332 



LECTURE XIX. 



spectrum of the chlorophyll colouring matter, • so very remarkable in itself. If 

 a parallel-walled vessel filled with an alcoholic solution of chlorophyll is placed in 

 the path of a beam of light (p. 303) which affords a spectrum band by means of 

 a prism, the whole of the violet and blue disappears from the spectrum, as do also 

 those ultra-violet rays which become luminous when falling upon a solution of quinine. 

 At the same time, a broad, black stripe becomes visible in the red part of the spectrum 

 and in the yellow region also a feeble absorption of light occurs. The green colouring 

 matter is also strikingly fluorescent : if the focus of a burning-glass is allowed to fall 

 in a solution of chlorophyll, the white sunlight appears blood-red. As Stokes showed 

 long ago, this phenomenon is caused by the fact that the highly refrangible 

 light-rays become changed into lowly refrangible red ones. By an erroneous 

 inversion of the correct state of affairs — that the light-rays effective in the as- 

 similating chlorophyll must be destroyed as such — Lommel arrived at the fallacy 

 that the rays destroyed in the chlorophyll spectrum are those active in assimilation. 

 In this he left out of account that the rays absorbed in a solution of chlorophyll 

 are the same as those absorbed in a living green leaf. In a solution of chlorophyll, 

 however, no separation of oxygen from carbon dioxide takes place : according 

 to Lommel, also, it must be the most strongly absorbed red rays which bring about 

 assimilation, whereas all direct observations show that the maximum evolution of 

 oxygen takes place in yellow light ^. 



On the whole, investigations on the spectrum of chlorophyll have hitherto 

 yielded no facts of any physiological value — i. e. we should know quite as much 

 of the physiological function of chlorophyll if its spectrum were absolutely un- 

 known to us. 



' Cf. Sachs' ' Lehrbuch,' p. 732. 



