The present paper describes a number of experiments made in connection with Christiansen's experiment in which a beam of light is passed through a transparent insoluble powder immersed in a liquid, with the result that light of the particular colour for which the indices of powder and liquid are the same passes unaffected, while light of all other colours is scattered. In this communication only the simplest case of the above is dealt with, that, namely, in which a flat piece of glass, ground on one side, takes the place of the powder, the rugosities of the ground surface representing a single layer of grains, and air takes the place of the liquid. In such a case there is, of course, no colour of light for which the indices of solid and liquid are alike, and indeed it was found that the colour of the light made very little difference to the results. On the other hand, however, it soon became apparent that different ways of grinding the glass surface led to very different effects. Accordingly, as no previous work appears to have been done in this field, and as it seemed a promising one, it was decided to make a systematic study of the various cases. Each specimen of glass employed was photomicrographed, and had its polar light distribution measured by a photometer. Two methods of characterising the particular scattering power of a screen soon suggested themselves and have been formally defined—the one connected with the Angle of Maximum Total Emission, and the other with the Equivalent Cavity.
It is hoped to continue the investigation not only on the above lines, but also in the direction of ascertaining the effect of a number of plates, i.e. of successive layers of light-scattering particles; and in investigating the polarisation effects, which some rough preliminary experiments have shown to be marked.
I am glad of this opportunity of acknowledging the help I have received from the Trustees of the Carnegie Trust in the form of grants for the construction of the special apparatus necessary; and desire to tender my grateful thanks to Professor MacGregor for the many facilities for carrying on the work which he kindly placed at my disposal.