386 Mr. E. Edser. An Extension oj 



Since both c t and c 2 are directly proportional to n the number of 

 molecules per unit volume, it f ollows that in a medium which can be 

 compressed without appreciably altering TJ and T S we shall have 



/r -1 cc density. 



When ju, is very nearly equal to unity, this may be written 

 JLI 1 cc density, 



which is Gladstone and Dale's well-known law. 



Further, the refractive index for infinitely long waves is obtained 

 from the equation. 



Ac = 



_ 4.M 

 ~KT 



which is the value obtained for the specific inductive capacity of a 

 medium by (4). 



Further, we may re-write (15), 



Also if Vo^ = AJ, , V T 2 = X 2 , V O T = X, 



this may be written 



~"\ 



where c' = Ci^ 2 , c" = c 2 T 2 3 . 



This is Ketteler's dispersion formula, which he has shown is capable 

 of representing the optical properties of a large number of substances 

 over a great range of values of X. It is the same as that obtained by 

 Mr. Glazebrook in his paper " On the Extension of Lord Kelvin's 

 Contractile j^Bther to include Dispersion, &c.,"* and by a slight 

 alteration in form reduces to Lord Kelvin's dispersion formula. f 

 As its properties have been fully discussed in the papers referred to, 

 nothing further need be said about it here. 



The molecular constitution considered above is, of course, the 

 simplest imaginable. With a more complicatedj-rnolecute, since each 



* E. T. Glazebrook, ' Phil. Mag.,' December, 1888. 

 f Lord Kelvin, Baltimore Addresses, 188 i. 



