432 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 uuto 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 identi- 

 cally 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 atmos- 

 phere 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. AVe 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 dark- 

 ness. Now this teils us tliat 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 ex- 

 periments 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 



