The methods for the demonstration of Weber's law, of relations between psychical quantities, whether elementary compound, are usually called psycho-physical methods. is unsuitable, however, because the fact that physical here employed is not unique, but holds for all the experimental psychology. They could better be capable, for the measurement of psychical quantities". With these methods it is possible to follow one of two courses in relations mentioned as favorable for judgment. A direct mode of procedure is as follows: one of two psychical quantities A and B, as, for example, A is kept constant, and B is [p. 257] gradually varied until it stands in one of the relations mentioned, that is, either equals A or is just noticeably greater or smaller, etc. These are the adjustment-methods. Among these we have as the method most frequently applied and that which leads most directly to conclusions, the "method of minimal changes", and then as a kind of modification of this for the case of adjustment until equality is reached, the "method of average error". The second mode of procedure is to compare in a large number of cases any two stimuli, A and B, which are very little different, and to reckon from the number of cases in which the judgments are A = B, A > B, A < B, the position of the relations mentioned, especially the difference-threshold. These are the reckoning - method. The chief of these is the method known as that of "right and wrong cases". It would be more proper to call it the "method of three cases" (equality, positive difference, and negative difference). Details as to this and the other methods belong in a special treatise on experimental psychology.
There are two other interpretations of Weber's law still met with besides the psychological interpretation given above; they may be called the physiological and the psycho-physical theories. The first derives the law from hypothetically assumed relations in the conduction of excitations in the central nervous system. The second regards the law as a specific law of the "interaction between body and mind". The physiological interpretation is entirely hypothetical and in certain cases, as, for example, for temporal and spacial ideas, entirely inapplicable. The psycho-physical interpretation is based upon a view of the relation of mind which must be rejected by the psychology of to-day (cf. § 22, 8).
11. As special cases in the class of apperceptive comparisons generally falling under Weber's law we have the comparison of quantities that are the relatively greatest sensational differences or, when dealing with feelings, opposites. The phenomena that appear in such cases are usually gathered up in the class-name contrasts. In the department where contrasts have been most thoroughly investigated, in residual sensations, there is generally an utter lack of discrimination [p. 258] between two phenomena which are obviously entirely in origin, though the results are to a certain extent related. We may distinguish these a physiological and psychological, contrasts. Physiological contrasts are closely connected with. the phenomena of after-images, perhaps they are the same (p. 68 sq.). Psychological contrasts are essentially different; they are usually pushed into the background by the stronger physiological contrasts when the impressions are more intense. They are distinguished from the physiological by two important characteristics.
First, they do not reach their greatest intensity, when the brightness and saturation are greatest, but when they are at the medium stages, where the eye is most sensitive to changes in brightness and saturation. Secondly, they can be removed by comparison with an independent object. Especially the latter characteristic shows these contrasts to be unqualifiedly the products of comparisons. Thus, for example, when a grey square is laid on a black ground and close by a similar grey square is laid on a white ground and all is covered with transparent paper, the two squares appear entirely different; the one on the black ground looks bright nearly white, that on the white ground looks dark, nearly, black. Now after-images and irradiations are very weak when, the brightness of the objects is small, so that it may assumed that the phenomenon described is a, psychological contrast. If, again, a strip of black cardboard which is covered with the transparent piper., and therefore exactly the same grey as the two squares, is held in way that it connects the two squares the contrast is removed entirely, or, at least, very much diminished. If in this experiment a colored ground is used instead of the achromatic, the grey square will appear very clearly in the appropriate complementary color. But here, too, the contrast can be made to disappear through comparison with an independent grey object.
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