1857.] of Gold (and other Metals) to Light. 415 



bright sunlight it will appear light brown and almost opake. 

 From behind, the same fluid may appear of a pure blue in both 

 lights, whilst from the side it may appear amethystine or ruby. 

 These differences result from the mixture of reflected and 

 transmitted lights, both derived from the particles, the former 

 appearing in greatest abundance from the front or side, and the 

 latter from behind. The former is seen by common observation 

 in a purer state if a black background be placed behind the 

 fluid ; when a white background is there, much of the trans- 

 mitted light from that source comes to the eye, and the ap- 

 pearance is greatly altered. A mode of observing the former 

 by a strong ray of light and a lens has been already described ; 

 but even in that case some effects of transmitted light are 

 observed if the focus is thrown deep into the fluid ; and it is 

 only the particles near the surface, whether illuminated by the 

 base or the apex of the cone, which give the nearly pure effect 

 of reflexion. In order to observe the transmitted ray in an 

 unmingled state, a glass tube closed at one end was surrounded 

 with a tube of black paper longer than itself, and with the 

 black surface inwards. When a fluid (or the particles in it) 

 was to be examined, it was put into this tube, and a surface of 

 white paper illuminated by daylight or the sun, regarded 

 through it, other light being excluded from the eye ; or the 

 tube was sometimes interposed between the eye and the sky, 

 and sometimes the rays of the sun itself were reflected up to 

 the eye through it. In speaking hereafter of the tints of the 

 light transmitted by the particles (which will of course vary 

 with the proportion of different rays in the original beam of 

 light), a pure white original light is to be understood, but 

 occasionally differently-tinted papers were employed with this 

 tube as sources of different coloured lights. 



The very oblique angle at which reflected light comes to 

 the eye from the diffused particles, is well seen when the lens 

 cone, or a direct ray of the sun, is passed into the fluid and 

 observed from different positions ; it is only when the eye is 

 behind and nearly in the line of the ray, that the unmixed 

 transmitted ray is observed. In the dark tube I think that no 

 reflected light arrives at the eye : for if half an inch in depth 

 of water be introduced, white light passes ; if a drop of the 

 washed deposit, to be hereafter described, be introduced, the 



