The spectrum of molybdenum K α rays scattered by graphite at 45 • , 90 • and 135 • has ben compared with the spectrum of the primary beam. A primary spectrum line when scattered is broken up into two lines an "unmodified" line whose wave-length remains unchanged, and a "modified" line whose wave-length is greater than that of the primary spectrum line. Within a probable error of about 0.001 A, the difference in the wave-length (λ − λ0) increases with the angle θ between the primary and the

more »

... ered rays according to the quantum relation (λ − λ0) = λ(1 − cos θ), where λ = h/mc = 0.0242 A. This wave-length change is confirmed also by absorption measurements. The modified ray does not seem to be as homogeneous as the unmodified ray; it is less intense at small angles and more intense at large angles than is the unmodified ray. An X-ray tube of small diameter and with a water-cooled target is described, which is suitable for giving intense X-rays.