444 Prof. C. Timiriazeft'. [Apr. 30, 



Now, it is a fact testified by thousands and thousands of observations 

 that the principal chlorophyll band lies between B. and C. and not 

 between C. and I). On the contrary, Acworth's curve representing the 

 sensitising effect has its maximum between B. and C. nearer to C. 

 exactly where innumerable observations unanimously place the principal 

 absorption band of chlorophyll, and in this respect all we know only 

 confirms the exactness of Mr. Acworth's experiment. Beginning with 

 the late Edmond Becquerel's first experiment and ending with my 

 recent photographs, we always see the maximum of the sensitising 

 effect corresponding to the band between the lines B. and C. It follows 

 that Mr. Ac-worth places the maximum of photographic effect in the 

 right place, but the maximum of absorption in an evidently wrong 

 place, and it is only by means of this evident error that chlorophyll is 

 made to agree with the general rule. 



In reality the two points perfectly coincide, and as a further conse- 

 quence chlorophyll must be considered an exception to the general rule 

 of the shifting of the maximum of chemical effect towards the red end 

 of the spectrum. Now it seems to me that the exception as well as 

 the rule might find an explanation in the simple admission that the 

 photo-chemical effect must be attributed not only to the degree of 

 absorption of a certain group of rays of which the band is composed, but 

 at the same time to the energy or the amplitude of the corresponding 

 vibrations. In every absorption band (in the limits of the visible 

 spectrum) the side nearer to the red end will be composed of radiations 

 possessing a greater amount of energy, and as a consequence of the 

 supposition just made, the maximum effect must be shifted that way 

 until we arrive at that part in the red where the maximum of energy 

 and absorption coincide. At this point there will be no reason for any 

 shifting of the photographic maximum, and so we get an explanation of 

 the curious anomaly presented by chlorophyll as a photographic 

 sensitiser. 



Of course the experimenium cruris for testing my hypothesis would be, 

 using Sir William Abney's wonderful method of infra-red photography, 

 to see whether in the infra-red part of the spectrum Acworth's rule 

 would be reversed. But this kind of research is quite out of the reach 

 of a botanist. 



Meanwhile it seems to me that the facts we have just seen concerning 

 the relative effects of yellow and blue light, may afford a new argument 

 in favour of this supposed relation between the chemical process and 

 the energy of the active rays. In fact we may consider it a limiting 

 case, since it allows us a direct comparison of the relative effects of 

 two groups of absorption bands lying at the opposite ends of the 

 visible spectrum, and consequently possessing the greatest difference 

 possible as to the amount of energy of the corresponding radiations. 

 We have seen that this relative effect was represented by 100 for th( 



