42 PHOTOCHEMICAL PRINCIPLES 



for each wavelength. Then one computes the number of quanta incident 

 during the required exposure as q ^ = I»t/hv, so 



^=^A (3) 



^x nx 



If the thickness is concealed one may prefer to observe the number of 

 units transformed per unit area instead of per unit volume, say 

 j<Ig — Mx, and we have 



^ = ^A (4) 



q\ n 



As Dr. French has pointed out in the preceding paper, a weak 

 absorption or "thin sample" may be desirable and will enable us to 

 further analyze A/x. In a homogeneous system 



If the exponent is small because of the thinness of the sample {x or c 

 small) 



— = aiCl + a-2C2 + • • • OCnCn 

 X 



where «„ is the molecular extinction coefficient and c„ the concentra- 

 tion of the type n molecule, the sum being extended to all absorbing 

 molecules. 



Under these conditions a^c^ may be the portion initiating our reac- 

 tion and 



, aiCi 



9 = 



aiCi -f a-iC'i + • • • anCn 



in the case of competition without screening. Thus in an ideal case of 

 a thin homogeneous sample, equation (2) may be written 



N Nhv 1 ... 



qx hi n 



In the actual case of a biological system one deals not only with 

 organized structure but often screening layers such as skin and cell 

 wall. Here (f) is further reduced and may be characteristically wave- 

 length dependent. 



Where one can manipulate the concentration and thickness, it may 



