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XII. Colours in Metal Glasses and in Metallic Films. 



By J. C. MAXWELL GARNETT, B.A., Trinity Collage, Cambridge. 



Communicated ly Professor J. L ARMOR, Sec.R.S. 



Received April 19, Read June 2, 1904. 



Introduction. 



1. THE present paper contains a discussion of some optical properties of a medium 

 containing minute metal spheres. The discussion is divided into two Parts : the first 

 Part dealing with colours in metal glasses, in which the proportion of volume 

 occupied by metal is small ; the second Part dealing with metal films, in which this 

 proportion may have any value from zero to unity. 



In Part I. the observations of SIEDENTOPF and ZSIGMONDY beyond the limit of 

 microscopic vision (' Ann. der Phys.,' January, 1903) are discussed. It is shown that 

 the particles seen in a gold ruby glass are particles of gold which, when their 

 diameters are less than 0'1/x, are accurately spherical. I have endeavoured to show 

 that the presence of many of these minute spheres to a wave-length of light in the 

 glass will account for all the optical properties of " regular" gold ruby glass, and that 

 the irregularities in colour and in polarisation effects sometimes exhibited by gold 

 glass are due to excessive distance between consecutive gold particles or to excessive 

 size of such particles, the latter, however, involving the former. It is also shown 

 that the radiation from radium is capable of producing in gold glass the ruby colour 

 which is generally produced by re-heating. The method adopted enables us to 

 predict from a knowledge of the metal present in metallic form in a glass what 

 colour that glass will be in its " regular" state. 



In Part II. the optical properties, and the changes in colour on heating, of the 

 silver and gold films observed by Mr. G. T. BEILBY (' Roy. Soc. Proc.,' vol. 72, p. 226), 

 and of the potassium and sodium films deposited on glass by Professor R W. WOOD 

 ('Phil. Mag.,' p. 396, 1902), are discussed, with a view to showing that they can be 

 accounted for by supposing the films to be composed of minute metal spheres of 

 varying sizes. 



VOL. CCIII. A 370. 3 D 17.8.04 



