The Action of Light on Chlorophyll. 



389 



tion of the chlorophyll solution on glass plates and in glass tubes and 

 flasks. 



The bleaching of chlorophyll can be conveniently demonstrated by 

 exposing the half of a strip of paper tinged with chlorophyll to the light, the 

 other half being kept in the dark. In sunlight the bleaching takes place 

 very rapidly, but very slowly in diffused light. 



The action of the different rays of the spectrum can be shown by exposing 

 a piece of paper tinged with chlorophyll or a glass plate covered with a 

 layer of chlorophyll to a sunlight spectrum, and it will be seen that the 

 bleaching takes place as described in Herschell's experiments and more 

 recently by Reinke* and by Dangeardf in those parts of the spectrum where 

 the light is absorbed. A convenient method of showing the different effects 

 of the principal parts of the spectrum is to make use of filters through 

 which definite wave-lengths are transmitted. The Wratten and Wainwright 

 filters are suitable for this purpose, and the action of light is much more 

 rapid than with the pure spectrum. The disadvantage of filters is that the 

 different colours absorb varying proportions of the light which they are 

 supposed to transmit. Thus whilst a red filter may transmit 78 per cent, of 

 the light, a blue filter may transmit only 16 per cent, of it. 



The tricolour set of filters supplied by Messrs. Wratten and Wainwright 

 divide the visible spectrum into three nearly equal parts — red, green, and 

 blue — with some slight overlapping, but as this green allows rather more of 

 the yellow and blue ends of the spectrum to pass than is desirable, it is 

 better to add to the green another one which limits its range. The different 

 parts of the spectrum transmitted through the three filters which I have used 

 are as follows : — 



Eed— Standard tricolour filter, X 710-590. 



Green — Standard tricolour plus green (two filters), about X 550-480. 

 Blue — Standard tricolour filter, A, 510-400. 



The bleaching of chlorophyll takes place very rapidly through the red 

 filter, much more slowly through the green and blue filters. If, however, 

 the light is allowed to act for a longer time through the blue and green 

 filters, the bleaching then becomes as pronounced through the blue as 

 through the red filter. Thus in bright sunlight it takes approximately 

 8-10 times as long to bleach chlorophyll paper through the blue filter as 

 through the red. This seems to indicate that the different effects of the red 

 and blue ends of the spectrum are proportional to (1) the absorption of light, 



* ' Bot. Zeit.,' 1885. 



t ' Le Botaniste,' 1912 and 1913. 



