﻿636 Prof. J. A. Pollock on a 



than of a gas, and the diffusely scattered energy may then 

 be small compared with v 2 Jtt<t times the energy regularly 

 reflected. Thus the lamina, with increasing density, may be 

 expected to approximate in behaviour to a specular reflector 

 more rapidly than is indicated by the theory in its simplest 

 form: true absorption, as before, being absent. 



27. None of the incident plane-wave energy is diffusely 

 scattered by an extensive ordered arrangement of secondary 

 vibrators, such as that investigated by Lord Rayleigh * (who 

 points out, however, that the scale of the arrangement must 

 not be too great if we wish to avoid complications from 

 spectra of various orders). This leads to a considerable 

 divergence between the properties of plane assemblages of 

 vibrators, according as the distribution is ordered or wholly 

 irregular. For in the latter case, when the vibrators are 

 tuned as resonators, (9), (9 a) indicate that the secondary 

 plane waves emitted are always directly opposed in phase to 

 the incident train. Since the vibrators ex hypotliesi absorb 

 no energy, there must evidently be another energy-term in 

 question ; and this term, as we have ?e^n, corresponds to the 

 disturbance irregularly scattered. When the arrangement 

 of resonators is orderly and not too open, so that the only 

 secondary disturbances are normally propagated plane waves, 

 the phase of these must always be such as to keep the total 

 wave-energy unchanged. 



The second part of this paper deals in some detail with 

 the aspects of the problem of § 18 when the thickness L is 

 infinite, as well as with the modifications which have to be 

 made in the various formulae when it is desired to apply them 

 to the solution of optical problems. 



Boar's Hill, Oxford, 

 6th March, 1915. ' 



LXVII. A New Type of Ion in the Air. By J. A. Pollock, 

 D.Sc, Professor of Physics in the University of Sydney^. 



Introductory. 



IN an address to the members of {Section A of the Austral- 

 asian Association for the Advancement of Science at 

 Brisbane, in 1909, 1 mentioned that observations of atmo- 

 spheric ionization, made at the Physical Laboratory of the 

 University of Sydney, indicated the presence in the air of 



* Phil. Ma^. loc. cit. 



+ Communicated bv the Author. 



