146 Dr. C. A. Sadler and Mr. P. Mesham on Rontgen 



and from (2) 



and hence 



e X ' X2 [m l m 2 -n 1 n 2 ] — ^ 1+V ' r2 [??2. 2 — ?? 2 ] -m 1 mj ! 7i 1 + m 2 ?? 2 n 1 = 0. (3) 



IB ,r l = .?' 2 = .?', ?. £., if the same thickness of absorbing material 

 be used in obtaining- the same successive absorptions, 



m x = ?>? 2 = ni 



and tii — n 2 — n 



and equation (3) reduces to the simpler form, 



or \'x -. . r o .r(X + X')n O X'ff , (X-"-X').r A 



All the quantities in this equation are known, and since 



m = e , 



\ Y the unknown absorption coefficient of the second radiation 

 may be calculated. 



To test the order of accuracy to be obtained by using this 

 formula, a mixed radiator of Cu and Fe was made up and 

 subjected to the radiation from As, the mixed tertiary beam 

 in this case consisting essentially of the homogeneous radia- 

 tions characteristic of Fe and Cu. A sheet of paper, placed 

 in the path of the tertiary beam, gave a first absorption of 

 37'0 per cent. A similar sheet of paper was then placed in 

 the path of the beam, and a second absorption found with 

 the first sheet of 35'5 per cent, (this method ensures that x 

 is the same for each absorption). Taking the absorption 

 coefficient of the Cu radiation as known to be 129, and 

 supposing that of the Fe radiation to be unknown, we have 



e Xx = 1-5873 ; e X ' x = 1*5504 ; n = 1-3890, 

 substituting these values in the above equation, 



m = 1-904. 

 From this, we get as a value of the absorption coefficient 

 for the Fe radiation, 



129 X log (1-904) 

 ~ log (1-3890) 

 = 253 

 against a known value for Fe radiation of 239. 



