NEW APPLICATIONS OF LIGHT. 43 



the sun shines upon them; while the complete band is 

 produced by the numerous raindrops, the colored rays 

 from which form the rainbow. Newton examined the 

 colors of the spectrum very carefully, and explained 

 them on the theory that light of different colors has dif- 

 ferent refrangibilities or, as we now say, different wave- 

 lengths. He also showed that a similar set of colors can 

 be produced by the interference of rays when reflected 

 from the two surfaces of very thin plates, as in the case 

 of what are termed Newton's rings and in the iridescent 

 colors of thin films of oil on water, of soap bubbles, and 

 many other substances. 



These color-phenomena, although very interesting in 

 themselves, and giving us more correct ideas of the 

 nature of color in the objects around us, did not lead to 

 anything further. But in 1802, the celebrated chemist, 

 Dr. Wollaston, made the remarkable discovery that the 

 solar-spectrum, when closely examined, is crossed by 

 very numerous black lines of various thicknesses, and at 

 irregular distances from each other. Later, in 1817, 

 these lines were carefully measured and mapped by 

 Fraunhofer; but their meaning remained an unsolved 

 problem till about the year 1860, when the German 

 physicist, Kirchhoff, discovered the secret, and opened up 

 to chemists and astronomers a new engine of research 

 whose powers are probably not yet exhausted. 



It was already known that the various chemical ele- 

 ments, when heated to incandescence, produce spectra 

 consisting of a group of colored bands, and it had been 

 noticed that some of these bands, as the yellow band of 

 Sodium, corresponded in position with certain black 

 lines in the solar spectrum. Kirchhoff's discovery con- 



