•All Prof. E. Ketteler on the Dispersion of Light 



was now driven by an auxiliary battery. The value of n was 

 50. The galvanometer (resistance about 68) was put in the 

 primary as above described. 



The above calculation represents the case thus realized pretty 

 closely ; for although the self-induction of the galvanometer 

 has been neglected and the resistance of the battery only 

 roughly estimated, yet neither of these affects the important 

 term. The result, therefore, of experiment ought to be nearly 

 as above predicted. It icas so as nearly as could be seen, 

 taking account of unavoidable experimental errors. 



It appears, therefore, that the above theory stands so far the 

 test of experiment. When I can get the use of a sine-inductor 

 or a sufficiently delicate electrodynamometer (both of which 

 will probably soon be added to the collection of instruments at 

 the Cavendish Laboratory), it will be easy to test the theory 

 still further. 



If it be accepted, it seems to me that an interesting conclu- 

 sion follows, viz. that, of the total induced magnetism which a 

 given field of force is capable of generating in any body placed 

 in it, a very considerable fraction must be developed in a 

 time very much less than ^J^ of a second. 



Perhaps a method for measuring the inductive capacities 

 for temporary magnetism of strongly magnetic substances 

 might be built on the experiments I have described ; but this 

 can hardly be done until it is better known what degree of 

 accuracy can be ascribed to the law 



Call 



Possibly by sufficiently increasing the speed of revolution we 

 might with a sine-inductor be able to introduce the element of 

 time into magnetic measurements, and thereby get new light 

 on the difficult subject of magnetic induction. 



Cavendish Laboratory, Cambridge, 



~ 1876. 



LI. Attempt at a Theory of the (Anomalous) Dispersion of 

 Light in Singly and Doubly Refracting Media. By Professor 

 E. Ketteler. 



[Continued from p. 345.] 



7. HTF we now make the attempt to extend our theory to an- 

 -*- isotropic media also, only one procedure will lead to the 

 end in view, and that totally different from the usual one. Ac- 

 cording to Fresnel's method, namely, the mathematical treat- 

 ment has hitherto been restricted exclusively to the differential 

 equations of the vibratory motion of the 93ther perpendicular 

 to the normal of the waves ; and by means of them the 



