200 THE POPULAR SCIENCE MONTHLY. 



Helinholtz, however, again showed, in 1852, that a color to our un- 

 aided eyes identical with white w r as produced by combining yellow 

 with indigo. At that time yellow was considered to be a simple color, 

 and this, therefore, was regarded as an exception to the general rule, 

 that a combination of three simple colors is required to produce white. 

 Again, it was, and indeed still is, the general impression that a com- 

 bination of blue and yellow makes green. This, however, is entirely a 

 mistake. Of course, we all know that yellow paint and blue paint 

 make green paint ; but this results from absorption of light by the 

 semi-transparent solid particles of the pigments, and is not a mere 

 mixture of the colors proceeding unaltered from the yellow and the 

 blue particles ; moreover, as can easily be shown by two sheets of col- 

 ored paper and a piece of window-glass, blue and yellow light, when 

 combined, do not give a trace of green, but if pure would produce the 

 effect of white. Green, therefore, is after all not produced by a mixt- 

 ure of blue and yellow. On the other hand, Clerk Maxwell proved in 

 1860 that yellow could be produced by a mixture of red and green, 

 which put an end to the pretension of yellow to be considered a primary 

 element of color. From these and other considerations it would seem, 

 therefore, that the three primary colors if such an expression be re- 

 tained are red, green, and violet. 



The existence of rays beyond the violet, though almost invisible 

 to our eyes, had long been demonstrated by their chemical action. 

 Stokes, however, showed in 1852 that their existence might be proved 

 in another manner, for that there are certain substances which, when 

 excited by them, emit light visible to our eyes. To this phenomenon 

 he gave the name of fluorescence. At the other end of the spectrum 

 Abney has recently succeeded in photographing a large number of 

 lines in the infra-red portion, the existence of which was first proved 

 by Sir William Ilerschel. 



From the rarity, and in many cases the entire absence, of reference 

 to blue, in ancient literature, Geiger adopting and extending a sug- 

 gestion first thrown out by Mr. Gladstone has maintained that, even 

 as recently as the time of Homer, our ancestors were blue-blind. Though 

 for my part I am unable to adopt this view, it is certainly very remark- 

 able that neither the " Rigveda," which consists almost entirely of 

 hymns to heaven, nor the " Zendavesta," the Bible of the Parsees or 

 fire-worshipers, nor the Old Testament, nor the Homeric poems, ever 

 allude to the sky as blue. 



On the other hand, from the dawn of poetry, the splendors of the 

 morning and evening skies have excited the admiration of mankind. 

 As Ruskin says, in language almost as brilliant as the sky itself, the 

 whole heaven, " from the zenith to the horizon, becomes one molten, 

 mantling sea of color and fire ; every black bar turns into massy gold, 

 every ripple and wave into unsullied, shadowless crimson, and purple, 

 and scarlet, and colors for which there are no words in language, and 





