COLOURS IN METAL GLASSES AND IN METALLIC FILMS. 417 



It has therefore been shown that all the observed colour changes in gold films are 

 in accordance with the theory and numerical results set forth in this paper. 



The points corresponding to /m = 1, referred to on p. 410, which were plotted in for 

 red and yellow, were 



UK = 4-89 for p. = 734 for red, 



riK = 3-80 p. = '685 yellow. 



Let us now consider silver films. 



The results for thin films are not of much interest, as probably none of the films- 

 observed came into this class. We may, however, notice that, according to the 

 graphs of n z K, the thin film should start by being more yellow than red. There is an 

 absorption band in the red about p. = '822, for which value of //., TI~K = 71. '4 for red. 

 There is great absorption in the yellow for p. = 795 when H~K = f>2'9. The nearness 

 of these vahies of /j. for the maxima of the absorption of yellow and red suggests that 

 thin films of silver should be blue or else very opaque when /x is about "S. The thin 

 film should turn more red than yellow for a slightly > - H and remain red down to 



t/ C7 



small values of /A, at least as far as p. = -1. 



Passing to thick films, for which the absorption is measured by UK \. we observe 

 from the graphs that as p. diminishes from unity the absorption at first increases 

 rapidly. This may be correlated with the increased conductivity manifested by a 

 silver film in the early stages of annealing. Shortly before p, = "8 the film becomes 

 more red than yellow, and although by the time ^ = '(> the absorption has already 

 become extremely small, the film remains more red than yellow until p. vanishes. 



Putting p = I/a, we find the additional points on the graphs of UK 



for red .... UK = 8'5\ when ^ = '822. 

 ,, yellow . . . n-K = 7 "99 .. /J. = 795. 



The red colour of silver films for low values of /A is observed in those obtained by 

 depositing silver on glass in the manner described by Professor WOOD (' Phil. Mag.,' 

 August, 1903). It is also often seen in fogged photographic plates. 



12. We proceed to consider the potassium and sodium films discussed by 

 Professor R, W. Wood, in the ' Phil. Mag.,' 1902, p. 396, ct s&j. Owing, however, 

 to the inavailability of the numerical values of the constants for potassium or 

 sodium for more than one colour when p. = I, the numbers used are those given by 

 DRUDE (loc. cit.) for "potassium-sodium," for blue and yellow light. Consequently 

 the same degree of numerical accuracy as for gold and silver has not been aimed at. 



The yellow and blue curves for II-K are plotted from the numbers tabulated iu 

 Table IT., p. 406. 



The graph of UK has been constructed to pass through the uutabulated points 



Yellow . . . HK = 3-811 for p. = I/a = '484, UK = 2'18 for /* = !.. 

 Blue . . . . K = 2-225 /* = l/ = '306, nic =178 p = 1. 



VOL. CCIII. A. 3 H 



