586 WHEELER P. DAVEY 



trometer or in terms of the Christen "Half Value layer." The 

 other factors may be given in terms of the reading of a Kienbock 

 strip or a Holzknecht pastille, etc. ; or better, 



b) The voltage and distance are given directly and the prod- 

 uct of the current and time is given, thus, 



"100 milliampere-minutes at 25 cm. distance at 50 kilovolts." 

 This is usually contracted to read 



MAM 

 100-^ at50KV. 



252 



It will be noticed that the distance is expressed in terms of 

 its square. This is because the intensity of X-rays varies in- 

 versely as the square of the distance. Too much stress can not 

 be laid upon the necessity for recording the voltage, and for 

 keeping the voltage-reading constant, for not only does the pene- 

 trating power of the X-rays depend upon the voltage, but even 

 the quantity of rays given off by the tube per milliampere depends 

 very largely upon the voltage. 



In figure 2 B, 'days life' is plotted against the logarithm of 

 the X-ray dose. The 10 per cent, 25 per cent, 50 per cent, 75 

 per cent and 90 per cent points of the curves for 500, 1000, 2000 

 and 4000 KV. He on a family of straight lines Y = A - B log. X 

 where X is the X-ray dose and Y is the number of days life 

 after raying. All these lines meet the zero line at the point 



(64,000). The points for 8000 and 15,500 ^^^ at 50 KV. 



do not lie on these lines, but when taken along with the points for 



4000 ^ at 50 KV., they are found to form a new family of 



curves of the same type as the first, but with a steeper slope. 

 The interpretation of this is given later. It should be noted 



here, however, that beetles rayed more than 4000 — —— at 50 



KV. were unable to move their legs and antennae easily, but 

 that this effect was not noticed in beetles rayed less than this 

 amount. For this reason it was difficult at the higher dosages 

 to obtain data as accurate as that obtained at the lower dosages. 



