galton.org 25
Sequence of Test Weights
25
which the operatee habitually employs his senses. It does not measure his
capacity for discrimination, because the discriminative faculty admits of
much education and the test results always show increased delicacy after a
little practice. However, the requirements of everyday life educate all our
faculties in some degree, and I have not found the performances with test
weights to improve much after a little familiarity with their use. The
weights have, as it were, to be played with at first, then they must be tried
carefully on three or four separate occasions.
I did not at first find it at all an easy matter to make test weights so
alike as to differ in no other appreciable respect than in their specific
gravity, and if they differ and become known apart, the knowledge so
acquired will vitiate future judgments in various indirect ways. Similarity
in outward shape and touch was ensured by the use of mechanically-made
cartridge cases; dissimilarity through any external stain was rendered of
no hindrance to the experiment by making the operatee handle them in a
bag or with his eyes shut. Two bodies may, however, be alike in weight
and outward appearance and yet behave differently when otherwise
mechanically tested, and, consequently, when they are handled. For
example, take two eggs, one raw and the other hard boiled, and spin them
on the table; press the finger for a moment upon either of them whilst it is
still spinning: if it be the hard-boiled egg it will stop as dead as a stone: if
it be the raw egg, after a little apparent hesitation, it will begin again to
rotate. The motion of its shell had alone been stopped; the internal part
was still rotating and this compelled the shell to follow it. Owing to this
cause, when we handle the two eggs, the one feels
quick and the other
does not. Similarly with the cartridges, when one is rather more loosely
packed than the others the difference is perceived on handling them. Or it
may have one end heavier than the other, or else its weight may not be
equally distributed round its axis, causing it to rest on the table with the
same part always lowermost; differences due to these causes are also
easily perceived when handling the cartridges. Again, one of two similar
cartridges may balance perfectly in all directions, but the weight of one
