1903.] The Cosmicat Function of the Green Plant. 439 



sensitised by an admixture of chlorophyll, so that in less than a year 

 ^fter the publication of my first experiments I was able to give them 

 A rational explanation in the light of Vogel's new theory. This idea, 

 that chlorophyll plays in the living organism the part of an optical 

 sensitiser, whicli I advanced for the first time in 1875, is now generally 

 accepted, but I suppose very few botanists have had an opportunity 

 of seeing the photographic effect of chlorophyll on a sensitised plate. 

 The late Edmond Becquerel in 1877 kindly showed me his beautiful 

 collodion plates ; but the collodion process being now somewhat out 

 of fashion, I tried to find a simple way of applying the sensitising 

 action of chlorophyll to the gelatino-bromide plates now in general 

 .use. Alcoholic solutions are -not practicable in this case, and I found 

 it very convenient to substitute a potassium or sodium chlorophyllate, 

 which is easily soluble in water. 



Here is a plate (Ilford ordinary), half of which has been immersed 

 in a bath of chlorophyllate of sodium, and then exposed to a spectrum 

 reduced by means of a yellow filter to its least refrangible part (fig. 8). 



FIG. 8. 



B.C. D. 



It may be seen that whilst on the ordinary plate the effect stops short 

 of the line D., in the part that has been immersed in the sensitising 

 bath there is a bright band corresponding exactly to the principal 

 Absorption band of chlorophyll between the lines B. and C. 



I tried to push the analogy between the effect of chlorophyll in a 

 sensitised plate and in the green leaf a step further, by showing that 

 chlorophyll is an optical sensitiser, not only in Vogel's but in Sir 

 William Abney's acceptance of the word. 



Sir William Abney admits that an optical sensitiser is as a rule a 

 fugitive dye, i.e.,- one that rapidly fades in the light. Sir John 

 Herschel's classical researches on the bleaching of paper tinged with 

 chlorophyll, prove that chlorophyll belongs to the above class. I have 



