1903.] The Cosmical Function of the Green Plant. 445 



le<s refrangible part and by only 54 for the more refrangible rays. 

 But we ought to take into account the different degrees of absorption 

 in these two halves of the spectrum. It would seem that the simplest 

 way would be take into consideration the directly visible absorption 

 bands presented by the colouring matter in the same state of con- 

 centration as in the leaf (we shall see later how such solutions may be 

 obtained). This was, in fact, the way I first adopted. But this 

 estimate must be considered as not free from error, since the eye is a 

 poor photometer for slight and gradual differences of absorption. 

 This is the reason why in this class of researches, as already mentioned, 

 we must have recourse to the spectro-photometric method. 



The choice of the instrument is far from being immaterial for 

 instance, Vierordt's photometer with a double slit, generally used by 

 German physiologists, is not to be trusted. As the most practical 

 form I consider d' Arson val's spectro-photometer as furnished by 

 M. Pellin. Here are, for instance, two photometrical curves for 

 solutions of normal and modified chlorophyll of equal concentration. 

 And here (fig. 6 A) we have the spectro-photometric curve correspond- 

 ing to the absorption of light by a single leaf of maple, such as was 

 used for the experiments with yellow and blue light. Reduced to the 

 normal energy spectrum the relative amounts of absorption are 

 measured by the areas a, b, c, d, e, and e, r/, /, r/, h. The total 

 amount of energy of the spectrum I take from Mr. Langley's paper 

 on "Energy and Vision" in the 'Philosophical Magazine' for 1889. 

 The relative values of the energy absorbed by the leaf, as represented 

 by these areas a, b, c, d, e and e, d,f t g, h, will be 100 for the yellow and 

 70 for the blue (fig. 6, B). If plotted together with the previous analytical 

 data (100 and 54) the results of these calculations furnish a new argu- 

 ment in favour of the supposition that the chemical process stands in 

 direct relation to the energy of the radiation. 



All things considered, it seems to me that there is a certain amount 

 of evidence for admitting that, so far at least as chlorophyll is con- 

 cerned, the photo-chemical effect of a radiation depends not only on the 

 degree of its absorption, but at the same time on its energy or amplitude 

 of vibrations ; or, to put it in other words, that of two rays equally 

 absorbed, the one possessing the greater amplitude of vibration will 

 produce the greater effect. From this point of view chlorophyll, since 

 of all radiations it absorbs those possessing the greatest energy, may be 

 considered not only a sensitiser but perhaps the best of sensitisers, 

 specially adapted to its function. 



I feel that I cannot dismiss the subject without a word of remark 

 concerning the attitude taken towards my researches by the German 

 physiological school, as represented by Julius Sachs and Professor 

 Pfeffer. The latest opinion on the subject expressed by Sachs wa* the 

 following: "Alle direkte Beobachtungen zeigen, dass das Maximum 

 VOL. LXXII. 2 I 



