1903.] The Oosmical Function of the Green Plant. 44$ 



700 millionths of a millimetre, let us say 666 (a figure easily retained 

 in the memory, it being the number of the Beast). The maximum oft 

 absorption is somewhere about the two-thirds between the lines B. 

 and C., between 686 and 656, consequently again about the wave-length 

 666-millionths of a millimetre. At a later date Mr. Langley furnished 

 data which I have plotted above (fig. 6, A and B), and we shall soon 

 have occasion to apply them. It must be remembered that Mr. Langley's 

 numbers refer to what he calls " high sun." For the mean intensity of 

 the whole day the coincidence would probably be still closer. We may 

 consider the same question from another point of view. I have made- 

 a rough attempt to represent the relative amplitudes of the vibrations 

 corresponding to the different rays of the spectrum, and it may be seen 

 that the greatest disturbances correspond to that region of the spectrum; 

 where the principal absorption band of chlorophyll, and therefore the 

 maximum chemical effect on the carbon dioxide, are situated. 



But we may perhaps find a still more convincing argument in favour 

 of the supposition that the photo-chemical effect of a radiation is a 

 function of its energy, in a comparison of chlorophyll with the other 

 sensitisers, and still more in a comparison of the relative effects of 

 yellow and blue light in the experiment just cited. 



There is an important fact, based on the testimony of such able 

 observers as Eder and Vogel, and made especially conspicuous in a very 

 elaborate paper of Mr. Acworth, the fact that the two maxima, the 

 maximum of absorption of light by the sensitiser and the maximum of 

 photograghic effect on the sensitised plate, do not strictly coincide 

 the latter, as a rule, being shifted towards the red end of the spectrum. 

 No explanation has been put forth for this fact, nor does Mr. Acworth 1 

 offer any, holding at the same time that the fact in itself is past any 

 doubt. But so far as chlorophyll is concerned a curious error seems to 

 have crept into Mr. Acworth's statements. He admits that chlorophyll 

 makes no exception to the general rule, i.e., that the maximum of 

 chemical effect does not correspond to the maximum of absorption in 

 the first band but is shifted to the red end of the spectrum though air 

 attentive glance at the figure relating to chlorophyll will easily convince- 

 one that there is something wrong about it. As in the other cases, 

 Mr. Acworth gives the two curves, the absorption curve and the curve- 

 of the sensitising effect on the photographic plate, in order to show 

 the shifting of the latter towards the red end of the spectrum. But 

 in fact his statement (in both text and figure) concerning the absorp- 

 tion spectrum of chlorophyll is inexact. Here are his very words r 

 "Die Absorption dieser Emulsion ist, was die Haupt- und weniger 

 brechbare Bander betrifft, deutlich ausgepragt. Dieses beginnt etwas- 

 vor C., erreicht bei G|. D. ein Maximum; jenseits desselben ist die- 

 Absorption nicht hinlanglich scharf zu verfolgen, um sie genau zeichneiL 

 zu konnen." 



