The light that shines back to us from a sheet of water, has not penetrated through its substance, certainly. But now, let us be Tritons, or sea-nymphs, and let us live in a cool crystal grot under the waves. We don't live in the dark, unless we be unmitigated deep-sea Tritons. The deeper we go, the darker we find it. Why? Now, let us be absurd, and suppose that it is possible for light to be measured by the bushel. Ten bushels of light are poured down from the sun upon a certain bit of water; six of these, we will say, reflected from its surface, cause the glittering appearance, which is nothing to us Tritons down below. But light can pass through water; that is to say, water is a transparent substance; so the other four bushels soak down to illuminate the fishes. But this light, so soaking down, is by the water (and would be by any other transparent substance) absorbed, altered, partly converted into heat--when we understand exactly what Mr. Grove calls the Correlation of Physical Forces, we shall understand the why and how--we only know just now the fact, that all transparent bodies do absorb and use up light; so that the quantity of light which entered at the surface of our water suffers robbery, becoming less and less as if sinks lower down toward our coral caves. Furthermore, beside reflection and absorption, there is one more thing that light suffers; and that we must understand before we can know properly why skies are blue, and stars are twinkling. That one thing more is called Refraction. A horse trots fairly over the stones, but slips the moment stones end, and he comes upon wood pavement. A ray of light travels straight as a dancing-master's back, so long as it is in air, or water, or glass, or any other "medium," as the books say, of a certain unvarying thinness or thickness, fineness or coarseness, or according to the school-word "density." But if a ray that has been traveling through warm and light air, suddenly plunges into air cold and heavy, it is put out of the way by such a circumstance, and in the moment of making such a change, it alters its direction. Still more, a ray of light that has been traveling in a straight line through air, is put out of its course on entering the denser medium of water; it is dislocated, refracted very much, alters its course, and then continues in a straight line on the new course, so long as the new medium continues. In the same way, a ray of light which travels through a medium that becomes denser and denser very gradually would be perpetually swerving from its straight path, and would travel on a curve. Our atmosphere is heaviest upon the surface of the earth, and becomes lighter and thinner as we rise; the ray, therefore, from a star comes to us after traveling in such a curve. But we see all objects in the direction of a perfectly straight line continued in the direction which the rays sent from them took at the moment of falling upon our sense of sight.