468 BIOLOGICAL EFFECTS OF RADIATION 



mental influences, and the influence of the time factor. Their easy avail- 

 ability at all times, in large numbers, makes them exceptionally good 



test objects. 



The experiments of Packard are summarized in Fig. 1. Each point 

 is determined from the average of several tests involving several hundred 

 eggs. The doses were measured b^ open ionization chambers, the 

 intensity of the softest radiation (1.5 A) being obtained with special care. 

 In this instance every source of inaccuracy was investigated and the 

 necessary corrections made (5). The intensity determined by the bio- 

 logical method differed from that obtained from ionization readings by 

 less than 2 per cent. The hardest radiations, generated at 550 kv. could 

 not be measured with equal precision, but the biological results are in 

 fair agreement with those obtained at lower voltages (27). Henshaw 

 (16), who has studied the effectiveness of 700-kv. X-rays, finds that it does 

 not differ significantly from that of much softer radiation. These results, 

 together with those of Glocker (14), give no evidence that there is any 

 point of maximum or minimum effectiveness throughout this very wide 

 range of wave-lengths. 



A comparison between the activity of gamma rays and X-rays has 

 been made by Simon (37). Virgin Drosophila were irradiated, then 

 mated, and the percentage of fertile eggs determined. The intensity of 

 radiation was measured in ergs/cc. according to the method of Stahel. 

 The effect produced by equal doses of both radiations, when given in 

 equal lengths of exposure, was the same. But short exposures proved 

 to be more effective than long, the total doses being equal. 



The course of the survival curve of Drosophila eggs remains constant 

 regardless of the quality of radiation used as a lethal agent (29) . This is 

 true also for the mortality curves of Ascaris, of yeast, algae, and bacteria. 

 But according to Glocker (14) there are a few test objects whose mortality 

 curves are affected by changes in the wave-length of the incident beam, 

 the soft rays being more effective in small doses while the reverse is true 

 for large doses. This phenomenon he attributes to the size of the 

 quanta. A discussion of the quantum theory is presented elsewhere in 

 this volume. 



One more test of a different kind may be mentioned. Fricke and 

 Petersen (9) have investigated the action of hard and soft rays on hemo- 

 globin. The laked blood is exposed to measured doses of three wave- 

 lengths. The hemoglobin is gradually transformed into methemoglobin, 

 the exact amount being determined by a^ spectrophotometer. They 

 conclude that radiations of 0.75 and 0.54 A produce identical results, 

 while shorter rays (0.25 A) are very slightly more effective. In further 

 experiments Fricke and Morse (8) irradiated acid solutions of ferrous 

 sulphate with homogeneous beams of approximately the same wave- 

 lengths previously used. Analysis showed that in each case the amount 



