371 
1898-99.] Lord Kelvin on Reflexion and Refraction. 
the “ change of phase ” discovered by Fresnel and investigated 
dynamically by Green for a procession of periodic waves of simple 
harmonic motion experiencing “total internal reflexion.” (See 
§ 20 below.) 
§ 9. Meantime we take up the problem of the four reflected 
and refracted waves produced by a single incident wave of purely 
distortional character, in which the motion is in a plane perpen- 
dicular to the five wave-fronts. Taking this for XOY, the plane 
of our diagram, let YOZ he the interface between the two 
mediums. We shall first consider one single incident wave, I, 
of the purely distortional character. By incidence on the inter- 
face, it will generally introduce reflected and refracted waves I', 
I,, of its own kind, that is purely distortional, and J', J /5 reflected 
and refracted waves of the condensational-rarefactional kind. The 
diagrams represent, for two cases, sections of portions of the five 
waves by the plane XOY. F and R show the front and rear of 
each wave ; and the lines of shading belonging to it show the 
direction of the motion, or of the component, which it gives to the 
medium. The inclinations of the fronts and rears to OX, being 
what are ordinarily called the angle of incidence and the angles of 
reflexion or refraction of the several waves, will he denoted by 
