410 ME. J. C. MAXWELL GAENETT ON 



The (comparatively) small effect of the coefficient 



on the colour by transmitted light will be considered later. For the present it is 

 sufficient to observe that M () becomes small when /A = I/a for any colour, and hence 

 that the variations of M intensify the absorption bands, which will be shown to 

 occur for gold and silver near /x = 1. 



10. In order to illustrate the discussion of the colours exhibited by films of 

 metal for various values of p., graphs are given of UK and n~K for gold, for silver, and 

 for the amalgam, potassium-sodium, the constants of which for ju, = 1 were given by 

 DRUDE (lo<\ cit.}. The graphs of UK and H~K for gold and for silver when the incident 

 light is red or yellow are plotted from the vahies given in the accompanying table, 

 with the help of the additional point corresponding to /xa = 1. 



This last point is easy to plot, for we see from equations (18) and (19) that 



2 K = and ir (/c I) = 2 when u = I/a. 



4p.ft 4ft 



( lonsequently for 



p. == I/a, >r(/c- + 1) = 2 V //7V + 1, 



so that . 



r / ' \ > i 



II-K- -- v/V/c 2 -f 1 + 1 and HK = j v( 8 fl) + 1 + 1 [ 



This point is also very near the maximum of H~K, owing to the smallness of /8 in 

 comparison with , and is also, in the graph of UK, not far from the maximum, and in 

 the graph of M not far from the minimum. It will be shown that for each of these 

 reasons there is in general an absorption band in the colour whose a = I//JL. 



The graph for H~K when blue light is incident on gold is surmised; i.e., it is 

 constructed on the supposition that the constants n and K for gold, when /A = 1, are 

 continuous from red through yellow to blue. The curve for UK for gold under blue 

 light is made of the same shape as for yellow and red, the value ot HK for p. = a" 1 

 being plotted from the maximum value of ITK assumed in that graph. 



The curves for H-K for potassium-sodium are plotted, again with the help of the 

 points for /A = I/a, the incident light being blue or yellow. The graph of H~K for 

 red is again surmised. 



The graphs of 11 K for potassium-sodium are shown by analogy with those for gold, 

 the only points plotted being for /j. = I/a, /A = 1. The red curve is constructed from 

 that for n~K in the same manner as the blue curve for UK for gold was got from the 

 assumed curve for n~K for gold. 



