s of our
discrimination, and that this degree of keenness varies, as in the case
of the weights just cited, with the magnitude of the exciting cause.

Weber determined to see whether these common experiences could be
brought within the pale of a general law. His method consisted of making
long series of experiments aimed at the determination, in each case, of
what came to be spoken of as the least observable difference between the
stimuli. Thus if one holds an ounce weight in each hand, and has tiny
weights added to one of them, grain by grain, one does not at first
perceive a difference; but presently, on the addition of a certain
grain, he does become aware of the difference. Noting now how many
grains have been added to produce this effect, we have the weight which
represents the least appreciable difference when the standard is one
ounce.

Now repeat the experiment, but let the weights be each of five pounds.
Clearly in this case we shall be obliged to add not grains, but drachms,
before a difference between the two heavy weights is perceived. But
whatever the exact amount added, that amount represents the stimulus
producing a just-perceivable sensation of difference when the standard
is five pounds. And so on for indefinite series of weights of varying
magnitudes. Now came Weber's curious discovery. Not only did he find
that in repeated experiments with the same pair of weights the measure
of "just-{p}erceivable difference" remained approximately fixed, but
he found, further, that a remarkable fixed relation exists between
the stimuli of different magnitude. If, for example, he had found it
necessary, in the case of the ounce weights, to add one-fiftieth of an
ounce to the one before a difference was detected, he found also, in the
case of the five-pound weights, that one-fiftieth of five pounds must be
added before producing the same result. And so of all other weights; the
amount added to produce the stimulus of "least-appreciable difference"
always bore the same mathematical relation to the magnitude of the
weight used, be that magnitude great or small.

Weber found that the same thing holds good for the stimuli of the
sensations of sight and of hearing, the differential stimulus bearing
always a fixed ratio to the total magnitude of the stimuli. Here, then,
was the law he had sought.

Weber's results were definite enough and striking enough, yet they
failed to attract any considerable measure of attention u