42 REPORT— 1876. 



I have described the phenomenon with silver, which shows it hest ; but specidum- 

 metal, gold, and copper show it very well, while with steel it is far less conspicuous. 

 When the coloured metals gold and copper are examined by the light of a pure 

 spectrum, the ring is seen to be better formed in the less than in the more refi'an- 

 gible colours, being more intense when at its best ; while with silver and speculum- 

 metal there is little difference, except as to size, in the different colours. Hfematite 

 and iron pyrites, which approach the metals in opacity and in the change of phase 

 which they produce by reflection of light polarized parallel relatively to light polarized 

 perpendicularly to the plane of incidence, do not exactly form a ring isolated in a 

 bright field ; but the spot seen with light polarized perpendicidarly to the plane of 

 incidence is abnormally broad just about the limit of total reflection, and rapidly 

 contracts on increasing the angle of incidence. 



It seemed to me that a sequence may be traced from the rapidly contracting 

 rings of diamond seen in passing the polarizing angle of that substance, through the 

 abnormally broad and rapidly contracting spot seen with iron pyrites just about 

 the limit of total reflection, and the somewhat inconspicuous ring of steel seen a 

 little beyond the limit, to the intense rapidly contracting ring of silver seen consi- 

 derably beyond the limit. K so, the full theory of the ring will not be contained 

 in the usually accepted formulae for metallic reflection, modified, as in the case of 

 transparent substances, in accordance with the circumstance that the incidence on 

 the first surface of the plate of air is beyond that of total reflection. 



MacOullagh was the first to obtain the formulae for metallic reflection, showing 

 that they were to be deduced from Fresnel's formulae by making the refractive 

 index a mixed imaginary, though they are usually attributed to Oauchy, who 

 has given formulae differing from those of MacHullagh merely in algebraic detail. 

 As regards theory, Cauchy made an important advance on what MacCullagh had 

 done in connecting the peculiar optical properties of metals with their intense ab- 

 sorbing power*. Now Fresnel's formulae do not include the phenomena discovered 

 by Sir George Airy, which are seen in passing the polarizing angle of diamond, 

 and which have been more recently extended by M. Jamin to the generality of 

 transparent substances ; and if these pass by regular sequence to those I have de- 

 scribed as seen with metals beyond the limit of total internal reflection, it follows 

 that the latter would not be completely embraced in the application of Fresnel's 

 formulae, modified to suit an intensely absorbing substance and an angle of inci- 

 dence given by a sine greater than unity f. 



Electricity. 

 On the Contact Theory of Voltaic Action. By Professors Atrton and Perrt. 



On a new Form of Electrometer. By Prof. J. Dewae, F.B.S.E. 



On a Mechanical Illustration of Electric Induction and Conduction. 

 By Oliver J. Lodge, B.Sc. 



The paper describes the construction of a model which illustrates Prof. Clerk 

 Maxwell's theory of electric action on the hypothesis of stress in a dielectric me- 



* The apparent difference between MacCullagh and Cauchy as to the values of the 

 refractive indices of metals is merely a question of arbitrary nomenclature. 



t It was long ago observed, botli by Professor MacCullagh and Dr. Lloyd, that when 

 Newton's rings are formed betweeu a glass lens and a metallic plate, the first dark ring sur- 

 rounding the central spot, which is comparatively bright, remains constantly of the same 

 size at high incidences, although the other rings, like Newton's rings formed between two 

 glass lenses, dilate greatly as the iucideuce becomes more oblique. See ' Proceedings of the 

 Eoyal Irish Academy,' vol. i. p. 6. 



