first part, which is not so influenced, but which follows the 

 law of $ rays strictly. 



It is easy to show, by comparing corresponding strata 

 at the front and back of one plate, that the incidence radia- 

 tion should be somewhat less than p tim.es the transmitted 

 radiation. Somewhat less, because the cathode radiation, 

 which is turned back, is scattered and softened in the process. 



To sum up : — On the ether-pulse theory we ought to find 

 perfect symmetry in the secondary radiations from the two 

 sides of a plate. But experiment shows nothing of the kind. 



On the material or neutral-pair theory, the "incidence" 

 radiations should follow the ^ ray law. This is known to 

 be the case. The emergence radiations should not follow the 

 )8 ray law, and experiment shows that they do not. If the 

 density law held for both )8 and y rays, and if the y rays 

 were homogeneous, the emergence radiations should all be 

 equal. As already explained, experiment shows that the dh- 

 served inequalities give promise of ready explanation, on the 

 ground that no one of these suppositions is quite true. 



It is, perhaps, better not to extend the preliminary ac- 

 courit of these experiments by any lengthy discussion of the 

 issues arising from them. Many points that invite considera- 

 tion have been discussed already in the papers first referred 

 to. Moreover our own further experiment are incomplete, 

 and their full interpretation is not yet certain. We will, 

 therefore, confine ourselves to one or two questions which seem 

 of special interest. The X-rays resemble the y rays so closely 

 that it is practically inconceivable that the two radiations 

 should be essentially different. The secondary cathode radia- 

 tions, which are set free w-hen X-rays impinge on any mate- 

 rial, must therefore have been part of the X-ray stream, and 

 must start their independent existence by moving on in the 

 line of the X-ray motion. Their velocity is much smaller than 

 that of the secondary cathode rays due to y rays, and they 

 are much more readily scattered. It may still remain an open 

 question whether or no the X-ray stream contains ether pulses. 

 Perhaps their existence must be supposed in order to explain 

 the velocity experiment of Marx, and the diffraction experi- 

 ment of Haga and Windt. Possibly they are also required 

 in order to explain Barkla's polarization experiments : but 

 we do not think that the experiment described by Barkla in 

 "Nature" (October 31, 1907) is in any way decisive. 



It seems proper to consider a possibility that the nega- 

 tive particle, when it moves on in the original line of motion 

 of the pair from which it came, retains also its original velo- 

 city. It is a striking fact that the cathode particle, due to 

 the y rays, has the same speed, very nearly, as the $ particle 



