478 RADIATION BIOLOGY 



caused by one ionization or activation or, at most, by a minute natural 

 cluster of them receives strong support from two other series of results, 

 concerned with the voltage (or wave length) of the radiation used and 

 with its timing (or intensity), respectively. After the early experiments 

 of Hanson (1928), which indicated that the mutagenic effectiveness of 

 7 rays and of ^ rays of radium on Drosophila spermatozoa is the same for 

 the same dose, tests carried on in Russia by Schechtmann (1930) and 

 by Efroimson (1931) indicated that hard and soft X rays were of like 

 mutagenicity in this material. Following this work, very exact studies 

 by Timofeeff-Ressovsky and his co-workers (Pickhan, 1935; Timofeeff- 

 Ressovsky and Zimmer, 1935b; and Zimmer et at., 1937), again on sex- 

 linked lethals induced in Drosophila spermatozoa, demonstrated the equal 

 effectiveness of ordinary X rays, of y rays of radium, and of ^ rays of 

 radium, while Wilhelmy et al. (1936) found that X rays of the Grenz-ray 

 wave length of 2.0 A on up through the shortest wave lengths gave, for 

 equal doses, no statistically significant differences. It is true that, with 

 the longest rays tested (2.0 A), there was a suggestion of a slightly lower 

 effectiveness. However, results soon afterward obtained by Fricke and 

 Demerec (1937) showed no difference between the effectiveness of 2.2- 

 and 0.94-A radiation. Similarly, Stubbe (1933) showed the mutagenic 

 equivalence of X rays from the level of 10 kv to that of 175 kv in Antir- 

 rhinum. Thus within very wide limits the manner of scattering of the 

 ionizations and activations had no detectable influence on the result. 

 The conclusion was thereby indicated that the effective agents were either 

 the single ionizations and/or activations or the small clusters of them 

 formed in 5 rays or at the ends of electron tracks, inasmuch as these 

 clusters occur with roughly equal frequency for a given dose in the case of 

 all the radiation in this entire range. 



In timing experiments with X rays, it was found by Patterson (1931) 

 and by Timofeeff-Ressovsky and Zimmer (1935a) that fractionated doses, 

 separated by intervals of days or weeks, produced sex-linked lethals in 

 Drosophila sperm at the same frequency as did the same total dose 

 applied in one treatment. In Timofeeff-Ressovsky and Zimmer's work, 

 weak but long-continued treatments also were found to have the same 

 effectiveness as strong but brief treatments which delivered the same 

 total dose. The time and intensity values in this work differed by a 

 factor of 300. In work conducted by Muller and Ray-Chaudhuri 

 (Muller, 1939a, b, 1940; Ray-Chaudhuri, 1939, 1944) the intensity was 

 reduced to approximately 0.01 r/minute without causing the effectiveness 

 of a given total dose of 7 radiation to be less (or more) than that of 

 X rays delivered at 100 r/minute — an intensity difference of 10,000. 

 Finally, in the work of Uphoff and Stern (1949) the range was extended 

 down by another order of magnitude, to approximately 0.001 r/minute, 



