1912-13.] The Scattering of Light. 281 
imagine all the flat places sloped off into one or other of the adjoining 
cavities, then we should produce an excess of slightly sloping area. F urther 
discussion of this point, however, had better be postponed until the 
publication of the succeeding paper. 
§ 4. Summary. 
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. 
Department of Physics, 
Edinburgh University. 
( Issued separately July 16 , 1913 .) 
