516 Prof. Faraday on the Experimental Relations 
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 trans- 
mitted lights, both derived from the particles, the former appear- 
ing 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 transmitted 
light from that source comes to the eye, and the appearance is 
greatly altered. A mode of observing the former by a strong 
ray of hight 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 ex- 
cluded 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 here- 
after 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 under- 
stood, 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 ra 
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 intro- 
duced, white light passes; if a drop of the washed deposit, to be 
hereafter described, be introduced, the light transmitted is either 
blue or ruby, or of other intermediate tint, according to the cha- 
racter of the deposit ; but if water be then added until the column 
is six inches or more in length, the quantity of light transmitted 
does not sensibly alter, nor its tint; a fact which I think excludes 
the idea of any light being reflected from particle to particle, and 
finally to the eye. 
If a given ruby-tinted fluid, containing no gold in solution, 
be allowed to stand for a few days, a deposit will fall from which 
