1886.] A General Theorem in Electrostatic Induction. 423 



assumed only in a field of force which is not uniform from point to 

 point. 



Dr. Kerr has made the remarkable discovery that some dielectrics 

 become optically "positive," others "negative," when subjected to 

 electric stress. I think it may be inferred from Prof. Quincke's 

 experiments just referred to, that those bodies which Dr. Kerr found 

 to be "positive" have their index of refraction decreased by electric 

 stress ; " negative " bodies on the contrary have their index of 

 refraction increased. I am not aware that this point has been decided, 

 but I hope shortly to investigate it in the laboratory of University 

 College, London. 



The sign of the change of index of refraction is not essential to 

 the present discussion. We will assume, however, simply for con-' 

 venience of statement, that a " positive " dielectric experiences a 

 decrease, and a "negative" dielectric experiences an increase of index 

 of refraction when placed in a field of electric force. 



Now, whatever opinion may be held concerning the electro- 

 magnetic theory of light, there can be no doubt that along with 

 change of index of refraction of a dielectric, there goes always 

 change of specific inductive capacity. With the supposition we have 

 made above regarding the sign of the change of index of refraction 

 produced in the dielectrics examined by Dr. Kerr, his results when 

 expressed in electrical terms translate into the statements that : a 

 " positive " dielectric has its specific inductive capacity decreased by 

 electric force; a "negative" dielectric bas its specific inductive 

 capacity increased by electric force. In view of the theorem proved 

 in Part I of this communication, this form of statement is very 

 important. 



It means that if the specific inductive capacity of a " positive " 

 body be decreased in presence of a field of force, then the electric 

 forces assist this change — work is done by these forces. .On the other 

 hand, if the specific inductive capacity be increased, work is done 

 against the forces of the field. 



We get corresponding statements for " negative " bodies by 

 changing signs. 



Let us return now to equation (6). It is — 



h=-(^ + V.^\. 

 \dV dKj 



Let us suppose that the dielectric is placed in a field of zero force. 

 Then, with the disposition of apparatus that we assumed at the 

 beginning of Part I, V = 0, and the second term of the right hand 



number is zero. But the first term ^ need not necessarily vanish 



