On the diffraction of light by particles comparable with the wave-length

This research was undertaken to investigate an effect first noticed by one of us (A. W. P.) when experimenting with the light scattered by suspensions of finely divided sulphur. These were obtained, in the well-known way, by depositing sulphur from a solution of thiosulphate of soda by means of a weak acid. When such a suspension is placed in the path of a beam from an arc lamp focussed on a screen, the image of the carbon is usually red, of greater or less depth according to the size and number of the diffracting particles. The production of this red colour has been satisfactorily explained by Lord Rayleigh as due to the selective scattering of blue light by particles which are small compared with the wave-length of the light. One of us noticed, however, that if time be given for the particles to increase in size (and possibly in number also) the solution after becoming nearly opaque becomes transparent again, but in this new stage an excess of blue is transmitted, which attains at one stage a deep indigo tint, this afterwards changing through various blue-green and green tints, to white.