37 0 Proceedings of Eoyal Society of Edinburgh. [sess. 
by von Helmholtz, H. A. Lorenz, J. J. Thomson, FitzGerald, 
and Rayleigh.* None of them has quite dared to say that the 
physical action represented by his formulas for this case is a to- 
and-fro motion of the ether perpendicular to the plane of inci- 
dence, reflexion, and refraction ; nor has any one, so far as I 
know, absolutely determined whether it is the lines of electric 
force or of magnetic force that are perpendicular to that plane in 
the case of light polarized by reflexion at the surface of a trans- 
parent medium. For the action, whatever its physical character 
Fig. 1. 
may be, which takes place perpendicular to that plane, they all 
seem to prefer “electric displacement, ” of which the only con- 
ceivable meaning is motion of electricity to and fro perpendicular 
to the plane. If they had declared, or even suggested, definitely 
this motion of ether, they would have been perfectly in harmony 
with the undulatory theory of light as we have it from Young and 
Fresnel. We shall return to this very simple problem of reflexion 
and refraction of purely distortional waves in which the motion is 
perpendicular to the plane of the three rays, in order to interpret 
in the very simplest case the meaning, for a solitary wave, of 
* See Glazebrook’s “ Report on Optical Theories” to British Association, 
1885. 
