264 
Proceedings of the Royal Society of Edinburgh. [Sess. 
XXIV. — The Scattering of Light. By James Robert Milne, D.Sc. 
(With Two Plates.) 
(Bead March 21, 1910. MS. received May 1, 1913.) 
§ 1. Introduction. 
In most optical text-books a reference will be found to “ Christiansen’s 
experiment.” * Christiansen passed a beam of light through a liquid 
containing a transparent insoluble powder, the refractive index of the 
powder being the same as that of the liquid for one, and only one, colour 
of light. For that colour, accordingly, the mixture is optically homogeneous, 
but for all other colours it is highly heterogeneous and the light is 
scattered, with the result that the directly transmitted light is approxi- 
mately all of one wave-length, and with increasing thicknesses of mixture 
becomes more and more so. Lord Rayleigh has shown experimentally 
that the selective action of such a mixture may be surprisingly efficient, 
and in an experiment carried out by him with improved arrangements and 
a layer of mixture 15-20 mm. thick, he estimates that the extreme 
differences of wave-length in the transmitted light amount to no more 
than two and a half times the distance between the two D lines.-)* 
It has for some time seemed to me that it might be worth while to 
follow up this experiment, and further consideration has only served to 
show how wide is the field thus suggested. Not only is the choice of 
materials, both solid and liquid, very large ; but the conditions of the 
experiment, as regards temperature, thickness of layer, size of particles, 
etc., admit of almost infinite variation. In each case, however, it is 
clear that the results must depend on quite definite laws, and although 
these may be somewhat complicated, it is reasonable to expect that they 
could be elucidated by a sufficient series of well-considered experiments. 
No particular difficulty need be anticipated in carrying out the necessary 
photometric and other measurements. And the outcome of such an 
investigation would probably be of considerable interest both from a 
theoretical, and also perhaps from a practical point of view. Hitherto 
nothing of this kind appears to have been done, at least of a systematic 
nature ; such investigations on the subject as I am aware of have all 
* JVied. Annal ., Band xxiii. p. 298, Nov. 1884. Description by Christiansen of his 
original experiment. 
t “ On an Improved Apparatus for Christiansen’s Experiment,” Lord Rayleigh, Phil. 
Mag., xx. p. 358, 1885; also Nature, lx. p. 64, 1899. 
