5i6 The American Naturalist. LJ^ne. 
One child knows a letter or figure, remembers it, under- 
stands it, on seeing it once; another requires twenty times; 
while the learned pig can only spell or count on being shown 
the same letter or figure, a hundred, maybe a thousand times. 
Animals can be taught many mental operations. I recall 
the performing elephants, dogs, bears, monkeys, birds, even 
the fly and the flea, while Sir John Lubbock has ascertained 
the sensibility of the ant in the matter of sight, hearing, smell, 
and has shown that he can receive impressions through these 
senses which lead up to perception, understanding and deci- 
sion. Sir John discovers that the ant has a goverment, and 
consequently a governor. He will emigrate to any other coun- 
try, can organize an army, make raids, fight battles, take pris- 
oners whom he enslaves. It is highly interesting to count the 
needed sensations, impressions, and perceptions required to 
perform all these mental operations. 
The same system by which this can be measured or counted 
in our experiment with the animal can be applied to man. 
What are school examinations or college commencements 
but tests by measurement or counting of the mental capabili- 
ties of the students? They may be only for comparison one 
with another ; but that comparison is made by measurement 
more or less indefinite. At West Point and Annapolis the 
comparison is made with an absolute standard, in which lOOO 
is perfection. The system may be anomalous, for unblacked 
shoes and unkempt hair may so reduce it as to defeat his pro- 
Color blindness and astigmatism are measured by the oculist. 
They may be diseases or caused by defective mechanism, but 
these are only determined by measuring the sensations pro- 
duced in the mind. 
The measurement of a mental sensation is something accom- 
plished, and I would pursue the same line of investigation to 
its end. Examine the candidate for the truthfulness, the fidelity 
of his sensation. First as to his sense of color. The question is, 
what sensation is produced upon his mind by the sight of a 
strand of worsted of a given color ? Does green give the 
proper sensation, or purple, or red ? This is a test of the cor- 
rectness of the impression as to color. The operation is done 
leisurely. Rapidity is not now required. It is accuracy 
which is now being tested. As his examination progresses 
mark on the chart his successes or his failures. The fault in 
his sense of color may result from a species of disease. But 
