XIV. X RAYS AND X I R R A D I A IM O \ 



463 



rays on the tissues. The survival curves, observed when bacteria 

 or other forms were irradiated, could be explained by a rationalization 

 of the Poisson equation (see Sect. G). The X rays were visualized 

 as distributed over the bacteria or, in general, the target as discrete 

 particles and in a random manner. Absorption of these X-ray par- 

 ticles was regarded as causing point-heat lesions in the tissues. For 



MEAN LETHAL DOSE {7) 



60 120 180 240 50 100 150 200 250 



DURATION OF EXPOSURE TO CHROMIUM X RAYS, sec. 

 Arithmetic scale Aritti.- log. scale 



Fig. 0. Effect of irradiating Drosophila spfrm with X rays from chromium 

 target tube: at left, survival data are plotted on linear percentage scale with doses 

 of X rays in r.; at right, the ordinate is logarithm of survival. Actual data curve 

 is irregular line. Best fitting Poisson equation is smooth curve. Advantage of 

 arith.-log grid at right is immediately evident in that Poisson equation on this 

 grid gives straight line plot. 



a given organism the chance that one of these absorptions would 

 fall in a spot vital to the organism was constant for the same amount 

 of X rays applied under the same conditions. Similarly, this chance 

 was identical with that of another bacterium of the same species 

 and imder the same conditions. For a unit dose of X rays, the chance 

 of death would be the percentage receiving these point-heat absorp- 



