n6 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 Fraun- 

 hofer, who, several years later, with much improved apparatus for col- 

 lecting 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 por- 

 tion 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 ultra- 

 violet region and lettered the principal lines from L to P in continua- 

 tion of those already lettered by Fraunhofer. 



