438 HAROLD F. BLUM 



We have defined /o as the incident intensity in terms of number of 

 ciuanta per imit time, and /o' as the incident intensity in terms of 

 energy vmits per unit time. It follows from ecpiation (13) that, for 

 unit time : 



.V = /o cc /o'X (14) 



In comparing the effectiveness of diffeient wavelengths, when one 

 has no means of estimating the actual munbers of cjuanta — as is most 

 often the case — it is convenient to use either of the reciprocals, 

 1 (QX) or l/(/oA) as the index oi relative effectiveness. Over short 

 wavelength ranges this correction may be relatively unimportant 

 but, when the action spectrum covers a wide spectral range, as for 

 example in photosynthesis, considerable error may be introduced if 

 the correction is not made. 



3. Estimating Absorption 



In photochemical experimentation it is common practice to ar- 

 range the apparatus for "complete" absorption of the incident radia- 

 tion, that is, the dimensions of the i-eaction chamber and the concen- 

 tration of the light absorber are chosen so that the entering beam is 

 reduced to a negligible intensity in passing through the system. 

 Provided scattering is negligible or can be corrected for, the ab- 

 sorbed radiation can be, in such cases, taken as equal to the incident 

 radiation. This greatly facilitates the measurements; and efficiency 

 can be calculated in these terms. The success of this sort of arrange- 

 ment depends upon, among other things, the maintenance of vir- 

 tually uniform concentration of the reactants throughout the system, 

 usually by stirring. It is sometimes possible to use the same device 

 in studying a photobiological process, for example, in photosynthesis 

 by a microscopic organism such as the alga Chlorella, since the or- 

 ganisms, suspended in an aqueous medium, may be stirred in the re- 

 action chamber in the same way as more simple chemical reactants, 

 maintaining for them a homogeneous environment (15). Suspensions 

 of bacteria have been treated in the same way (IS). In such cases 

 stirring must be adequate. The process of photosynthesis lends itself 

 further to study by photochemical methods, since one can measure 

 the oxygen production, from which may be calculated the amount of 

 carbon dioxide reduced to carbohydrate. 



The situation is not quite the same when one measures photosyn- 



