Page:Popular Science Monthly Volume 78.djvu/442

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THE POPULAR SCIENCE MONTHLY

Sky red at night
Is the sailor's delight."

But in many ways the most interesting of all those proverbs that have to do with red sunrise and red sunset is the one which, according to Matthew, Christ used in answer to the Pharisees and Sadducees when they asked that He would show them a sign from heaven.

"He answered and said unto them, When it is evening, ye say, It will be fair weather: for the sky is red.

"And in the morning. It will be foul weather to-day: for the sky is red and lowring. "

It would seem, too, that Christ sanctioned these views, for it does not appear reasonable that He would teach by illustrations which He knew to be false. Then, too. He follows the above with these words:

"O ye hypocrites, ye can discern the face of the sky; but can ye not discern the signs of the times?"

But whether or not Christ accepted these weather signs as being good, we feel certain that those to whom he spoke must have known and believed in them. It is, therefore, worth while to search, even though the search be a somewhat tedious one, for the physical explanation of these phenomena, and to see how it is possible, if it really is, for identically the same colors of the sky to have for the evening one moaning, and for the morning another entirely different.

To clear the way for this explanation it is necessary, first, to tell something of the composition of sunlight, and a little about the atmosphere through which it passes on its way to the surface of the earth.

We know that rain drops are colorless, and we know, too, that when we are between a falling shower and the bright sun they give us the exquisite coloring of the rainbow. We are also aware that prism-shaped, colorless and transparent objects will receive a ray of white sunlight and emit all the rainbow's brilliant hues, from the faintest violet to the deepest ruby: and that when these are recombined the result is white light like the original. Through such experiments and observations we infer that sunlight is composed, in part at least, of all pure colors, and that they gradually merge the one into the other.

Again, it is possible to obtain two sources of light of the same color and intensity such that at certain places they produce more than twice—in fact up to fourfold—the intensity of one alone, and at certain other places intensities less than that of just one, even to utter darkness. Now this tells us that in some respects two lights behave in a manner similar to two trains of water waves, for these may combine so as at some places to produce exceptionally large waves and at others practically smooth water. Indeed, it has been shown by numerous experiments that light has several properties in common with water waves; one of these being wave-length, that is, the distance from a point in one