462 BIOLOGICAL EFFECTS OF RADIATION 



while those of gamma rays, which represent voltages of the order of 

 1000 kv. and more, are very uncertain. 



Quantitative biological effects are determined by measuring the 

 magnitude of the reaction of various test objects to the rays. First to 

 be used for this purpose was the human skin, a natural choice because the 

 first radiologists were concerned almost wholly with clinical effects. 

 Because at that time no accurate means of measuring intensities had been 

 devised, the erythema reaction itself was used as a unit of dosage. It is 

 still employed widely although it lacks precision. The degree of redden- 

 ing of the skin is subject to considerable variation depending on the 

 natural pigmentation of the skin, on the part of the body which is exposed, 

 and on the age and health of the patient. Furthermore, the magnitude 

 of the reaction varies with the area under exposure and with the intensity 

 of the beam. For these reasons an absolute erythema dose cannot be 

 defined in terms of physical units. 



The older literature shows little uniformity of opinion regarding the 

 effectiveness of different wave-lengths in producing a typical skin reac- 

 tion. Many of the conclusions arrived at before 1928 when the inter- 

 national unit, or roentgen, was adopted, are questionable because of inac- 

 curate measurement of dosage, and because no account was taken of the 

 part played by differences in intensity. The general opinion was that 

 soft radiations are more effective than hard. Recent observations, 

 however, have shown that this is not true (36). When the erythema 

 doses for beams of various degrees of softness are measured by a standard 

 instrument they are found to be identical. So also Hess (17) whose first 

 measurements indicated that rays generated at 60 kv. were twice as 

 effective as those produced at either 9 or 185 kv., discovered when his 

 dosimeter was properly calibrated, that all three doses were actually 

 the same. Hickey and Pohle (18) and others give evidence to show that 

 this equality of effect extends over a wide range of wave-lengths, when 

 the area of exposure is small and the duration of exposure is constant. 

 But Reisner who reviews the subject in detail comes to the conclusion 

 that while within the region of moderately hard rays (0.14 to 0.22 A) 

 all wave-lengths are equally effective, still softer radiation is much more 

 active. In opposition to this view are Failla and Henshaw (7) who state 

 that the hard gamma rays of radium are more effective than 160-kv. 

 X-rays. In measuring these two qualities they used both physical and 

 biological methods. The latter consist in determining the doses of both 

 radiations which kill the same proportion of Drosophila eggs. Equal 

 doses thus measured are not equally effective on the skin, the gamma 

 rays being more active. 



Without doubt inaccurate measurement is in part responsible for 

 these wide variations in opinion. But an important source of error lies 

 in the neglect of the time factor (20) . It is now known that the same total 



