38() ltAi>IAT10.\ l5lOL(){JV 



racy or can l)«' asisunicd to he o(juivalent to the relative extinction eocffi- 

 eients of tlie su.sp(>('t(>cl substances. 



5. Th(^ reciprocity law is xalid i'oi' t lie I inics iMuploycd in t lu'cxpcriincnt . 



Since in many cases the \ali(lity ol all lliese assumptions is difficult or 

 impossible to demonstrate, the action-spectrum techni(|ue can be sug- 

 gestive, l)ut the results must be interpreted with caution. Nevertheless, 

 much useful information has been gained by this technicjue. Giese (1945) 

 has summarized the data on action spectra of various photobiological 

 effects and has listed seven general types. The observation of the maxi- 

 mum efficiency of bactericidal effects at ^2600 A early focused attention 

 on the purine and pyrimidine constituents of nucleic acids. 



SHAPE AND SIGNIFICANCE OF SURVIVAL CURVES 



Less uniformity in the form of survival curves has been observed with 

 ultraviolet than with ionizing radiations. 



Coblentz and Fulton (1924), working with thickly seeded plates of 

 E. coll, observed distinctly sigmoidal survival curves with a long threshold 

 exposure before any bactericidal effects were noticed. In some cases the 

 threshold exposure was nearly half the exposure recjuired to kill 90 per 

 cent of the organisms. It is doubtful if the bacteriological techniques 

 employed by these investigators were sensitive enough to detect small 

 amounts of inactivation. Similarly, Gates (1929a, b, 1930) observed sig- 

 moidal survival curves which he interpreted to indicate differences in the 

 sensitivity of individual cells. He computed the extremely skewed distri- 

 bution necessary to account for the observed curves on this hypothesis. 



Baker and Xanavutty (1929) observed survival curves with increas- 

 ingly steeper slopes when plotted semilogarithmically. They postulated 

 a cumulative action of a toxic substance produced by the ultraviolet 

 radiation as the mechanism. 



Wyckoff" (1932) obtained exponential survival curves when E. coli cells 

 were irradiated on the surface of agar plates with monochromatic ultra- 

 violet. Extending the analysis applied to his results Avith ionizing radia- 

 tion, Wyckoff suggested that the exponential curve indicated that the 

 absorption of a single quantum in a vital structure was sufficient to kill the 

 cell, but calculations using absorption data of Gates (1930) showed only 

 one in about 4 million quanta absorbed by the cell was effective. In 

 general, the quantal efficiency of ultraviolet killing of bacteria is very low. 



HoUaender and Glaus (1936) observed some deviations from exponen- 

 tial killing but, if their data were corrected for the known proportion of 

 double cells determined by microscopic examination, the observed sur- 

 vival could be fitted best by an exponential curve, and their theoretical 

 analysis was based on such a curve. Hercik (1937) observed exponential 

 killing with monochromatic ultraviolet irradiation of Bacillus megatherium 

 spores and vegetative cells. 



