84 LIMIT OF VISIBILITY FOR MINUTE MASSES. 
fect Mayer’s value of the limit of visibility can be deter- 
mined by experiment. The question is whether the eye 
can detect a smaller magnitude by the positive action of 
light received from it, than by the absence of action due 
to the loss of light which it intercepts. 
Experiments to answer this question were instituted 
upon Mayer’s’* plan, considerably modified, as follows: 
Two blocks of wood, each eight inches square, were pre- 
pared with smooth surfaces made dead black. A disc of 
white paper, 4mm in diameter, was cemented upon the 
face of one near its middle point. With this exception, 
the two blocks were as nearly as possible alike. Upona 
large lawn, under a clear sky, the tests were made as fol- 
lows: An assistant would hold the two blocks side by side 
while the observer, with his back toward the sun, would 
find a position so far in front of them that he could no 
more than detect the white spot with certainty, which 
was known by his being able to declare unerringly which 
block carried the spot, the blocks being changed in posi- 
tion, so that the observer could never know whether the 
disc was on the one at the right or left, above or below. 
The distance between the observer and the block was 
then measured. The vision of thirty young women, con- 
stituting a class in physics, was tested in this way, and 
the average distance was found to be 247 feet. 
A disc 4mm in diameter, at a distance of 247 feet, sub- 
tends the same angle of vision as one of 0.013495 mm at 
a distance of 10 inches—the average distance of distinct 
vision for small objects. Accordingly, for the average 
eye among the thirty individuals tested, the smallest 
white object that can be detected on a black ground is 
one whose diameter is 0.001349cm. Forsuch an eye, this 
is the value of @ in the formula (2), which thus becomes 
1«‘Minute Measurements in Modern Science.”—Sci. Am. Sup., Vol. 
ITI., page 879. 
St 
