14 THE STORAGE OF ENERGY 



phyll, erythrophyll, etiolin and carotin ; botany of green plants, 

 distribution of chlorophyll in leaves ; variegation in leaves ; and 

 structure of chloroplasts) (see Fig. 1). 



I2.OOO IOOOO SOOO 6OOO 4-OOO 2OOO 



HEAT VISUAL ACTINIC 



FIG. 1. Curves showing relative energy and luminosity of different regions of the 

 spectrum. (Abney.) The wave-lengths are given in Angstrom units. An Angstrom 

 unit (A.U.) = oue ten millionth of a millimetre = 0' 1 MM- 



Absorption Spectrum of Chlorophyll. 



An optical property of primary importance in a substance is 

 its differential absorption of light. The wave motion of the 

 ether, which constitutes white light, is very complex, consisting 

 not only of transverse vibrations in every plane, but of waves 

 of different lengths associated together. Now each wave length 

 produces a definite colour-sensation in the optical receiving 

 mechanism. The shortest wave length optically perceptible to 

 man is 4/10,000 mm. (violet), the longest 7/10,000 mm. (red). 

 Between these extremes there are blue rays 470 /x/x, green 510 /u/z, 

 and orange 620 /UL/UL. (These are average figures, e.g. the various 

 lengths classed under orange vary from 595 to 645 n/j. [IUL/UL = 

 1/1,000,000 mm.].) Beyond these visible rays exist both shorter 

 (ultra-violet) and longer (infra-red) rays. From physics one learns 

 that the ultimate constituents of matter exist, not in a state of 

 rest, but in a state of ordered motion. That is, the electrons 

 (or other ultimate complex, see colloids, p. 75, and radiant 

 energy, p. 117) vibrate with a definite periodicity dependent on 

 the intrinsic energy of the system to which they belong. From 

 the study of the emission spectra of gases, a relationship may 

 be deduced between the period of vibration of a particle and the 

 position of the light bands emitted by the element in gaseous 

 form. Conversely, if one views the absorption spectra of that 

 gaseous element, black bands will be found in place of the light 

 emission bands. To take a concrete example, if a substance 

 when emitting light produced rays consisting for the most part 

 of waves 680 /u/x long, the colour of the light evolved would be 



