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POPULAR SCIENCE MONTHLY

THE RELATION OF COLOR AND CHEMICAL CONSTITUTION
By WILLIAM J. HALE, PH.D.

UNIVERSITY OF MICHIGAN

AMONG the many branches of scientific learning whose early development we owe to the analytical mind of Sir Isaac Newton, none can show a more beautiful discovery than that different colored rays of light suffer unequal amounts of refraction or bending when passed through a prism; and that sunlight itself by similar means is resolved into a series of colors, the order of which, beginning at the red and ending with the violet, corresponds with a gradual increase in refraction. It was this that gave us our first spectrum and proved at once the composite nature of white light.

After these discoveries by Newton, a hundred years and more elapsed before Wollaston in 1802 observed the spectrum of a ray of sunlight admitted through a narrow slit in a window-blind. In this simple experiment, which gave a better distribution of the spectrum colors than could be obtained with the pencil-like rays of Newton's time, certain black lines were seen to cross the spectrum parallel to the slit. The investigation of these lines, however, was left to Fraunhofer, who, several years later, with much improved apparatus for collecting the light rays and projecting the same upon a screen, succeeded in definitely establishing the existence of a large number of black lines in the solar spectrum. In other words, the light from the sun was shown to be incomplete by reason of the absorption of certain of its rays, as was indicated by over 700 of these dark lines. To the principal lines, still called by Fraunhofer's name, he assigned letters beginning in the red with A and ending in the violet with H.

That there also existed an invisible portion of the spectrum lying to either side of the visible spectrum, was early pointed out. Sir William Hershel in 1800 observed the great heating effect of that portion beyond the red, while Sir John Herschel in 1840 demonstrated the existence of Fraunhofer lines in this infra-red region. Ritter and Wollaston showed that silver chloride blackened readily in the invisible portion of the spectrum beyond the violet, a fact readily inferred from Scheele's observation in 1777 that silver chloride turned dark more readily in violet than in red light. E. Becquerel, however, in 1842 succeeded in identifying many of the Fraunhofer lines in this ultraviolet region and lettered the principal lines from L to P in continuation of those already lettered by Fraunhofer.