LECTURE  IV

§ I. The Just Noticeable Sens.ation. § II. Upper and Lower Limit
OF Weber's Law. § III. Psychological Interpretation of the
Law.

§1

FOR the solution, of all the problems which may arise in
any definite sense-department, there are required, as we
have seen, two kinds of measurement. First, w6 must
know the constant relation in which alteration of sensation-
intensity stands to alteration of the intensity of stimulus ; and,
secondly, the magnitude of the just noticeable sensation must
be determined. The first of these measurements we have
carried out; the second now remains to be performed.

Pressure-sensations afford us the simplest conditions for our
investigations. We lay upon that portion of the skin whose
sensibility is to be tested small weights, preferably of cork or
pith, and seek to ascertain what magnitude of weight is
necessary for the production of a just noticeable sensation.
Observations made in this way have shown that the sensibility
of the skin at different parts of its surface is very far from being
uniform. The most sensitive portions are the forehead, temples,
eyelids, the outer surface of the fore-arm, and the back of the
hand. We can usually sense on these parts weights of only
yi-0- gramme. Less sensitive are the inside of the fore-arm, the
cheeks, and the nose, and very much less sensitive than these
the palm of the hand, the abdomen, and the thigh. Here the
sensibility sinks to about -^ gramme. On some specially
protected parts,—e.g., the nails and the heel,—the just noticeable
weight rises as high as a whole gramme.

Far more adequate for the apprehension of weak stimuli is
our organ of hearing.    A mere touch of the external auditory