842 METHODS OF KINETIC MEASUREMENTS CHAP. 25 



matic" red light, isolated b}^ filters (or monochromators with wide slits), 

 since their sensitivity droj s by a factor of 10 between 600 and 700 m^ 



(fig. 25.2A). 



For all these reasons, if white light is used for photosynthetic work, the 

 best way of characterizing its intensity is to measure it by means of a ther- 

 mopile protected from infrared light by a suitable filter. This will give an 

 adequate picture of the quantity of light available for photosynthesis, and 

 enable one to determine correctly the proportion of this light absorbed by 



the plants. 



The desire for greater sensitivity often will force the investigator to use 

 a photoelectric cell, instead of a thermopile, despite all the shortcomings 

 associated with its selective sensitivity ; this should be done only in full 

 realization of the errors that can be introduced in this way. Only in work 

 with truly monochromatic light are the photocells entirely reliable (as- 

 summg that the linearity of their response has been ascertained by fre- 

 quent comparison with a thermoelement). 



In addition to the problem of a reliable photometric instrument, dif- 

 ficulty arises in the determination of the quantity of light absorbed by 

 leaves, algae or cell suspensions, because of the scattering phenomena dis- 

 cussed in chapter 22. 



The scattering by cell suspensions is comparatively weak, and that by 

 leaves can be reduced by injection with water, or— still better— with gly- 

 cerol (by evacuation under the liquid), thus eliminating the most effective 

 source of scattering— the liquid-air interfaces (c/. fig. 22.8 and 9). How- 

 ever, figure 22.2 shows that, even in Chlorella suspensions, enough scatter- 

 ing is present to cause a marked error in the determination of absorbed light 

 energy — an error of only a few per cent in the region of strong absorption, 

 but of 100% or more in green or in the far red, where tme absorption is very 



weak. 



In measurements in these spectral regions, as well as in precision ex- 

 periments in other parts of the spectrum, it is necessary to measure the 

 total scattered flux {i. e., the diffusely transmitted and diffusely reflected 

 fluxes, Ta and Ra, together with directly transmitted and specularly re- 

 flected fluxes Ts and Rs) and to use the complete formula: 

 (25.2) A = h - Ts - Td - Re - Rd = h - S 



for the determination of the absorbed light energy, A. 



The neglect of both T^ and R^ in the determination of A must lead to 

 entirely erroneous results, because, for all leaves and thalli, Ts is only a 

 fraction — and often a small one — of the total transmitted flux. 



The fact that optical work with leaves requires a consideration of scat- 

 tering was clear to Maquenne (1860) and Simmler (1862) ; but some inves- 

 tigators—not only botanists like Sachs (1864) and Detlefsen (1888), but 



