388 



ME. J. C. MAXWELL GARNETT ON 



It appears from equations (6) or (7) that such a small sphere, in common with any 

 other minute system whose moment is proportional to the electric vector of the 

 incident light, emits light with an intensity proportional to the inverse fourth power, 

 of the wave-length, provided that N is independent of X. It is this property which, 

 as Lord RAYLEIGH has shown, accounts for the blue colour of the light received from 

 the sky. 



\ 



Fig. 1. Fig. 2. 



3. In the ' Annnlen der Physik ' for January, 1903, H. SIEDENTOPF and 

 R ZSIGMONDY publish some observations on the metal particles in gold ruby glasses. 

 By their method of illumination they were able to see particles whose dimensions 

 were of the order of from 4 to 7 //./A, where /A/J. represents 10~ 6 millim. 



The arrangement consisted of a system of lenses following a strongly illuminated 

 and very narrow slit. The system of lenses, of which the last is a low power 

 microscopic objective, serves as a condenser and forms a very narrow image of the slit 

 inside the glass under observation. This image of the slit may not be more than 

 one or two wave-lengths thick. 



The observation is made with a microscope having the tube perpendicular to the 

 incident light, so that only the light emitted by the metallic particles travels up the 

 tube. This is the light the electric vector of which has been distinguished by the suffix 

 unity in the preceding analysis. The image of the slit, which is parallel to Ox in 

 fig. 2, comes directly under the microscope tube, which is in the direction Oy ; thus 

 only the particles illuminated at the image of the slit send light up the tube. The 

 diffraction discs do not pile up on top of one another if the average distance between 

 two metal particles is greater than the thickness of the image of the slit. In this 

 case, then, the number of particles per unit area can be counted. 



On pp. 1 1 and 1 2 of the paper referred to, SIEDENTOPF and ZSIGMONDY discuss the 

 appearances in the second focal plane of the microscope when the light incident in 

 the glass is plane polarised. The figs. 3-6 above are reproduced from their paper. 

 In fig. 3 the plane of polarisation of the incident light was that of incidence, the 

 plane of incidence being the plane containing the axis of the microscope and that of 



