Biological Effects of Penetrating Radiations 93 



and is suspended in air so that scattered radiation reaching it 

 is at a minimum, the absorption of energy is uniform throughout 

 the object irradiated and is directly proportional to the intensity 

 of the radiation beam. For example, if a large number of Droso- 

 pliila eggs is exposed to an X-ray beam of unknown intensity 

 for 10 minutes and if, as a result, half the individuals fail to 

 hatch, then 180 rontgen units have been delivered at the rate of 

 18 r/min.^^ The constancy with which such quantitative experi- 

 ments yield the same result is perhaps one of the most striking 

 features of this type of investigation. With Drosophila eggs the 

 error is not more than 3%,^^^- ^*^^ and this order of accuracy is 

 obtained with other types of biological material under laboratory 

 conditions. 



A great variety of organisms has now been used as biological 

 indicators of radiation action by many observers, and each ma- 

 terial has its advantages and its limitations. The most important 

 consideration is that the experimenter shall be familiar with the 

 material chosen for experiment, and be able to distinguish with 

 certainty the changes produced by radiation and those uncon- 

 nected with it. 



These indicators are of particular use where the biological 

 effects of two different types of radiation, with no physical unit 

 of measurement in common, are being compared ; for example, a 

 comparison of the biological effects of X-rays and neutrons.*^- ^^"^ 

 If the biological response can be matched, then a useful com- 

 parison of the physical conditions of irradiation is obtained. 

 Biological indicators are also useful to establish the relationship 

 between injury produced by radiation and other types of injury, 

 e.g., to determine whether the effects of two agents are additive, 

 equal, unrelated, or whether one is capable of potentiating the 

 other. ^^*-^ The indicators should be small in size, easily available 

 in large numbers at all times, they must show only a small and 

 definite amount of normal variation, and the reaction to radia- 

 tion must be sharp and easily measured.^'' Some investigations 

 may be simplified by using a response which is independent of 

 the time factor. Since radiosensitivity varies enormously with 

 stage of development, it is essential that the greatest care is taken 



