58 



BIOLOGICAL EFFECTS OF RADIATION 



plotted points on these curves are from the corresponding curves of 

 Fig. 6, each point on the extreme left indicating the thickness of copper 

 (about 0.1 mm.), which would make up the difference in effect of the walls 

 of the two tubes. As may be expected, this thickness varies somewhat 

 with the excitation potential, because the spectral absorption in glass is 

 not the same as in copper. 



From the consistency of this correlation, in which it is obvious that 

 absorption curves corresponding to different excitation potentials do not 

 fit, it is to be concluded that the shape of the absorption curve is char- 

 acteristic of the excitation potential. On the other hand, the coordinate 



Table 5.— Aluminum Equivalents of Lime Glass for Radiations Excited by 

 Various Constant Potentials and Filtered by 0.45 mm. of Lime Glass 



displacement of the given curve from that of the basic curve is char- 

 acteristic of the filtration between the target and the point of measure- 

 ment. Having a family of such curves (as in Fig. 5), in which the 

 constant excitation potential varies by steps of practicable magnitude, it is 

 clear that instead of using the complete absorption curve to specify the 

 quality of the radiation, we may use the more convenient magnitudes: 

 excitation potential (constant) and initial filtration. Since the radiation 

 from various wave forms of excitation potential may be simply and 

 adequately equated to that of some equivalent constant-potential excita- 

 tion, the quality of any given radiation may he specified hy its equivalent 

 constant excitation potential and the initial filtration. This initial filtration 

 is desirably expressed in terms of the equivalent copper or aluminum. 



Absorption in Tube Walls.— Y or the purpose of evaluating the absorp- 

 tion of radiation by the glass tube walls, copper and aluminum equivalents 

 of lime and pyrex glass are given in Tables 4 and 5. At least in the case 

 of the thick pyrex glass tubes now commonly used, the tube-wall filtration 

 should be taken into consideration when describing the radiation quality. 



