7 o THE MECHANISM OF LIFE 



length are unequally refracted by a gelatine lens. Hence 

 rings of different wave-length which, originating at the same 

 spot, are at first concentric, are no longer parallel after passing 

 through a gelatine lens. A convergent lens which will change 

 the long spherical incident waves into shorter plane waves, 

 will transform the short incident waves into concave waves 

 whose curvature is opposite to that of the original waves, i.e. 

 it will transform a divergent into a convergent beam. This 



is an illustration of what 

 is called the aberration of 

 refrangibility. 



In the same way we may 

 demonstrate the course of 

 diffusion waves through a 

 gelatine prism, showing the 

 refraction on their incidence 

 and again on emergence. 

 The prism is made of a 

 stronger gelatine solution, 

 which is more refractive than 

 the gelatine around it. The 

 waves of diffusion whilst 

 traversing the prism are 



retarded, and this retarda- 

 FIG. ,4-Trans.ormation ol a sphericaJ ^ ^ 



wave-front into a plane wave-front by & 



a conveigent diopter. where the passage is longer. 



Hence the wave-front is 

 tilted towards the base of the prism, and this tilting is re- 

 peated when the wave-front leaves the prism. 



If we examine diffusion waves of different wave-length on 

 their emergence from the gelatine prism, we shall sec that 

 they cut one another. With a dense prism, the wave-front 

 of the shorter waves is more tilted towards the base than the 

 wave-front of the longer waves. For diffusion as for light 

 the shorter waves are the most refracted. Both refraction 

 and dispersion are due to the unequal resistances of the 

 medium to undulatory movements of different periodicity. 

 Diffraction. — When light traverses a minute orifice, instead 



