Williams |
The chief purport
of this classical book of the German psycho-physiologist was the elaboration
and explication of experiments based on a method introduced more than twenty
years earlier by his countryman E. H. Weber, but which hitherto had failed
to attract the attention it deserved. The method consisted of the measurement
and analysis of the definite relation existing between external stimuli
of varying degrees of intensity (various sounds, for example) and the mental
states they induce. Weber's experiments grew out of the familiar observation
that the nicety of our discriminations of various sounds, weights, or visual
images depends upon the magnitude of each particular cause of a sensation
in its relation with other similar causes. Thus, for example, we cannot
see the stars in the daytime, though they shine as brightly then as at
night. Again, we seldom notice the ticking of a clock in the daytime, though
it may become almost painfully audible in the silence of the night. Yet
again, the difference between an ounce weight and a two-ounce weight is
clearly enough appreciable when we lift the two, but one cannot discriminate
in the same way between a five-pound weight and a weight of one ounce over
five pounds.
This last example, and similar ones for
the other senses, gave Weber the clew to his novel experiments. Reflection
upon every-day experiences made it clear to him that whenever we consider
two visual sensations, or two auditory sensations, or two sensations of
weight, in comparison one with another, there is always a limit to the
keenness 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 until they were revived and extended by Fechner and brought before
the world in the famous work on psycho-physics. Then they precipitated
a veritable melee. Fechner had not alone verified the earlier results (with
certain limitations not essential to the present consideration), but had
invented new methods of making similar tests, and had reduced the whole
question to mathematical treatment. He pronounced Weber's discovery the
fundamental law of psycho-physics. In honor of the discoverer, he christened
it Weber's Law. He clothed the law in words and in mathematical formulae,
and, so to say, launched it full tilt at the heads of the psychological
world. It made a fine commotion, be assured, for it was the first widely
heralded bulletin of the new psychology in its march upon the strongholds
of the time-honored metaphysics. The accomplishments of the microscopists
and the nerve physiologists had been but preliminary - mere border skirmishes
of uncertain import. But here was proof that the iconoclastic movement
meant to invade the very heart of the sacred territory of mind - a territory
from which tangible objective fact had been supposed to be forever barred. |
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