LE ROY ©. COOLEY. 79 
This value will vary with the substance, inasmuch as 
molecular absorption by different kinds of matter is not 
the same. Moreover, it will vary with the sensitivity of 
the eye. Butif in any case this value can be ascertained, 
it will, for that case at least, be the limit of visibility for 
mass. 
The limit of visibility for linear magnitudes of various 
kinds has been estimated. For example, according to 
Tait, a difference in position of two lines on a vernier 
equal to ,j55 Of an inch can be detected. Now we can- 
not declare that the particles of a coloring matter in so- 
lution, even when the quantity taxes the sensibility of 
the eye, are actually separated by this distance. But we 
are entitled to say that if the quantity dissolved in a 
given volume of solution be broken into particles whose 
masses have a certain small and uniform value, and be sep-. 
arated by distances uniformly equal to this limit of visi- 
bility, then the eye receiving light transmitted by the so- 
lution will perceive a color, the intensity of which it is 
just able to detect. Moreover, if the particles lie in 
closer proximity, as, in fact, they probably do, they must 
be more numerous, and their masses must be less in ex- 
actly the same proportion, so that the total mass within 
the distance of zotoo of an inch—the limit of visibility— 
is the same. In every colored solution, the divisibility 
of the coloring matter must extend, at least, to particles 
whose mass is equal to the sum of all the smaller parti- 
cles which, possibly, lie within the boundaries of the 
space whose size does not exceed the smallest which the 
eye is able to perceive. What, in any substance, is the 
actual value of one of these small masses ? 
In what follows, I attempt to answer this question by 
showing 
1. How we may find the actual weight of the substance 
which, dissolved in a unit volume of solution, can be de- 
tected by the eye. 
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