718 LIMIT OF VISIBILITY FOR MINUTE MASSES. 
vent, separated by distances which depend on the num- 
ber of particles in a given volume, and which become less 
as the quantity of the solid increases. There is a limit 
to the sensibility of the eye in the perception of minute 
magnitudes, and when a small mass of coloring matter is 
dissolved in a large volume of liquid its particles may be 
so small and the distances between them so large that the 
eye can receive no impression from them: the solution is 
then colorless. Ina word, if the particles of a coloring 
matter in solution are too small to affect the eye sepa- 
rately, and the distances between them are not below the 
‘*limit of visibility,’’ or the smallest distance which the 
eye can perceive, then the presence of the substance can- 
not be detected by vision. 
By increasing the quantity of the coloring matter in a 
given volume of solution, its minute particles will be in 
closer proximity to one another. When the distances 
which separate the particles are reduced to the point at 
which they pass the ‘‘limit of visibility,’ then, if the 
masses of the particles are sufficiently large, the eye will 
perceive a continuous and uniform color. We may sup- 
pose the distances between these minute pieces of color- 
ing matter in solution to remain uniform and constant in 
value, and, in this case, the intensity of the color per- 
ceived by the eye which receives transmitted light, must 
depend upon the masses of the particles. If these 
masses are sufficiently small, the luminous energy ab- 
sorbed by their molecules will be too small in quantity 
to affect the eye, and the solution will be colorless ; but 
if, on the other hand, they are sufficiently large, the loss 
of luminous energy by absorption will be readily de- 
tected, and the corresponding color will be distinctly 
seen. It is evident that the masses may have a certain 
definite value, lying somewhere between the two values. 
just described, which will enable them to produce a’color 
of such intensity that the eye is just able to detect it. 
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