Therefore we see all stars in a part of the heavens where they really are not; we see the sun before it really rises. Light entering a denser medium is refracted from, entering a lighter medium is refracted toward, a line drawn at right angles to its surface. Light entering a new medium at right angles--that is to say, not aslant--continues its own course unaltered. There is but one more fact necessary to fill up the small measure of preliminary knowledge necessary for a general understanding of the phenomena produced by the mixing of light with air. Light in its perfect state is white, but the white light is a compound of other rays in due proportion, each ray being different in color and different in quality. So it takes place, because their qualities are different, that grass reflects the green ray and absorbs the rest, and therefore grass is green; while orange-peel reflects another ray, and swallows up the green and all the rest. These colors being in the light, not in the substance colored; in a dark room it is not merely a fact that we can not see red curtains and pictures; but the curtains really are not red, the paintings have no color in them, till the morning come, and artfully constructed surfaces once more in a fixed manner decompose the light. Beside the color of these rays, from which light is compounded, there are combined with them other subtle principles which act mysteriously upon matter. Upon the hard surface of a pebble there are changes that take place whenever a cloud floats before the sun. Never mind that now. The colored rays of which pure white light is compounded are usually said to be seven--Violet, Indigo, Blue, Green, Yellow, Orange, Red; and they may be technically remembered in their proper order by combining their initials into the barbarous word Vibgyor. These are called prismatic colors, because they were first separated by the passing of a ray of pure light through a prism. In that passage light is much refracted, and it happens that the contained rays all disagree with one another as to the extent to which they suffer themselves to be put out by a change of medium. Violet refracts most, and red least; the others stand between in the order in which they have just been named, the order in which you see them in the rainbow. So the rays after refraction come out in a state of dissension; all the rays--made refractory--having agreed to separate, because they are not of one mind, but of seven minds, about the degree to which they should be put out by the trouble they have gone through.