438 



I 'rot'. ('. Timiriazeff. 



;Apr. 30, 



FIG. 



Yeilov 



Blue 



ligkb. 



light. 



Whilst in the spectrum as we have just seen (fig. 5), the blue and 

 violet rays produced hardly any effect, here the effect of these rays 

 was quite prominent. 



The fact that the reduction of carbon dioxide as well as the pro- 

 duction of starch is due to the rays absorbed by chlorophyll, may be 



thus considered as fully established 

 in all its details, the more so that 

 an elaborate bolometric study of the 

 chlorophyll spectrum in the infra- 

 red by Donath, has proved that 

 there are no absorption bands in 

 this region. This accounts for the 

 fact established, as we have seen, by 

 Cailletat, that no reduction can be 

 attributed to the rays filtered 

 through Tyndall's iodine solution. 



We have now to consider the 

 second of the two points mentioned 

 above concerning the connection 

 between the photo-chemical process 

 and the absorption of light. We 

 have seen that Jamin, Edmond 

 Becquerel and, lastly, Lommel, ex- 

 pressed the opinion that Herschel's 

 law might be applied to our case. But it seemed to me that in this 

 reasoning there was a certain logical flaw, a link missing, between the 

 premisses and the conclusion deduced. Herschel's .Jaw means that the 

 photo-chemical effect is confined to those rays of light only which are 

 absorbed by the substance undergoing chemical change. Sir John 

 Herschel applied it himself to chlorophyll, showing that this substance 

 underwent a process of bleaching in exactly those rays of the spectrum 

 which correspond to the absorption bands. 



But in the reduction of carbon dioxide we have quite a different 

 case the substance undergoing decomposition is a colourless gas and 

 light is absorbed by another substance, chlorophyll. It was decidedly 

 impossible to see in this case a direct application of Herschel's law, and 

 that was one reason the more for my not insisting on this point at the 

 beginning of my researches. But the same reason accounts for my 

 being one of the first to acclaim the importance of Professor Vogel's 

 beautiful discovery of optical sensitisers. 



This brilliant achievement not only revolutionised the practice of 

 photography but furnished at the same time that missing logical link, 

 the absence of which did not permit of Herschel's law being applied to 

 ithe case of the green leaf. Vogel's researches on Eosine were shortly 

 followed by Edmond Becquerel's experiments on collodion plates 



