On the Theory of Illumination in a Fog. 443 
I need hardly say, in conclusion, that I do not in the least 
intend to convey the impression that the actual structure of 
the zxther is a bit like what I have described. What phy- 
sicists ought to look for is such a mode of motion in space as 
will confer upon it the properties required in order that it may — 
exhibit electromagnetic phenomena. Such a mode of motion 
would be a real explanation of these phenomena. I have only 
given a description of them. 
I think, however, that it is worth while considering these 
models, because in them the disturbance which represents light 
is not the same as the vibrations of an elastic jelly, for what 
represents an electric displacement is a change of structure of 
an element, and not a displacement of the element; and it 
seems almost certain that, notwithstanding the very high 
authority which seems to support the view that the ether is 
like an elastic jelly, nevertheless its vibrations are much 
more of the nature of alterations in structure than of dis- 
placements. 
XULVIIT. On the Theory of Illumination in a Fog. 
By Lord Rayueien, /.R.S.* 
A® a step towards a better understanding of the action of 
fog upon light, it seems desirable to investigate what 
the phenomena would be in the simplest case that can be 
proposed. Jor this purpose we may consider the atmosphere 
and the material composing the fog to be absolutely transpa- 
vent, and also make abstraction from the influence of obstacles, 
among which must be included the ground itself. 
Conceive a small source of radiation, e. g. an incandescent 
carbon filament, to be surrounded by a spherical cloud, of 
uniform density, or at any rate symmetrically disposed round 
the source, outside of which the atmosphere is clear. Since 
by hypothesis there is no absorption, whatever radiation is 
emitted by the source passes outward through the external 
surface of the cloud. The effect of the cloud is to cause 
diffusion, 7. e. to spread the rays passing through any small 
area of the surface (which in the absence of the cloud would 
be limited to a small solid angle) more or less uniformly over 
the complete hemisphere. 
Whether the total radiation passing outwards through the 
small area on the external surface of the cloud is affected by 
the existence of the cloud depends upon the circumstances of 
the case. If it be laid down that the total emission of energ 
from the source is given, then the presence of the cloud 
* Communicated by the Physical Society: read April 25, 1885. 
