266 BULLETIN OF THE UNIVERSITY OF WISCONSIN. 



Solving these, Ketteler finds : l 



V A* m / A m A m V A* m / 



(a^- 1 ) + g av ( 28a >- 



/V-X 2 -1 = C r 



m 



2 Mo X = C r 



V A 2 m ' A m A m V A- m / 



m /A 8 \ 2 „ A 3 



(l^- 1 ) + G ° A^ (28b). 



A new phase was given to the subject in 1893 when Helm- 

 holtz 2 produced his theory of dispersion based upon the electro- 

 magnetic theory of light. He postulates charged bodies im- 

 bedded in an ether transmitting electric disturbances, and that 

 the forces acting upon the ether during these disturbances can 

 set the material particles in vibration. He follows the electro- 

 lytic theory in assuming that electric charges of definite magni- 

 tude are accumulated at the center of affinity of chemically com- 

 bined ions, that these charges may be either positive or negative 

 but always have the same absolute value for every center of 

 affinity. "If the ether surrounding a pair of combined ions is 

 traversed by electric forces and thereby dielectrically polarized, 

 the oppositely charged ions will be subjected to stresses in the 

 direction of the lines of force, that is, to two forces of equal mag- 

 nitude but opposite in direction which together form a couple ; 

 this couple will not set the center of mass of the molecule in 

 motion, but it will tend to lengthen or. shorten the electrical axis 

 of the molecule and to deflect it towards or away from the direc- 

 tion of the lines of force." The pairs of ions as used by Helm- 

 holtz possess mass and inertia in addition to the usual properties 

 of dielectrically polarized molecules of insulating substances. 

 Hence, oscillations may throw them out of equilibrium and pro- 

 duce variations in the electrical moments due to these ionic 

 charges, f, g, h, forces, independently of the electrical moments 

 per unit volume, f, g, h, forces, of the free ether. 



i Ketteler, " Theoretische Optik," p. 100. 



» Helmholtz, Wied. Ann. XLVIII., pp. 389, 723; L. E. XXXVII., p. 404. 



