﻿416* Prof. Threlfall on the Scattering 



LIV. On the Scattering of Light by Metallic Particles. By 

 Richakd Threlfall, Professor of Physics in the University 

 of Sydney, N.S.W* 



THE problem of the scattering of light by small conducting 

 particles is treated by Prof. J. J. Thomson in his very 

 welcome ' Recent Researches in Electricity and Magnetism/ 

 It would of course be absurd for me to offer any remarks 

 on the formidable analysis by means of which Prof. Thomson 

 reaches his conclusions ; and therefore I shall limit myself to 

 the consideration of §§ 367 and 378, in which similar con- 

 clusions are reached by different methods. 



One of the most striking results is as follows (' Recent 

 Researches/ p. 449) : — " When non-polarized light falls upon 

 a small non-conducting sphere, the scattered light will be 

 completely polarized at any point in a plane through the 

 centre of the sphere at right angles to the direction of the 

 incident light. When the light is scattered by a conducting 

 sphere, the points at which the light is completely polarized 

 are on the surface of a cone w r hose axis is the direction of 

 propagation of the incident light and whose semi-vertical 

 angle is 120 degrees.''' 



Jt may well be considered questionable whether small 

 metallic particles can be regarded as the conducting particles 

 to which the investigation refers, especially if we bear in mind 

 the apparently excessive transparency of thin metallic films. 

 However, there is no difficulty in observing the scattering 

 from fine metallic particles ; and what follows deals with 

 some experiments I have made on the subject. To save time 

 for any one who does not care to follow the detail of experi- 

 mental w 7 ork, 1 will state at once that I have observed no 

 great difference in the effects produced by particles of gold, 

 copper, iron, silver, and gum mastic. The position of the 

 azimuth of maximum polarization with gold particles (which 

 are most easily observed) does not differ from the cor- 

 responding position w 7 hen mastic is employed by more than a 

 degree or two, if at all. The observations with metals other 

 than gold were nothing like so satisfactory ; but so far as they 

 go they confirm the results obtained with gold. 



As long ago as 1888 I examined the properties of a gold 

 " sky/'' In order that the gold particles might be suspended 

 in a non-conducting medium, 1 shook up some terebene with 

 a bit of solid phosphorus and added a very small quantity of 

 a solution of gold chloride in ether; in the course of a few 



* Communicated bv the Author. 



