CHAP, ii Ph of os) m thesis 315 



power of effecting photosynthesis in another lying below 

 the first. It is clear that the rays which are absorbed by 

 the chloroplasts of the first are the effective and not the 

 deleterious ones. 



The earlier methods of investigation of the influence of 

 different rays consisted in cultivating plants or exposing 

 leaves under glass of different colours, or guarded by hollow 

 screens containing definite coloured liquids, the chief of 

 these being chromate or bichromate of potassium and 

 ammonio-cupric sulphate. Such methods are open to criti- 

 cism, as the light passing through such media will not only 

 differ in quality, but almost certainly vary in intensity, 

 and the medium itself will not give well-isolated portions 

 of the spectrum. It is not very surprising, therefore, that 

 discordant results were obtained. The fact, however, of 

 the association at all was a great advance on previous 

 knowledge. 



The method of working with the spectrum which had 

 been introduced by Draper was taken up almost at the 

 same time by N. J. C. Miiller and by Timiriazeff. Miiller's 

 results lacked precision, on account of his working like 

 Draper with an impure spectrum. The difficulty of working 

 with such a narrow slit as to ensure a pure spectrum was that 

 the light that passed was of too low an intensity to cause 

 enough decomposition of carbon dioxide to be measured 

 by the apparatus then at the disposal of investigators. To 

 meet this difficulty Timiriazeff devised, in 1871, a modifica- 

 tion of Bunsen's gasometer of such extreme delicacy as 

 to allow him to estimate so small a quantity of the gas 

 as -ooi c.c. The leaves were put into small flat tubes 

 of such size as was necessary, and after exposure to the 

 light of the desired part of the spectrum the changes in 

 the air surrounding them ascertained by direct measurement 

 with the modified gasometer. 



In other experiments Timiriazeff took living plants of 



