Societies and Academies

Abstract

LONDON. Royal Society, April 21.—Sir Archibald Geikie, K.C.B., president, in the chair.—Lord Rayleigh: The incidence of light upon a transparent sphere of dimensions comparable with a wave-length. The investigation is on the basis of the electromagnetic theory of light, the transparent sphere being supposed to have a dielectric constant different from that of the surrounding medium. The case of a very small sphere, or of an obstacle of any size and shape under the restriction of very small refractivity, was treated in 1881. In the numerical calculations of the present paper the refractive index is supposed to be 1.5, and the ratio of circumference to wave-length has the values 1, 1.5, 1.75, 2, and 2.25. When the ratio in question is small and the incident light is unpolarised, the scattered light is polarised in all directions except, of course, those parallel to the incident ray; and the polarisation is complete at right angles to the primary ray. As the ratio increases, this condition of things is departed from. The maximum polarisation is now to be found in an oblique direction, inclining backwards. A little later the polarisation in certain directions is reversed, such changes occurring very rapidly as the ratio alters. Experiments similar to those made in 1881 upon sulphur particles precipitated from a dilute and acidified solution of “hypo” are described, and it is shown that a passage from red to blue light may reverse the polarisation, although there is no change either in the liquid or in the direction of observation.—Prof. Karl Pearson: The improbability of a random distribution of the stars in space.—Dr. R. D. Kleeman: The total ionisation produced in different gases by the kathode rays ejected by X-rays. The results are given in the annexed table, in which are also placed the total ionisations obtained by Prof. Bragg with the α particle. It will be seen that the two sets of values relative to air are very nearly the same. The energy spent in making an ion thus seems not to depend in any marked degree on the nature of the ionising agent.