Geological Society. 157 



tion of the electric displacement being at right angles to the mag- 

 netic disturbance, and both at right angles to the direction of the ray. 



The theory does not attempt to give a mechanical explanation 

 of the nature of magnetic disturbance or of electric displacement, it 

 only asserts the identity of these phenomena, as observed at our leisure 

 in magnetic and electric experiments, with what occurs in the rapid 

 vibrations of light, in a portion of time inconceivably minute. 



This paper is already too long to follow out the application of the 

 electromagnetic theory to the different phenomena already explained 

 by the undulatory theory. It discloses a relation between the induc- 

 tive capacity of a dielectric and its index of refraction. The theory 

 of double refraction in crystals is expressed very simply in terms of 

 the electromagnetic theory. The non-existence of normal vibrations 

 and the ordinary refraction of rays polarized in a principal plane are 

 shown to be capable of explanation ; but the verification of the theory 

 is difficult at present, for want of accurate data concerning the di- 

 electric capacity of crystals in different directions. 



The propagation of vibrations in a conducting medium is then con- 

 sidered, and it is shown that the light is absorbed at a rate depending 

 on the conducting-power of the medium. This result is so far 

 confirmed by the opacity of all good conductors, but the transpa- 

 rency of electrolytes shows that in certain cases vibrations of short 

 period and amplitude are not absorbed as those of long period would be. 



The transparency of thin leaves of gold, silver, and platinum can- 

 not be explained without some such hypothesis. 



The actual value of the maximum electromotive force which is 

 called into play during the vibrations of strong sunlight is cal- 

 culated from Pouillet's data, and found to be about 60,000,000, or 

 about GOO Daniell's cells per metre. 



The maximum magnetic force during such vibrations is '193, or 

 about J"q of the horizontal magnetic force at London. 



Methods are then given for applying the general equations to the 

 calculation of the coefficient of mutual induction of two circuits, and 

 in particular of two circles the distance of whose circumferences is 

 small compared with the radius of either. 



The coefficient of self-reduction of a coil of rectangular section is 

 found and applied to the case of the coil used by the Committee of 

 the British Association on Electrical Standards. The results of cal- 

 culation are compared with the value deduced from a comparison of 

 experiments in which this coefficient enters as a correction, and also 

 with the results of direct experiments with the electric balance. 



GEOLOGICAL SOCIETY. 



[Continued from p. 77-] 



December 7, 1864. — W. J. Hamilton, Esq., President, in the Chair. 



The following communications were read : — 



1. "On the 'Geology of Otago, New Zealand." By James 

 Hector, M.D., F.G.S. In a letter to Sir R. I. Murchison, K.C.B., 

 F.R.S., F.G.S. 



The south-western part of the province of Otago is composed of 



