304 LIGHT AND THE BEHAVIOR OF ORGANISMS 



with the above supposition there are, therefore, theoreti- 

 cally as many different color-tones in the visible solar 

 spectrum as there are different wave lengths. Practically, 

 however, these numerous theoretical hues are divided, usu- 

 ally into six classes, — • violet, blue, green, yellow, orange and 

 red. Authorities differ but little as to the point of division 

 in the spectrum between the different colors. Selecting 

 the classification most commonly found we shall refer to 

 wave lengths 390 to 430^" as violet, 430 to 490'"" as blue, 

 490 to 560''^ as green, 560 to 590^'' as yellow, 590 to 630^^ 

 as orange, and 630 to 760"" as red. 



I, Energy Distrihittion in the Spectrum 



The distribution of energy In the solar spectrum as well 

 as that in various artificial spectra has been thoroughly 

 investigated, as has also that of the effect of different 

 rays on a number of different chemical reactions and the 

 distribution of brightness as judged by the human eye. 

 In a recent paper Nichols (1905) has summarized much of 

 the work on the energy in the visible spectrum. He gives 

 the curves of distribution for the normal or grating spec- 

 trum of the following sources of light: Hefner lamp, 

 acetylene lamp, petroleum lamp, illuminating gas with 

 different burners, Welsbach mantle, various electric incan- 

 descent carbon filaments, magnesium flame, direct and 

 diffuse sunlight and a few others. While the distribution 

 of energy in the spectrum differs considerably with the 

 different sources of light, it corresponds in that the energy 

 toward the red end is much greater than that toward the 

 violet (Fig. 34). There is only one exception to this: in 

 case of magnesium oxide the energy is " greater in the 

 violet thaji in the yellow and green " {I.e., p. 159). 



In the normal gas spectrum the energy increases very 

 gradually from the beginning of the violet to the begin- 

 ning of the green at about 500^^, then it Increases very 

 rapidly and reaches its maximum at the end of the red. 



