5 mm., then his stimulus-limen is two olfacties, and his olfactus 1⁄2. The olfacty used by olfactometry becomes for each substance the unit of odorimetry. Odorimetry is correlated with photometry and phonometry, Both olfactometry and odorimetry are branches of “olfactology” (to anglicize another word used by Dr. Zwaardemaker). This again is correlated with optics, acoustics and haptics.
The interdependence of olfactometry and odorimetry is not unique. The unit of photometry, i. e., the unit for the measurement of light in the physical sense, is the illuminating power for sensation of the light of some standard candle. “We have no adequate objective method,” writes Prof. Külpe, “of ascertaining the intensity of the non-periodic and aperiodic concussions which form the substrate of simple or complex noises, independently of the statement of the observer whose sensitivity we are testing. The phonometric determination of sound intensities in psychophysical experiments is usually carried out upon a principle similar to that employed in photometry. As the objective stimulus-values in the apparatus employed,—say, elastic balls falling from a measurable height on a resisting plate,—are determined by way of a subjective comparison, the results are purely empirical, valid only for the material used, the special circumstances of the observation, ete.”[1]
The peculiarly unsatisfactory character of the determinations of olfactometry and odorimetry is due chiefly to the fact that olfactory qualities, unlike visual and auditory, are not demarcated. It is true that it is more difficult to keep uniform the duration and extension of smell-stimuli than it is to regulate these attributes for other stimuli, with the possible exceptions of temperature and taste. It is also true that the great gulf of psychophysics, our ignorance of the physiological processes which everywhere link the strictly physical to the psychological, is wider in the cases of temperature, taste and smell, than in the cases of vision, audition, pressure and strain, Yet, at best, the measurements of physics must always be in terms of sensation, and the measurement of sensation must always be in terms of physics.
It seems wise to emphasize at the outset the initial difficulty which makes all quantitative work in smell more or less unsystematic, viz., the indeterminateness of olfactory qualities, It is at present necessary to regard as a simple and separate quality the odor of every substance which from a physical point of view is unmixed ; yet, for several reasons, it is un-
- ↑ Outlines of Psychology, tr., p. 156.
