44 
of eyesight is the greatest distance at which a square of known 
size that has two of its sides vertical as in fig. i, can be 
Fig. i. Fig. 2. 
distinguished from another of the same size that has one of 
its diagonals vertical as in fig. 2. These two figures represent 
the opposite faces of the same card (or of painted zinc, which 
would be more durable), which is intended to be hung against 
a rather dark background. Procure three such pieces of card 
with squares on them of 3 cm., 2*5 cm., and 2 cm. in the side, 
respectively. An assistant displays and changes them, you 
stand by the person who is being tested, and, using an opera 
glass, note the greatest distance at which he can distinguish 
with certainty the two positions of the 2*5 cm. square. The 
object of the larger and of the smaller card is partly to dis¬ 
cover that distance quickly, and partly to serve as a check 
against the man’s apparent power of distinguishing the 
positions being due to lucky guesses. The cards must hang 
truly, and this will be the more easily ensured if weights 
are attached to holes in their lower ends. 
The testing must be performed when the light is perfectly 
good, but not dazzling. Always test yourself when you are 
testing others, because if your own efficiency comes up to its 
normal standard, it is fair evidence that the conditions of 
light, &c., are normal also, otherwise very probably they are 
not. 
F. G. 
