4 



Proceedings of the Royal Irish Academy, 



where 



= 1 + 2 



1 - 



0. 9s 



n = the index of refraction, 



A., = wave-length of ionic vibration, 



X = wave-length of light vibration, 



c = velocity of light 3x10^^ cm. per sec. 



= area of the ionic circuit, 



T, = period of the ionic circuit. 



It may be permitted here to briefly recall the physical meaning 

 of the expressions for n and cV. The summation of terms extends 

 over all the absorption-bands. The constant is characteristic of 

 the ions of the absorption-band, depending on their number, mass, 

 charge, and period. It is easily seen that it is equal to the contribu- 

 tion of the ions of the absorption-band to the dielectric constant by 

 making \= co. As is well known, there are a few substances for 

 which it is only necessary to take account of the ions in one ultra- 

 violet absorption-band. The rest of the absorption -bands lie so far 

 away on each side of the visible spectrum that their wave-length is 

 either very large or very small compared with the wave-length of 



light : therefore = 0. For such terms in the ultra-violet 

 = 0, and for the infra-red terms t-^ = 0. 

 In this case 



= 1 + + 



(2) 



