Diffraction Bands Formed by X-rays about the Photographic Image of a Slit. B. WALTER. (Ann. d. Physik, No. 15, I924. )-It was first of all necessary to get X-rays as nearly monochromatic as possible. After trying a molybdenum anti-cathode, copper was found to be better for the purpose because its two characteristic lines are of twice the wave-length of those of the former metal, so that the band,s from the radiation emitted by copper would be separated by a greater distance than in the case of molybdenum. Furthermore the interposition of a sheet of nickel .oI ram. thick in the path of the rays from the copper anti-cathode cut out one line almost entirely, thus leaving the transmitted ray nearly of a single wave-length. This radiation then passed through a slit .006 mm. wide and then through a second slit, 75 cm. distant from the first, that was in the form of a wedge, tapering in its length of I8 ram. from a width of .04 to .oo ram. To prevent too great a reduction in the intensity of the rays by absorption in air, two vacuous tubes were inserted in the path of the rays, one between the two slits and the other between the photographic plate and the wedge-shaped slit. The experiment is essentially an attempt to photograph the bands about the wedge-shaped slit, radiation of wave-length 1.54 ร IO -s cm. being used. Reproductions in 76-fold magnification of the photographs obtained by an exposure of thirty hours are given. It is quite easy to see the dark diffraction band nearest to the wide central band. The author claims to see the second also and from its distance from the central band calculates the wave-length of the radiation employed to be 1.3I x lO -s cm. This is about 15 per cent. less than the correct value. The intensities of the two bands are, it is stated, in accordance with theory.
The results of this investigation are of great importance because they confirm from an additional line of argument the identity of X-rays with light except in the single feature of difference in wave-length.
G.F.S. The Solar Constant and Weather Prediction. (Explorations and Field-work of the Smithsonian Institution in I923. Smithsonian Mis. Coll., Vol. 76, No. IO.)--" The late Secretary Langley used often to express his prevision that the study of the sun's heat, the losses that it suffers in passing through our atmosphere, the variations which it may be subject to, would, at length serve to forecast the changes of weather and climate which are so important for the agriculturist, and which in some parts of the world even lead to periods of disastrous famine. He used to speak of Joseph's seven years of plenty and seven years of famine, in this connection, and of the possibility that in the future the student of the sun might be in a position to emulate that ancient prophet." Dec., I924.] ยข PRESS OF J. B* LIPPINCOTT COMPANy PHILADELPHIA