ON COLOURS IN METAL GLASSES, ETC. 



261 



Now, however, suppose that v' = v, so that the light is reflected at the interface 

 between colourless glass and the same glass containing small spheres of metal. 

 Neglecting p. 3 , equation (13) then reduces to 



(15). 



Since this expression for R contains no large constant term, the light from the 

 interface will in this case be highly coloured in the case of those metals for which 

 a 3 +4/8 2 varies greatly for different values of X. 



If, however, the absorbing medium contain molecularly divided metal, equations (10) 

 are replaced by 



2 + " 2 - A (11). 



n" = 



Replacing 3/*/3 and 3/j.a. in equations (14) and (15) by (2 + v 2 )/v 2 . /3' and (2 + v 2 )/v 2 . a' 

 respectively, we obtain, as the intensities of the light reflect from the front face of 

 the stain and from the interface respectively, 



<>> 



(17). 



As before, it appears that when the stain is held towards the eye the reflection 

 R/ is almost as if the stain were not there ; while when the stained face is away from 

 the eye, the reflection is highly coloured. 



Sir WILUAM ABNEY has kindly measured for me the intensities R of light 

 reflected from the interface between the unstained and stained regions of one of 

 STOKES' specimens of silver glass. The values of R are given in the following 

 table : 



TABLE VI. Blue Reflection from Silver Glass. Measured Value of 



v at D Lines = T532. 



