86 LIMIT OF VISIBILITY FOR MINUTE MASSES. 
with water to about the same level. The colored solu- 
tion was then dropped from a burette, and thoroughly 
mixed with the water in one, until color could be fairly 
detected by contrast with the pure water beside it in the 
other. Asa mean of five fairly concordant experiments, 
it was found that 6.9 cc were required by the 200 ce of 
water. 
The calculations are as follows: The actual weight of 
the chromate, 2, which was added to the 200 cc of water 
was ve which is 0.69 mg. or 0.00069 gram. The vol- 
ume, 2, of the solution containing this weight was2 06.9 ce. 
The length, 2, of one side of the unit volume was the 
thickness of the stratum of fluid, or the distance between 
the walls of the tank, which was4cm. By substitution 
in formula (8), we have 
0.00069 x 4 0.0000018198 
C= ; SRS 
200.9 
It may be remarked that, in the construction of the ob- 
servation tank, the only measurement which needs to be 
known with exactness is the inside diameter from front 
to back. So long as the other dimensions remain con- 
stant, the same weight, a, of coloring substance, will 
impart the same tint to the solution, whatever the thick- 
ness, Z, of the stratum of fluid may be, but the volunte, 
, of the solution will vary directly as the thickness. 
The actual distance, therefore, between the sides of the 
tank may have any convenient value, but it is essential 
that this value should be accurately known. The front 
area of the tank is reasonably small in order that all 
points may be before the eye with equal distinctness at 
the same moment. 
_ The following results, all of which were obtained by 
the same observer, will still further illustrate the order 
of magnitudes reached by this method of testing the di- 
visibility of coloring substances. No doubt eyes of dif- 
ferent individuals would require some variation in the 
36 
= (),00000000002427 gram. 
