LE ROY C. COOLEY. 83 
But, in equation (1) we already have, for the weight 
of this mass, the expression se Dividing this by the 
f 
number of particles, an we have for the weight z of one 
particle, 
one i a. (2) 
In this expression the values of w, 7 and » depend on 
the nature of the coloring substance, the volume of wa- 
ter employed, and the dimensions of the unit volume 
employed. But d is aconstant: it represents the limit of 
visibility—the smallest linear magnitude which the eye 
can perceive. 
We have now to consider the most probable value of 
this factor. ‘ 
Several estimates of this value have been made. Tait 
gives zoiov inch as the smallest space which can be de- 
tected in reading the graduation of a vernier. Mayer 
has ingeniously shown’ that the eye can detect a line 
drawn on a dull surface when its width is sctoo inch, and 
that a black disc on a white surface can be detected when 
it measures sic inch in diameter. Itis evident, from these 
different estimates, that the limit of visibility depends on 
the kind of magnitude which is observed. 
Now the spaces between the particles in solution are 
certainly not lines. They are doubtless volumes. But 
as they lie before the eye they will present themselves as 
areas. They resemble Mayer’s discs. But, inasmuch as 
we see the color of the solution by transmitted light, the 
eye is affected by the luminous energy that filters 
through them, so that these spaces should be regarded 
rather as luminous discs on a dark ground, whereas in 
Mayer’s experiments the discs were black upon a ground 
of white. Whether this reversal of conditions would af- 
1‘* Minute Measurements in Modern Science.”—Sci. Am. Sup., Vol. 
III., page 879. 
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