356 RICHARDSON— DYNAMICAL EFFECTS OF [April 22, 



effective specific inductive capacity of the medium. Xow the spe- 

 cific inductive capacity of an insulating medium is equal to unity 

 plus the product of the numl^cr of electrons per unit volume by 

 their average displacement in unit electric field. When the material 

 is subjected to constant electric forces the displacements of the elec- 

 trons are always proportional to the forces and the specific inductive 

 capacity is therefore a constant quantity. When the force is an 

 oscillating one the matter is complicated by the fact that the elec- 

 trons try, as it were, to strike a balance between their own natural 

 period of oscillation and that of the force acting on them. They 

 end by oscillating with the same frequency as the force which 

 excites them but the distance thev travel from their e(iuilibrium 

 position depends a good deal on their natural periods as well. Thus 

 the specific inductive capacity for oscillating forces will not be a 

 constant quantity b'ut will depend to some extent on the frequency 

 of oscillation of the force. By the effective specific inductive ca- 

 pacity we mean the specific inductive capacity for electric forces 

 which oscillate with the frequency of the light under consideration. 



It is evident from what has been said that the refractive index 

 of an insulating substance depends upon the frequency or, in other 

 words, upon the color of the light. W^e see at once why a beam of 

 white light is split up by a prism into the constituent spectral colors. 

 For each ray is deviated by the prism according to the value of its 

 refractive index. 



Perhaps the most interesting question in this part of our subject 

 is that of the behavior of a substance towards light whose frequency 

 is close to that of the natural periods of the substance. In that case 

 the electrons are set into violent motion owing to the occurrence of 

 what are sometimes called sympathetic vibrations. The nature of 

 this phenomenon may best be illustrated by considering a simple 

 mechanical analogy. Imagine a spiral wire with a weight at one 

 end to be hung from a shaking support. If the weight is pulled 

 down and let go it will oscillate backwards and forwards with a 

 definite natural freffucncy which depends on the stift'ness of the 

 spring and the heaviness of the weight. If the shakiness of the 

 support arises from tremors in the building, to the walls of which we 

 will suppose it bolted, as a rule the frequency of its vibrations will 



