26 Stokes, on the Nature of the Rontgen Rays. 



at least as regards its amount, when we pass from point 

 to point in a normal direction, vanishing at the outer 

 and inner boundaries of the shell. 



As the disturbance we are concerned with is of the 

 distortional kind only, the disturbance at time / at a 

 point P in front of the shell maj- be obtained from that 

 at time o in the shell in its position which is taken as 

 initial by the last equation in Art. 22 of my paper on 

 diffraction already cited. Let R be a point in the shell 



of disturbance when in that position which is regarded 

 as initial, r, r' the distances P R, 0 R ; 0,0' their incli- 

 nations to OP; <jj the azimuth round Of of the plane 

 PRO. Then in the formula referred to ^cr=sin 0 dO d<p. 

 Also rdOxsin (0+0') = dr' ; and sin 0/sin (0+6')=r'IOP 

 = r' l(r-{-r') very nearly. 



Let 0 P cut the inner boundary of the shell in S, and 

 let a b or Q S, the thickness of the shell, be denoted by A. 

 In the equation referred to, the term arising from the 

 differentiation with respect to t of the t outside the sign 

 of double integration will be of the order \jr' as com- 

 pared with the others, and may, therefore, be neglected, 



0 



