Why Is The Sky Blue?

""Why is the sky blue?" is a question which haa often icen asked, but never gatisfactorily answered. Helmholtz offered an explanation which I cd 011 the renection of solar light by the air partióles in the atmosphere. These partióles bcing very minute would reflect preferoBly the shorter wave3 ot' light, i. c, blue vraves, vrhile they would allow the longer yraves, corresponding to green and red light, to pass through thcm ; just as a ot wood floating on the surface of still ■water would thro"VT oflf the tiny waveleta caused by a telling drop in it3 neighborhood, while the game log in long ocean swella would bo tosscd to and iVo without noticeably impeding the progress of the waves. Dr. E. L. Niehols (Phüosophical Magazine, December) has propounded another view, which lias much to recommend it. According to Youngand Helmholtz's thedry of color-impression, there are in tho cye three sets of nerve-tormini, one set chlefly influenoed by the red, another by the green, thg third by tho violet rays. The impressie1 of colora is the resultant of the ijtonsities of these toree rays. The impression apon the nerves is not directly proportional to the intensity of the ray, the different nerve-termini being subject to different ïtews. Fat veiy feeble rays the "violet," nerves are vcry sensitivc, while the "green" and "red" nerven scarcely sce at alt. As the light lacreases in intensity the "red'' and ""■reen" nérvea increase in activity, while the "violet" nerves beeome tircel and dazzlcfl. For rays of dozzling brilliancy, the "red" nerves are in their most sensitive condiüoii. Thus, ofthe simple colors, as the brightness iucreases, red and green chajjge to yellow, bino beeoine3 white. Daylight, at ordinary tntensities, affects the turee sets of nerve-termini oqually : the resultantimpresaioniswlnteness. Now, daylight is simply the light of the sun ■vrëakened by niaiiifold difl"use reflections. Tlie direct rays of the sun, as we let them fall apon any colorless object, appcar also a white light ; but, on attempting at noon, on a clear day, to gaze into the sun's face, the impreasion is of blinding yellow. It is not that the direct rays differ in composition from diffuse daylight, but that the "violet" nerves cannot transinit the action of sucli strong light. The moon, with enormously loss illuminating power than the sim, seeins bright, and is far brigliter than the open sky. In passing from the intensity of the moon's rays to those reaching us from a corresponding bit of the open sky, wc may, perhaps, take a step as great as that betweeu the brightness of the sun and moon. In general, white light will appoar bluer and bluer as its intensity diminishes, and this law will apply to thoskies; as the light they reilect becomes laiuter and fainter, they will increase in blueness. even though the light by the process of reflection suffer no chango in ion.-