SIGNIFICANCE OF ACTION SPECTRA 45 



peared from this spectrum, and instead we find an approximation to a 

 typical efficiency trend — maximum for the longest wavelength and 

 falling off as the quantum becomes unnecessarily large and there is 

 less close resonance. 



Assuming that curve A is proportional to (t>/nhv we divide B by A 

 and obtain a curve B A (Fig. 3), which now approximates the ab- 

 sorption which contributes to the lethality, as seen from equation (2). 

 Note the similarity between the observed absorption, the indirectly de- 

 duced total absorption 1/A^, and the action spectrum corrected for 

 efficiency shown by curve B/A . 



One is not often so fortunate in the control of optical density as in 

 the case cited. Reducing the population density often fails to produce 

 a typically "weak" absorption. While the transmission can be made 

 high, this may be the result of radiation which has not passed through 

 an organism, and hence only dilutes the transmitted radiation. Never- 

 theless, each cell may absorb heavily from the radiation which passes 

 through its volume, exhibiting the typical "thick sample" properties — 

 augmenting weak absorption and penalizing strong bands. This is 

 typical of spectra for unicellular organisms such as Euglena and Chlo- 

 rella as to chlorophyll absorption because of the great concentration in 

 the chloroplasts. In extreme cases strong bands may "saturate" and 

 exhibit reduced efficiency or reversal, owing to self screening. 



