514 



The Electromagnetic Theory of Light [OH. xvm 



IK 



magnetic theory the value of \ ^ ought to be identical with the refractive 



index for light of infinite wave-length. Unfortunately it is not possible to 

 measure the refractive index with accuracy except for visible light. 



IK 

 VK 



590. In the following table, the values of \/ ^- are mean values taken 



V J\.Q 



from the table already given on p. 132 of the inductive capacities of gases. 

 The values of n refer to sodium light. 



Authorities : 1. Mascart. 2. G. W. Walker (Phil. Trans. A. 201, p. 435). 

 3. Preston (Theory of Light, p. 137). 



From this it will be seen that for these substances there is quite good 

 agreement between theory and experiment, in spite of the failure of the 

 theory to take all the facts into account. In the case of vapours the agree- 

 ment is much less good, and for many solids and liquids there is no agreement 

 at all. For instance, the observed inductive capacity of water is 73'6 (see 

 p. 349), while the value of n is 1*33. 



J 



WAVES OF LIGHT IN NON-CONDUCTING MEDIA. 

 Solution of Differential Equation for Plane Waves. 

 591. The equation of wave propagation 



d 2 u _ 2 

 ~di?~ 

 has, as a particular solution, 



(547), 



provided / 2 + ra 2 + n 2 =l. This value of u is a complex quantity of which 

 the real and imaginary parts separately must be solutions of the original 

 equation. Thus we have the two solutions 



u = A cos K (Ix + my + nz at) .................. (548), 



u A sin K (Ix + my 4- nz at). 



