68 Mr Basset, A Provisional Theory of Kerr's [May 1, 



If the cylinders are of equal cross-sections a = /3, and the 

 above are the actual displacements, namely a rotation round 

 the axis of y of amount re coth a from x to z for the cylinder 

 r} =a, and from z to x for the cylinder 77 = — a. If the cylinders 

 are not equal, we have to add to the above displacements a ro- 

 tation rd round the axis of y, where d is the distance from the 

 origin of the centroid of the section. 



(2) On the Astronomical Theory of glacial periods. By Dr 



E. W. HOESON. 



(3) On Black Globes and some convenient uses of them. By 

 t). Y. Buchanan, M.A., Christ's College. 



(4) A Provisional Theory of Kerrs Experiments on the Re- 

 flection of Light from an Electromagnet. By A. B. Basset, M.A., 

 F.R.S., Trinity College. 



The experiments of Kerr on the reflection of light from an electro-magnet can 

 he explained by transforming the expressions for the amplitudes of the reflected 

 waves, when the medium is transparent [Phil. Trans. 1891, p. 371), by assuming 

 that in the case of a metal the refractive index is a complex quantity. 



1. Everyone who has devoted any attention to the history 

 of Physical Optics cannot fail to have been struck with the 

 fact that numerous theoretical results have from time to time 

 been obtained, which are in close agreement with experimental 

 facts, either by an imperfect theory or in some cases without 

 anything which in the proper sense of the word can be called 

 a theory at all. Cauchy, for example, towards the middle of 

 the present century arrived at certain formulae giving the ampli- 

 tudes of the waves reflected at the surface of a metal, which 

 the experiments of Jamin and others have shewn to be fairly 

 in accordance with experimental facts. Cauchy gave no proof 

 of his formulae, and until attempts were made in Germany by 

 von Helmholtz and others to construct theories based upon the 

 mutual reaction between ether and matter, nothing in the shape 

 of a plausible dynamical theory of metallic reflection could be 

 said to exist. At the same time, it will no doubt be conceded, 

 that the attempts made by Cauchy and others to elucidate the 

 difficulties connected with this subject, coupled with the fact 

 that formulae were actually obtained which may be regarded 

 as an empirical representation of the facts, were of distinct 

 scientific value, and have been, and no doubt will be in years 

 to come, of great assistance to men who are desirous of probing 

 this question to the bottom. 



Impressed with the importance of employing every possible 

 means of throwing light upon the curious and interesting pheno- 



