COLOURS IN METAL GLASSES AND IN METALLIC FILMS. 419 



It can further be proved that the variations with p. of the coefficients 



M.= 11J?_ Qrl 



- 3 



. 16 



= {(T+7*') 2 



are such that a change in (27) from M, to M/ would strengthen the absorption 

 bands. The complete analysis is somewhat lengthy ; 1 have therefore refrained from 

 reproducing it here. 



This result, however, shows that in general the absorption band should be weaker 

 when the incident light is polarised in the plane of incidence than when it is 

 polarised perpendicular to that plane. And tin's effect Professor WOOD observed in 

 almost every film. 



PART 111. 



14. Metallic media composed of small spheres of metal, many to a wave-length, 

 have many interesting properties in addition to those already referred to. The very 

 vivid colour effects which are exhibited according to the graphs given above for UK 

 for gold, silver and "potassium-sodium" when light traverses such media, in 

 consequence of the different absorptions of different colours, suggest enquiry whether 

 metals in bulk have ever been obtained giving brilliant colours by transmitted and 

 reflected light, such metals being ordinary metals with /j. less than unity. For 

 instance, have any of the metals we have discussed been obtained in states in which 

 the specific gravity was not the normal value for that metal and in which the colour 

 changed with the specific gravity ? 



I hope in the near future to examine CAREY LKA'S \vork in detail with a view to 

 finding out whether his allotropic silver is a medium of the type we have con- 

 sidered silver with //. less than unity. But the first glance at his papers (' American 

 Journal of Science,' L889) shows the following remarkable correspondence between 

 the properties he observed and the properties which should, according to our 

 calculations for yellow and red light, be possessed by silver with /A < 1 : 



(i.) CAREY LEA'S silvers were obtained from solution ; and we have shown that 

 gold, and therefore, presumably, silver, crystallises out of solutions into 

 particles which are spherical if they are very small. Our silver (/* < 1) is 

 composed of minute spheres. 



(ii.) CAREY LEA'S silver can be changed by pressure or heating into normal silver. 

 We should expect ^ to be increased by pressure. 



(iii.) The specific gravities of the two principal forms of allotropic silver were 

 appreciably less than that of normal silver. 



{iv.) From our graph of HK for silver we see that red and yellow light are about 

 equally, and very powerfully, absorbed when ^ = '81. The ratio of the 



