THE arguments contained in Dr. Barkla's letter to NATURE ot May 7 do not directly affect the position which I have taken in respect to the nature of the γand X-rays. I have shown that all the striking phenomena of the secondary kathode radiations are simply and completely explained on a neutral pair theory, but not on the older theory of ether pulses. Dr. Barkla refuses to consider this large body of evidence on the ground that it is well to deal with the simple and then proceed to the complex, and he would consider only the secondary X-rays. I grant this principle, of course, but I object entirely to the application which he makes of it. It is the γrays which give the simpler effects, and the hardest γrays which give the simplest, for the obvious reason that such rays ignore atomic structure altogether even in the case of the heaviest atoms. The X-rays are soft, and therefore atomic structure influences and complicates the effects to a remarkable degree, as Dr. Barkla's own work shows. A true application of the principle would lead us to work out the laws of the hard γrays first, and then to consider the X-rays in the light of the knowledge we have obtained. This is what I have tried to do. The γrays suggest a corpuscular hypothesis, and on turning to the X-rays it is at once clear that a large proportion of the effects which they show may also be simply explained on the same hypothesis.