PERIODICITY 



69 



give us a very beautiful representation of the simultaneous 

 propagation of undulations of different wave-length in the 

 same medium. 



Like waves of light and sound, these waves of diffusion are 

 refracted when they pass from one medium into another of a 

 different density, where they have a different velocity. When, 

 for instance, a diffusion wave passes from a 5 per cent, solu- 

 tion of gelatine into a 10 

 per cent, solution, the 

 wave-front is retarded, the 

 retardation being propor- 

 tional to the length of the 

 path through the denser 

 medium. Hence the wave- 

 front is flattened, the cur- 

 vature of the refracted 

 wave being less than that 

 of the original wave of 

 diffusion. The contrary is 

 the case when the wave- 

 front passes into a medium 

 where its velocity is 

 irreater. The middle of 

 the wave-front now travels 

 faster than the flanks, and 

 the curvature is increased. 



These diffusion rings 

 furnish us with most ex- 

 cellent diagrams of refrac- 

 tion at a " diopter," i.e. a 



spherical surface separating two media of different densities. 

 Fig. 14 shows the refraction at a convergent diopter, i.e. a 

 surface where the denser medium is convex. The diffusion 

 waves in this case emanate from the principal focus of the 

 diopter, and therefore become plane on passing through the 

 convex surface of the denser gelatine. 



These periodic diffusion rings also illustrate the phenomena 

 of colour diffraction. Diffusion waves of different wave- 



FlG. 13. — Waves of diffusion refracted at 

 a plane surface on passing from a less 

 concentrated into a more concentrated 

 solution. The refracted wave-front is 

 flattened, the wavelength being less in 

 the denser medium. 



