NATUBAL PHILOSOPHY. 133 



analyze any colored light, and determine the proportions in whkh 

 the different homogeneous rays enter into it, and by means of a lens 

 we can recombine the rays and produce the original colored light. 

 Newton devised a plan for finding out the colors of a compound, the 

 quantity and quality of each color being given. Dr. S. Young origi- 

 nated a theory that the three elements of color are determined as 

 much by the constitution of the sense of sight as by anything ex- 

 ternal to us. He conceived that three different sensations may be 

 excited by light, but that the proportion in which each of the three is 

 excited depends on the nature of the light ; and conjectured that 

 these three primary sensations correspond to red, green, and white. 

 Sir David Brewster regards the actual colors of the spectrum as aris- 

 ing from the intermixture of three primary kinds of light red, yel- 

 low, and blue. Prof. Clark Maxwell, after referring to the above 

 theories, proceeded to explain the instrument by which he had prose- 

 cuted his researches. It consisted of a long box or tube, fitted with a 

 lens, and having adjustments for producing slits varying in width, 

 whereby he was enabled to separate any parts of the spectrum, and 

 recombine them. By means of this instrument he has been enabled 



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to ascertain that mixtures of the blue, red, and green rays produce 

 white ; blue and red, purple ; red and green, yellow, but if the 

 red predominates, orange ; while yellow and blue produce white in- 

 steacl of green, as in the case of pigments. Professor Maxwell has 

 verified, and is still endeavoring to verify, his results, by obtaining not 

 only the observations of persons of ordinary vision, but also of persons 

 wholly or partially color-blind. He has arrived at the following 

 among other conclusions : That there are three colors in the spec- 

 trum, red, green, and blue, by the mixture of which colors chromat- 

 ically identical with the other colors of the spectrum may be pro- 

 duced ; that the orange and yellow of the spectrum are chromatically 

 equivalent to mixtures of red and green ; that yellow has no preten- 

 sion to be considered a primary color. These conclusions were ex- 

 perimentally illustrated by means of the electric lamp, and by the* 

 apparatus expressly devised for the purpose by Professor Maxwell 

 himself. 



SPECTRUM ANALYSIS. 



Of all the contributions made to science during the last few years, 

 none are probably more important and generally interesting than those 

 which have recently resulted from the application (if not discovery), 

 by MM. Btinsen and Kirchhoff, of Germany, of the so-called pro- 

 cess of " spectrum " or "photo-chemical" analysis. Before proceed- 

 ing, however, to notice in detail the results attained to by these sci- 

 entists, it is necessary, at least for the non-professional reader, to 

 briefly refer to certain well-known and long-established principles in 

 optical science. 



When a ray of solar light is intercepted by, and caused to pass 

 through, a prism, it is refracted, and forms, when thrown upon a wall 

 or screen, a broad band of colored lights, which is known as the pris- 

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