and Refraction of Solitary Plane Waves. 



183 



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 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 perpendicular to the five wave-fronts. Taking this for 

 XOY, the plane of our diagram, let YOZ be the interface 

 between the two mediums. We shall first consider one 

 single incident wave, I, of the purely distortional character. 

 By incidence on the interface it will generally introduce 

 reflected and refracted waves 1', I y , of its own kind, that is 

 purely distortional, and J', J r reflected and refracted waves 



Fm. 1, 



of the condensational-rarefactional kind. The diagrams re- 

 present, 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 



02 



