28 Stokes, on the Nature of the Rovtgen Rays. 



Reverting to the expression for f given by the double 

 integral, we see that the only portion of the shell which 

 is efficient in producing a subsequent disturbance at P 

 lies between the sphere round 0 with radius 0 Q and 

 the sphere round P with radius PS. If /3 be the 

 distance from 0 P of the intersection of these spheres, 

 we have, considering the smallness of the obliquities, 



P 2 =— j— 7 ' 



' r-\-r 



If we suppose r and r' to be each 4 inches, and X the 

 millionth of an inch, we have /3= 0*002 inch, so that at 

 a distance not less than the one-25oth of an inch from 

 the projection of the edge of an opaque body intercept- 

 ing Rontgen rays coming from a point 4 inches off, and 

 received on a screen (fluorescent or photographic) 4 inches 

 on the other side, there would be full effect or no effect 

 according as we take the illuminated or the dark side 

 of the projection. We see then how possible it may be 

 to have an almost complete absence of diffraction of the 

 Rontgen rays if the pulses are as thin as above supposed ; 

 and as these rays are started in the first instance in a 

 totally different manner from rays of ordinary light, 

 namely, by the arrival of charged molecules from a 

 cathode at a target instead of by the vibrations of the 

 molecules of ponderable matter, we know of no reason 

 beforehand forbidding us to attribute an excessive thin- 

 ness to the pulses which the charged molecules excite 

 in the ether. 



