THE COLOUR OF THE SKY.

Press, Volume LII, Issue 9181, 12 August 1895, Page 4

THE COLOUR OF THE SKY.

Why is the sky blue ? Cliemi.ta at on* f time used to assume that water and the sky were really colourless. This fallacy with regard to water was exploded by Bunaen, , who proved that under a thickness of two yards water is of a bluish colour, and uncle, seven yards is bluish-green. Professor Spring, another eminent scientist, arrived at the same conclusion, and sliowed very clearly that, whatever may be the lengtn of the layers, if the liquid is absolutely pure, it has an unmingled blue colour. I. is only the shade that changes. The shade becomes deeper as the thickness increases. The greenish tintß an caused by yellow fluorescence, which is produced by the play of white light upon particles of solid matter, held in suspension by the water. This discovery induced Pro* feasor Spring to ascertain whether or not a similar miatake had not beeu made with regard to the sky, and whether the eky bad not really a bluish ool.ur. His original opinion was that the sky was blue, and that this colour was due to the p: esence of peroxide of hydrogen. He carried out several experiments, but found it very diffioult to arrive at any definite conclusion, aa it wm necessary to prepare more than a quart o( peroxide of hydrogen in an absolute statfl of purity before it could be examined under a sufficient thickness. Ultimately he succeeded, and was able to show that this substance could ba regarded axe, liquid ot the same colour as water, but of a much deeper shade, and more like the colour of oxygen, and especially ozone. His ingenious, hypothesis, however, was stopped here by this difficulty, that the atmosphere cou-. tains at least four blue substances under great thickness, oxygen, water, ozone, and peroxide of hydrogen, and it was impos-i-le for him to determine which of the four gave its or their colour to the sky. ' s: - :'. This same question, the clearness of the sky and the origin of its Rzure colour, has been studied by the greatett phystois'.s of the world. The late Professor Tyndall used to describe it as "The Enigma of Nature,:' and his theory to explain it was that colours of the shortest wave lengths are not reflected but transmitted through the air, wuereaß colours of the longest wave-lengths, such as blue, are reflected. Professor Spring, in a paper communicated a few weeks ago to the French Academy or Sci.nees, accepts this theory as the proper solution of tho problem. The blue colour i. produced by the reflection of the solar light, by means of Nicol's prism it is" shown to be polarised light. Solar light, as i» ' traverses through more or less thick layers, is continuously refracted, until the rays undergo total reflection.