lies in determining the particular sequence of weights that should
be employed. Mine form a geometric series, for the reason that when
stimuli of all kinds increase by geometric grades the sensations
they give rise to will increase by arithmetic grades, so long as the
stimulus is neither so weak as to be barely felt, nor so strong as
to excite fatigue. My apparatus, which is explained more fully in the
Appendix, consists of a number of common gun cartridge cases filled
with alternate layers of shot, wool, and wadding, and then closed in
the usual way. They are all identical in appearance, and may be said
to differ only in their specific gravities. They are marked in
numerical sequence with the register numbers, 1, 2, 3, etc., but
their weights are proportioned to the numbers of which 1, 2, 3, etc.,
are the logarithms, and consequently run in a geometric series.
Hence the numbers of the weights form a scale of equal degrees of
sensitivity. If a person can just distinguish between the weights
numbered 1 and 3, he can also just distinguish between 2 and 4, 3 and
5, and any other pair of weights of which the register number of
the one exceeds that of the other by 2. Again, his coarseness of
discrimination is exactly double of that of another person who can
just distinguish pairs of weights differing only by 1, such as 1 and
2, 2 and 3, 3 and 4, and so on. The testing is performed by handing
pairs of weights to the operatee until his power of discrimination
is approximately made out, and then to proceed more carefully. It is
best now, for reasons stated in the Appendix, to hand to the
operatee sequences of three weights at a time, after shuffling them.
These he has to arrange in their proper order, with his eyes shut,
and by the sense of their weight alone. The operator finally records
the scale interval that the operatee can just appreciate, as being
the true measure of the coarseness (or the inverse measure of the
delicacy) of the sensitivity of the operatee.