734 Mr. & Mrs. Soddy and Mr. A. S. Russell on the 



exponentially by the plate, the absorption coefficient being 

 represented by X in the sense of the equation 



It is required to find the quantity of radiation after it has 

 passed through the plate. Let E be the intensity of the 

 radiation at any point. Let I and I T , respectively, be the 

 measure of the total radiation over the angle of 180°, with 

 the absorbing plate absent and present. Within a solid 

 angle So) the intensity of the radiation is E8o> with no plate 

 present, and E . %<o . 6 ~ Xt / C0S 9 with the plate. 



Sco = S( — 2tt cos #), 



I 



■Xt/cos 9 



= f Ed(-2ircos0) = 2n-E, 



I T = E(« 



Jo 



^o-Jo 



)d(—2vcos0), 





Xt/cos 



^(-27TCOS^). 



Let \T/cos 



It 



Io 



The integral 



Jat y j « 



Xt 





(/y. 



— . dy is known as the Exponential 



Integral. It is expressed by the symbol ~Ei(x). Values 

 both for Ei(x) and Ei( — a?) have been tabulated by J. W. L. 

 Glaisher (Phil. Trans. 1870, clx p. 3G7). The result may 



