IONIZATION AND BIOLOGICAL EFFECTS 111 



that we can read most easily with the yellow light. We would then 

 conclude that this light has the greatest intensity. Let us now photo- 

 graph the printed page when illuminated by the different colored lights 

 under the same conditions as before, using the same exposure time. 

 Upon development the plate exposed to the red light would show hardly 

 any effect, whereas that exposed to the violet light might be completely 

 fogged. According to this criterion the light of highest intensity would 

 be the violet one. Measuring the true intensity of radiation of each 

 light by substituting a suitable thermopile for the printed page, we may 

 find that it is the same for all the colored lights. In the case of the visual 

 observation we determined the effective intensity of the light by its 

 physiological action on the retina. With the photographic test the 

 effective intensity was determined according to the ability of the radiation 

 to produce a change in the emulsion. The two values of the effective 

 intensity thus determined for each of the colored lights, differ between 

 themselves and are both different from the true intensity of the light. 

 Furthermore, it is obvious that the results depend also on the eye {i.e., 

 whether color blind or not) and on the kind of photographic plate used. 



SIGNIFICANCE OF IONIZATION MEASUREMENTS 



The fact that in the foregoing example the effective intensity is found 

 to depend on the effect and "apparatus" by which it is measured serves 

 to bring out another important point in ionization measurements. The 

 effect by which we have chosen to measure X-rays is ionization. As 

 ionizable medium we have adopted atmospheric air primarily because 

 its constituent elements do not differ materially in atomic number from 

 those of living matter which are chiefly responsible for the ionization. 

 But we have said nothing about the apparatus to be used. Since we are 

 interested in the determination of the number of ion pairs produced per 

 second in a definite mass of air by its interaction with the radiation at a 

 given point in a beam, it is evident that we must know the volume (also 

 the temperature and pressure) of the air in which the measured ionization 

 is produced. This may be done by means of a closed ionization chamber, 

 in which case the volume is that of the chamber, or by means of an open 

 ionization chamber of the standard type (cf. L. S. Taylor) in which the 

 ionized volume is determined indirectly. In the case of a closed chamber 

 it is obvious that the radiation cannot reach the air therein without first 

 traversing the enclosing structure. In so doing, it reacts with the mate- 

 rial of the enclosure and produces secondary electrons and photons. 

 Some of these reach the air in the chamber and produce ions. Conse- 

 quently, the ionization which is measured in the enclosed air is made up of 

 two parts: one which is due to the interaction of the radiation with the 

 air in the chamber, and the other which is derived from the interaction 



