244 



BIOLOGICAL EFFECTS OF RADIATION 



to the 50 per cent position, we may determine the lethal dose. For this 

 particular illustration the lethal dose, as read from Fig. 5, is approxi- 

 mately 0.40 Ti. 



While the procedure just outlined is obviously an approximate 

 method, it will determine the lethal dose as accurately as figures of this 

 sort warrant. If we also read from the graph the number of units of 

 dosage corresponding to the standard deviation, we have a measure of 



0.0 0.1 0.2 0.3 0.4 0.5 0.6 



D u ra t i n of R en d i a t i o n 

 Fig. 5. — Graph for the determination of lethal dose. 



0.7 



the individual variability of animals in their reaction to the radiation. 

 The value of the standard deviation so read is slightly higher than it 

 should be, for although the two assumptions given above do not disturb 

 the centering point, they do tend to increase the spread of the S-shaped 

 curve describing the lethal action. 



CURVE FITTING 



The effects of radiation will often be measured in terms of more than 

 one variable and a summary of the results of such cases will necessitate 



