242 



BIOLOGICAL EFFECTS OF RADIATION 



mental purposes is a bacterium, some protozoon form, or the well-known 

 fruit fly Drosophila melanogaster, it is easily possible to determine the 

 percentage of deaths at a variety of dosages with a high degree of accur- 

 acy, but if some of the larger forms are used, such as the rat or the rabbit, 

 the expense of the accurate determination of these percentages is usually 

 prohibitive. For that reason our best illustrations of lethal-dose curves 

 are found when the smaller forms are used, as for instance those given by 

 Packard (3, 4) for Drosophila eggs, two of which are given in Fig. 4. 



Since the change in percentage mortality is an S-shaped curve, 

 similar to that of accumulated frequencies of the normal probability 



I 



10 



Fig. 4. 



20 30 40 50 60 



Duration of Exposure (Minutes') 

 -Survivorship rate of Drosophila eggs exposed to X-rays of different intensities. 



curve, the probability paper previously described will be found useful 

 for its graphic representation. In some cases the observed points will 

 plot on such paper in essentially linear fashion and when a straight line 

 is drawn through the points the 50 per cent position representing the lethal 

 dose may be read directly from the graph. We may also read from such 

 graphs the standard deviation of the curve, which would be a measure of 

 the variation in the response of the organism to the radiation. In other 

 cases, such as those of Packard (3), referred to above, the percentage 

 mortality does not plot in linear fashion against the dosage on arithmetic 

 probability paper, due to the skewness of the S-shaped curve. This 

 skewness can sometimes be taken account of and a linear plot obtained 

 by using the logarithm of the dosage and making a plot on arithmetic 



