416 MR J. C. MAXWELL GAKNETT ON 



continuously diminishes from unity downwards so long as the metal is kept at a 

 temperature of about 400. 



Just before \t. has reached '9 the yellow begins to get through better than the red, 

 but the absorption of both increases rapidly. The value of ?? 2 /c for red becomes 

 equal to 24 when p, = '734 about. It almost immediately starts to diminish, being 

 only 15'88 when /j, = '7. There is thus a strong and quite narrow absorption band in 

 the red for /j. = 734. 



Similarly, ?t 2 /c, when the incident light is yellow, rises to a high value near 

 fj. = -686, and when p. has that value, ri*K =15 nearly. 



Between /x = 7 and /x = 734, red and yellow are absorbed to the same large extent. 

 It seems probable that blue will not be absorbed so greatly for this value of p.. The 

 film should therefore probably be blue. Mr. HKILBY finds (loc. cit., p. '228) that a 

 gold film turns blue or purple (absorption chiefly of yellow) in the earlier stage of 

 annealing, though, presumably, the films for which this effect was observed fall into 

 our class of thick films. The turning blue will therefore be again referred to when 

 we come to consider thick films of gold. 



When fj. "0, the red light is much less diminished in intensity than the yellow, 

 and probably less than the blue. The film is therefore pink, and remains pink down 

 to the dimensions of coloured glass for which p. is of the order of 10~ 5 . The thin film 

 observed by Mr. Bra LBV was rose pink after annealing (p. 'I'll]. 



The high transparency observed by FARADAY and by Mr. BEILBY corresponds to 

 the very small values of n~K for values of /i < '5. 



Consider next a thick film of gold. 



The absorption being now, according to the result given on p. 409, dependent 

 principally upon the value of HK/\, we see from the table for HK or from the graph 

 that, for the solid metal, yellow light is less absorbed than red. The colour of thick 

 gold leaf is, in fact, olive-green by transmitted light. As /u. diminishes the absorption 

 of both yellow and red increases, the latter more rapidly. Now when /x = 734, there 

 is a great absorption of red, according to the values of UK, which is intensified, since M 

 is for this value of // reduced to '177. The colour should then be more yellow than 

 red. and probably more blue than either. When /x is < 7, the colour is much more 

 red than yellow. If our assumed curve for UK for blue is correct, the colour of the 

 film should be blue between ^ = "85 and 7, purple at 7, and principally red from 

 Ij. -- -G5 through all the range of values of /JL from gold glass down to /x = 0. (If our 

 curve for blue is correct, the figure shows that the film is red when the blue curve 

 crosses the yellow.) 



According to Mr. BEILBY, a gold film, originally green, turned blue-purple after 

 annealing. Gold leaf turned, by annealing, pink with brown-green patches, the 

 latter, presumably, corresponding to large and the former to small values of /x. 



The rise in the absorption as tt begins to diminish from unity was noticed by 

 Mr. BEILBY (loc. cit., p. 232). 



