ON OPTICAL THEORIES. 209 



true, is implied in the original paper, and I regret that I misunderstood 

 the statement there. 



In consequence of these unknown forces the equations of stress are of 

 no use to us, and we are compelled to have recourse to Kirchhoff's prin- 

 ciple to arrive at the conditions. But this appears to me to aflPect in a 

 fundamental manner the whole of the theory. It ceases in consequence 

 to be a strict mechanical theory of light, for we are ignorant of what goes 

 on in the immediate neighbourhood of the boundary. There is a thin film 

 throughout which our equations of motion do not hold, for throughout it 

 the unknown forces A^^, &c., act. Unless we can show that this film is in- 

 finitely thin compared with the wave-length of light, we have no rio-ht to 

 assume that the displacements up to the boundary surface are given by 

 the expressions which hold in the interior of the medium. 



The actual displacements are, of course, continuous across the bound- 

 ary, but these displacements will, in addition to what we may term the 

 light motion, involve terms arising from the forces A^^, and such are 

 neglected in Professor Yoigt's theory. 



With regard to the electro-magnetic theory of dispersion developed 

 by Willard Gibbs, it should be remarked that H' ('Report,' p. 256, '20) 

 vanishes when E.', >/', i', the components of the irregular part of the motion, 

 vanish. Now this irregular part of the motion may be supposed to be 

 due to the presence of the matter-molecules, and will therefore disappear 

 in a vacuum ; so that in that case we should have H' zero, and there would 

 be no dispersion. 



First Report of the Committee, consisting of Mr. K. Etheridge 

 Dr. H. Woodward, and Mr. A. Bell, for the purpose of 

 reporting upon the ' Manure ' Gravels of Wexford. 



The area of the later Tertiary deposit of co. Wexford is described by the 

 late Sir Henry (formerly Captain) James as extending from Arklow to Kil- 

 more in a north to south direction, and inland to Ferns Gorey and Ennis- 

 corthy. The very short memoir upon this district (' Journ. Dubl. Geol. 

 Soc' vol. iii.) was accompanied by a list of the fossils obtained ; but the 

 localities from which they were collected not being stated, and the 

 differences in the nature and age of the various sands, gravels, and loamy 

 clays comprehended under the heading ' Post-Tertiary Deposits,' being 

 considerable, it is less useful than it might have been made. 



In Professor E. Forbes's well-known memoirs, the fossils are 

 simply recorded as from Wexford or Ireland, Post-Tertiary geology 

 being then in its infancy. These, with a few spare references in 

 papers contributed to the ' Geological Magazine ' by Messrs. Harkness 

 Kinahan, Hull, and the writer, in the 6-in. survey maps, and in two 

 volumes — one on the ' Physical Geology, &c., of Ireland,' by Professor 

 Hull, the Director of the Survey, and the other on the ' Geology of 

 Ireland,' by Mr. G. H. Kinahan, comprise the bibliography of the 

 subject. 



The so-called manure gravels consist of fine clean sharp sands without 

 stones or organic remains passing up into finely comminuted shell sand 

 and this, as the fragments become larger, forms a fine gravel containing 

 1887. p 



