Geological Society, 325 



foundation, appears approximately to represent the facts. The 

 question may indeed be raised whether it is possible so to 

 define an ideally matt surface that Lambert's law may become 

 strictly applicable. 



The conclusion drawn by Dr. Wright from his experiments 

 with compressed powders upon which I desire to comment 

 is that numbered (4) in his resume of results, viz. " A law 

 for the intensity of reflected scattered light cannot be. 

 symmetric in reference to i and e." It appears to me that 

 this statement is in contradiction to a fundamental principle 

 of reciprocity, of such generality that escape from it is 

 difficult. This principle is discussed at length in my book on 

 the 'Theory of Sound/ § 109. Its application to the present 

 question may be thus stated : — Suppose that in any direc- 

 tion (i) and at any distance r from a small surface (S) 

 reflecting in any manner there be situated a radiant point (A) 

 of given intensity, and consider the intensity of the reflected 

 vibrations at any point B situated in direction e and at distance 

 i J from S. The theorem is to the effect that the intensity is 

 the same as it would be at A if the radiant point were trans- 

 ferred to B*. The conclusion follows that whatever may be 

 its character in other respects, the function of i and e which 

 represents the intensity of the reflected scattered light must 

 be symmetrical with respect to these quantities. 



The actual departures from the reciprocal relation found 

 by Dr. Wright were not very large, and they may possibly 

 be of the nature of experimental errors. In any case it 

 seems desirable that the theoretical difficulty in accepting 

 Dr. Wright's conclusion should be pointed out. 



XXIX. Proceedings of Learned Societies, 

 GEOLOGICAL SOCIETY. 



[Continued from p. 158.] 



December 20th.— W. Whitaker, B.A., F.R.S., President, 

 in the Chair. 



r PHE following communications were read : — 

 -*- 1. ' On some Effects of Earth-movement on the Carboniferous 

 Volcanic Rocks of the Isle of Man.' By G. W. Lamplugh, Esq., 

 F.G.S., of H.M. Geological Survey. 



The author, since the completion of his survey of the Isle of 



* I have not thought it necessary to enter into questions connected 

 with polarization, but a more particular statement could easily be made. 



Phil. Maa. S. 5. Vol. 49. No. 298. March 1900. Z 



