opposite influence makes one’s numerical results more nearly correct than they would otherwise be. On the other hand, it makes the exact influence of each source of error more difficult to read from the figures. Vet it is not particularly difficult to detect the effect of the exhaustion when it is at all marked, and to exclude the most unreliable determinations. In our experience of thirteen different subjects, complete or marked anosmia from exhaustion, if it occurred at all, usually came on very suddenly.
Section 4. Pychophysical Methods Employed.
Before difference-determinations were made at all, the stimulns-limen was usually found as accurately as possible for the substance and subject concerned. The subject, starting with the end of the odorous cylinder even with the end of the inhaling-tube, moved the cylinder outward until he obtained a smell. If this smell seemed to him more than liminal, he moved the eylinder back for a short distance, and continued to move backwards and forwards until he had satisfied himself as to the point at which he obtained a just noticeable sensation. The method of moving steadily in both directions,—from a point considerably below to a point just above the limen, and from a point considerably above to a point just below the limen,—was tried, but was abandoned. It is often impossible, on account of adhesion in the tube or in the nasal passages, or on account of memory after-images, or cumulative stimulation, to move from a point of intensive stimulation to a point at which sensation entirely disappears. Memory after-images certainly occur, The existence of true aſter-images of peripheral origin has not been proved in the case of smell.[1]
The only difference-determinations for smell, so far on record, are a few which Zwaardemaker performed for yellow wax and vulcanized rubber. The method which he employed, and the method which so far seems practicable, is Fechner's rough and simple method of just noticeable differences. One gives the subject a standard stimulus, and then after an interval, which one makes as nearly uniform as possible, a second stimulus which is appreciably greater or smaller, He himself then moves the cylinder until he makes the stimulus just greater or just smaller than the standard. When in the neighborhood of the stimulus, he moves back and forth as he likes, until he has satisfied himself of the accuracy of the determination. Thus, as there is near the limen procedure in both directions, the method may be classed as a gradation-method. The interval between the two stimuli averaged in our experiments 21⁄2
- ↑ P. 260.
