs to the bottom of the bubble until its top becomes so thin as to burst. Can we tell the color of any object by touching it? No; we must see it in order to tell its color. How do we know this? If there are two coats, one blue and the other black, we cannot tell which is the blue one or which is the black one, by feeling them. What is necessary in order to see the colors? Light is necessary; because all colors come from the rays of light. Can we see the different colors in the ray itself? We cannot; because God has so blended them together that they cannot be seen by us. When most of the rays of light pass through a substance, what is said of it? The substance is said to be _transparent_. Name some transparent substances. Glass, ice, diamonds, air, and clear water. When only a few rays of light pass through a substance, what is said of it? The substance is said to be _translucent_. Name some translucent substances. Flint, isinglass, scraped-horn, and china-ware. When no rays of light pass through a substance, what is said of it? The substance is said to be _opaque_. Name some opaque substances. Wood, iron, coal, and granite. Can we see through a glass window? Yes; because the glass is transparent, and the light passes through it. Can we see through a looking-glass? No; because the back of the looking-glass is covered with quicksilver, which prevents the rays of light from passing through it. What becomes of the light falling on the looking-glass? It is thrown back from the glass. What is this called? It is called _reflection_. What objects are good reflectors of light? Those having smooth and polished surfaces; such as tin, silver, gold, and quicksilver. What objects are poor reflectors of light? Those having dull, uneven surfaces; such as iron, wood, cloth, leather, and calico. Of what is every ray of sunlight composed? It is composed of the seven colors of the rainbow. What does a looking-glass do with these colors? A looking-glass throws them all back from its surface. What does a glass window do with these colors? A glass window permits all these colors to pass through it. What do opaque substances do with these colors? They throw back or reflect some of them. Do they all reflect the same color? No; some reflect one color, and some another color. What color does the grass reflect? It reflects the green color; therefore, the grass is green. What does the grass do with the other colors? It absorbs or hides them in itself.