224 Prof. A. McAulay on the Wava- Surface and Rotation 



April 25. 



1 1023-35 



2 1027-30 



3 942-05 



4 1022-04 



5 1017-21 



6 1032-04 



7 1007-41 



8 1003-45 



9 1026-42 



The values of the proportional arms are :— 



April 27. 



1000 in X 947-92 



1000 in Y 994-21 



My object in giving the whole of the figures relating to 

 the resistance-coils is to show to what an extent bare wire 

 coils are to be trusted. Whether the coils retain their values 

 as well as the resistance standards of the British Association, 

 time alone can decide. There is no reason to suppose that, 

 if the coils are kept continuously in an oil-bath, and the oil 

 kept free from water, any considerable alteration can take 

 place. 



XXII. On the Wave-Surface and Rotation of Polarization 

 Plane in an Aeolotropic Electromagnetic Medium. By 

 Prof. A. McAulay, University of Tasmania *. 



IX the ' Philosophical Magazine ' for June 1885, p. 397 (also 

 ' Electrical Papers/ vol. ii. p. 1), Mr. Heaviside has in- 

 vestigated in a very instructive manner the properties of the 

 electromagnetic wave-surface and index-surface, and the 

 relations in a plane wave of the electric and magnetic forces 

 and the displacement and induction. 



In noticing that in one respect his investigation can be 

 simplified, I have been led to some simple connexions between 

 the electromagnetic surface and Fresnel's surface. The most 

 important result is that the former can in two ways by a real 

 pure strain be converted into a Fresnel surface ; the axes of 

 the strain being in the two cases those of permittivity and 

 inductivity. 



Thus all the peculiarities — such as double sheet, singular 



* Communicated by the Author. 



