Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: LECTURE IV § I. The Just Noticeable Sensation. § II. Upper And Lower Limit Of Weber's Law. § III. Psychological Interpretation Of The Law. 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, emust 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 -J-jj- gramme. Less sensitive are the inside ef 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 -fa 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 auditorymeatus or any contact with the tympanic membrane excites, as we all know, a fairly strong sound-sensation. And ...