EFFECTIVENESS OF X-RAY WAVE-LENGTHS 463 



dose, measured in roentgens, produces very different degrees of reaction 

 depending on the length of exposure, that is, on the intensity of the 

 beam. Holthusen (22) has shown that an increase in the duration ol 

 exposure must be accompanied by an increase in the total dose if the 

 same degree of erythema is to be obtained. Thus 500 r, delivered in 

 1 min. produce the same results as 900 r given in 50 min., or 1400 r given 

 in about 8 hr. In these tests Holthusen used both hard and soft X-rays 

 and also gamma rays. The latter were calibrated by a biological method 

 similar to that used by Failla and Henshaw, except that Ascaris eggs 

 were used. When equal doses of gamma and X-rays are given in equal 

 times, the reaction is approximately the same, the gamma rays being 

 slightly more active. 



One disadvantage of the skin as a test object is that it cannot be 

 uniformly radiated throughout. A varying amount of the incident 

 energy is absorbed in the outermost layers, the proportion depending 

 on the penetrating power of the rays. Furthermore, the effect produced 

 by the primary beam is augmented by scattered radiation, especially 

 in the case of the moderately hard rays. 



Better test objects are small, growing organisms such as eggs, seeds, 

 bacteria, and the hke, all parts of which receive approximately equal 

 amounts of radiant energy. In these also scattered radiation is negligible. 

 There are several criteria by which the usefulness of these various 

 organisms can be judged. 



a. The effect of the radiation must be clear-cut and easily measured. 

 In the case of seedlings it is the rate of growth or the proportion of 

 specimens whose growth is completely inhibited. For eggs, it is the 

 failure of the larva to hatch. 



b. A high degree of sensitivity is a valuable asset, for it permits 

 small doses and short exposures. The latter are especially desirable, 

 for during long exposures the organisms not killed immediately continue 

 their development and grow more and more resistant. In the accom- 

 panying Table 1 are shown the doses necessary to kill 50 per cent of the 

 samples of various organisms, or to reduce their growth to half of that 

 shown by the controls. 



Table 1 



R 



Axolotl eggs (25) 50 



Drosophila eggs (30) 190 



Vicia seedlings (13) 300 



Ascaris eggs (21) 1 'OOO 



Wheat seedlings (16) 1-200 



Mesotaenium (26) 9-000 



Saccharomyces (12) 42 ,000 



The figure given for Ascaris eggs is only approximate, for the eggs from 

 different worms vary in sensitiveness within very wide limits. The 



