Rediffusion of cathode rays.—The available evidence is reviewed and conclusions formulated on the rediffusion constants and distribution functions needed for the calculation of effects on x-ray emission intensities.

Thin-target x-ray intensities.—A previous paper gave the intensity of the $K$ lines from very thin Ag supported on Be, as a function of voltage, in terms of the intensity at twice the excitation voltage as a standard. In the same terms after correction for two opposing effects, diffusion of cathode rays within the thin target and rediffusion from the beryllium, the corrected values differ from the originals by not over 2 percent. Possible sources of error are reviewed, and it is concluded that none can increase this percentage very greatly. This answers a criticism on the theoretical significance of these data.

Comparison with theories.—Present wave-mechanics theories involve approximations not valid at low voltages, but subject to this qualification Bethe's theory compares very favorably with these data. The best classical-mechanics theory is Thomas's. Empirically, the $\mathrm{intensity}=\mathrm{const}.\mathrm{}\times U^{-0.8\mathrm{}} log U$, where $U=\frac{\left(\mathrm{tube}\mathrm{voltage}\right)}{\left(\mathrm{excitation}\mathrm{voltage}\right)}$.

Thick-target effects.—For line intensities percentage corrections for rediffusion, again taking the intensities at $U=2\mathrm{}$ as standard, are more than twice those for thin targets at all voltages in the above range, and are of opposite sign. On continuous spectrum intensities averaged over all directions, these calculations confirm Kramers' suggestion, that most of the non-linear term in the intensity formula is due to rediffusion.

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