,370 U. LUMHY AND J. D. SPIKKS 



where F/ is the experimentally determined velocity of the Hill reac- 

 tion resulting from the light intensity averaged through the reaction 

 cell, I a: k and K are constants. If the true rate law obtaining through- 

 out an infinitesimal depth of reaction system (microscopic law) were: 



ki I -^ ko 



where v is velocity through an infinitesimal reaction cell depth along 

 the axis of propagation of the light beam, k^ is the rate constant for 

 the hmiting light step, ko is the composite rate constant at infinite 

 light intensity, and / is the constant intensity within the reaction 

 layer, we would have expected a better fit to the experimental data 

 from the following equation which averages velocity through a cell of 

 finite depth 



^ kh [1 - e-^/cca ,3. 



K -f 7o [1 - e-«"]/«a 



K and k are constants, h is the incident intensity, a is cell thickness, 

 and a is the absorption coefficient of the suspension. The fact that the 

 use of average light intensity provides a better rate law than the use 

 of average velocity indicates the presence of a complication previously 

 suggested, but now of greater importance in view of certain newly 

 established ramifications of flashing-light kinetics (see B. Kok's 

 paper in this volume). The observed beha\'ior in the Hill reaction as 

 described above is consistent with the interpretation that there is a 

 time lag so great between photon absorption and product formation 

 that the chloroplast fragments can pass through regions of very dif- 

 ferent light intensity during the lag. Equation 2 would thus be ex- 

 pected to yield the rectangular hyperbola of equation 1. In such a 

 situation one really observes flashing-light effects dependent on the 

 rate of stirring in supposedly steady-state experiments. We have 

 always found rapid stirring necessary to produce maximum rates of 

 reaction at high light intensities. Simple calculations show that the 

 decrease in intensity of the light beam in passage through a single 

 chloroplast fragment or an algal cell w'ould be sufficient to produce 

 flashing-light effects even if the chloroplast fragment or algal cell 

 merely rotated in place. Only experiments at the very high light in- 

 tensities sufficient to produce saturation throughout the chloroplast 

 fragment or the cell would thus give the true steady-state picture. 



