A. C. REDFIELD AND E. M. BRIGHT 37 



can be compared with a measure of the same beam obtained by some 

 standard physical method. Since we have shown that the volume 

 of the membrane is a direct function of the logarithm of the intensity 

 of radiation through a considerable range of membrane volumes, 

 it is a simple matter to calibrate each lot of eggs so that the intensity 

 of radiation can be approximated from any known membrane volume.® 

 To do this a series of measurements was made of the physiological 

 effects of various intensities of the entire radiation from a tube of 

 radium emanation, the intensity being determined by the distances 

 between the tube and the eggs according to the formula of Wood and 

 Prime. ^° From data obtained in this way a curve, such as that shown 

 in Fig. 3, was drawn relating the intensity of radiation with the re- 

 sulting membrane volume. This curve was then used to determine 

 the relative intensity of physiologically effective radiation in each 

 beam of homogeneous /3-rays by finding on the curve the intensity 

 which corresponds to the average volume of the membranes of each 

 lot of eggs. 



Having obtained in the preceding manner a measure of the rela- 

 tive intensity of the physiologically effective radiation in each beam 

 of i8-rays of homogeneous velocity it became necessary to determine 

 the total intensity of each beam by some standard physical method. 

 Although the relative number of ;S-particles of various velocities 

 given off by tubes of radium emanation in equilibrium with its prod- 

 ucts has been determined with precision,^^' i- two considerations made 

 it desirable to obtain this information with the apparatus used for 

 the physiological determinations. In the first place the slit, S, used 

 to separate the rays into homogeneous beams was of finite dimensions' 

 so that it was only possible to obtain an approximation to homogene- 

 ity. Since our physiological method of measurement was relatively 

 insensitive it was necessary to keep the distances through which the 

 rays travelled short and the slit wide in order that the effect upon 

 the eggs might be great enough to be measured accurately. Another 

 source of error in the method was due to the fact that a considerable 



1° Wood, F. C, and Prime, F., Jr., Ann. Surg., 1915, Ixii, 751. 

 " Wilson, W., Proc. Roy. Soc. London, Series A, 1909, Ixxxii, 612. 

 ^^ Wilson, W., Proc. Roy. Soc. London, Series A, 1911, Ixxxv, 240. 



