418 MK. J. G. MAXWELL GARNETT ON 



The films made "by Professor WOOD were obtained by the condensation of the 

 vapour of the evaporated metal on the insides of exhausted glass bulbs. We should 

 therefore expect the film in its original form to be in drops, which, in accordance 

 with Part I., 3, when very small, approximate to the spherical form. 



The absence of conductivity in these films supports this view of their structure. 

 The eii'ect of heating up to and beyond the melting point would be to fuse these 

 drops into continuous metal, and although surface tension tends to a re-formation into 

 spheres, it is probable that p. will generally be considerably increased by the fusion. 



It appears, from our graphs of n~K and UK, that thin or thick " potassium-sodium " 

 films should transmit more blue than yellow light, provided that //. > '4, there being a 

 very strong absorption of yellow for /x = '49 (about) in both cases. It is interesting 

 to note that Professor WOOD always refers to the yellow absorption bands as 

 particularly strong. As //, increases, the absorption of yellow relative to blue 

 increases in both thick and thin films. 



If now we introduce our hypothetical curves for H~K and for HK for red light, we 

 find that for /x < '4 the film should be red. Near the greatest absorption of yellow 

 (^i = -4i>), red and blue should be equally absorbed and the film be purple. As /t 

 increases further, red should be most absorbed, and blue least, so the film should be 

 blue. Thus, in general, the film should turn from purple to blue when heated, as 

 was the case with most of the films observed. Professor WOOD (loc. cit., p. 407) 

 further states that the particles which he observed were distinctly closer in the blue 

 than in the pinkish-purple part, thus again suggesting that a change from purple to 

 blue accompanies an increase in /JL. 



So far, then, as they go, our results are in good accordance with observation. 

 When, however, numbers can be obtained for n and n/c for potassium and for sodium 

 for blue, yellow, and red light, it may be possible to state with more certainty that 

 our explanation of the colours of the films and of the changes in colour, due to 

 heating is the true one. 



Sj l-'i. By considering the oblique incidence of plane-polarised light on thick films of 

 metal by the method adopted in 9 in the case of thin films, it can be shown that 

 equation (26) is replaced by : 



(1.) When the incident light is polarised in the plane of incidence 



(2.) When the incident light is polarised perpendicular to the plane of incidence 



I n 1 2 _ 1 G ('- + v' 2 ) w ,, . cos e ., ^ 



.((i +u 'Y + r'^ e 



where u, v and u', v' are certain functions of n, K, 6 such that when the angle of 

 incidence, 9, is zero, 



it =. u' = and v = v' = UK. 



