596 c. STUMPF'S TONPSYCHOLOGIE, i. jective credibility, those in judgments of Class II. to defective sensibility, inore particularly discriminative. We come now to the measurement of the credibility or degree of trustworthiness of judgment. Objective credibility is measured by the well-known psycho-physical series of trials. These deter- mine in the first place nothing respecting the relations between the stimulus and the sensation (as was first erroneously supposed), but simply " how large the difference or what the ratio of two stimuli must be in order to bring about judgments of a certain degree of probability (Cl. I.) or of exactness (Cl. II.) under definite circumstances, e.g., in a definite region of tone in the case of a particular individual ". We may, however, measure the separate factors, namely, sensibility and subjective credibility. The discussion of the second, which, as the author remarks, is of much the greater psychological value, is particularly interesting both by way of showing what can and what cannot be done. The number of psychical circumstances which complicate the judgment is well brought out. Thus, the author refers to a distinguished painter who was quite incapable of judging of colour in the manner required by the physiologist and psychologist. The measurement of sensibility presupposes that we know the subjective credibility. We ascertain this by deducting from the objective credibility, which is all that is directly measurable, that which is due to subjective credibility. For this purpose, judg- ments of Class II. and individuals of the highest subjective credibility are most suitable. The discussion of the measurement of discriminative sensibility leads on to a consideration of the knotty point whether the just noticeable differences of sensation in different regions of stimuli are to be regarded as equal or only as " equally noticeable ". The relation of the author's view of the measurement of judgment to Fechner's psycho-physical theory is then touched on. This last is said to constitute a chapter and, in the order of investigation, the last chapter in a doctrine of measurement of judgment. The author's conception of the general methods to be pursued in such measurement is unfolded more fully in an appendix to this section. Next follows a special section on attention, practice (Uebung), and fatigue. With respect to the nature of attention all that can be said is that it is identical with interest, and that interest is a feeling. The conditions of attention are briefly dealt with, after which its effects are considered. The author takes special pains to show that attention does not intensify or add to the strength of an impression, by arguing that if this were so it would tend to confuse rather than aid judgments respecting intensity. By practice, is meant the result of exercise, an acquired disposi- tion to judge more rapidly, definitely and with greater credi- bility. The effects of practice show themselves in two directions the improvement of the power of reproducing impressions, and of the power of concentration. The general course of practice, that
