* 3 
Suppose the bottom of the containing cylinder to be per- 
fectly flat. Owing to the adhesion of the liquid to the 
sides, and the consequent elevation there above the general 
level, the colour will be more intense near the sides. This, 
however, if the cylinders are of moderate radius, may be 
remedied by covering the bottom with a black plate having 
a small aperture in the centre ; in this way the rays coming 
from the sides are cut off. But, even supposing this done, 
the colour will not be exactly the same over the whole 
area. Suppose an eye to be placed at A in the axis of the 
cylinder, and let CH represent the section by a vertical 
plane of the circular area at the bottom admitting light. 
Let AE be the ray which passes through the centre ; the 
path of this ray through the liquid will be EB. Let AFG 
be another ray which reaches the eye ; the path of this ray 
through the liquid is FG ; and as this is greater than EB, 
the absorption will be greater. Hence the colour will 
gradually increase in intensity as we pass from the centre 
