ON COLOURS IN METAL GLASSES, ETC. 



247 



TABLE III. Copper. 



We have now to compare the observed optical properties of various media coloured 

 by gold, silver, and copper with those corresponding properties which, according to 

 the Tables I., II., and III., would be exhibited by the media if the colouring agent 

 were the metal itself, in either the molecularly subdivided or the granular state. 



The simplest of the optical properties to observe and to measure is the absorption of 

 light by the medium. Although the absorptions of colloidal solutions of various 

 metals, and even of suspensions of metals in gelatine, have already been measured for 

 several values of the wave-length, X, of light, no such measurements of the absorption 

 of glasses coloured by metals appear to have yet been made.* Owing, however, to 

 their permanence, such media seemed likely to yield the surest information as to the 

 chemical and physical nature of their colouring agents. The absorptions of a series 

 of glasses coloured with gold, silver, and copper have therefore been measured for me 

 at the National Physical Laboratory, under the supervision of Mr. F. J. SELBY. The 

 silver glasses consisted of a silver stain on one side of a colourless glass, the refractive 

 index of which was, for sodium light, equal to v = 1'579. The gold ruby glasses 

 were flashed on to colourless glass. Both the silver and gold glasses were, to 

 ensure purity of materials, specially prepared at the Whitefriars Glass Works by 



* Except two gold glasses, the absorption curves for which are given by ZSIGMONDY ('LiEB. Ann.,' 

 vol. 301, pp. 46-48). 



