Equal appearing intervals—Just perceptible distances—The interpretation of Weber's Law—Indirect methods of measurement—The approach to measurement by means of grading magnitudes and their differences.

EQUAL APPEARING INTERVALS

THE pre-conditions of measurement in any sphere of experience are (1) the homogeneity of the phenomena, or of any particular aspect of it, to be measured, (2) the possibility of fixing a unit in terms of which the measurement may be made, and of which the total magnitude may be regarded as a mere multiple or sub-multiple. These pre-requisites are satisfied in the cases of spatial and temporal magnitudes, in terms of which, directly or indirectly, all the measurements of the physical sciences are expressed. It was thought by Fechner that they are also satisfied in the case of the strictly psychical phenomena of sensationintensity, i.e. it was assumed that any given sensation-intensity might be regarded as made up of a sum of unit sensation-intensities. This view has been definitely rejected by many later psychologists in whose opinion every sensation-intensity is qualitatively distinct from every other sensation-intensity. “ To introspection, our feeling of pink is surely not a portion of our feeling of scarlet; nor does the light of an electric arc seem to contain that of a tallow:candle in itself” (James). Such writers contend that Fechner's mistake was due to a confusion of produce them.

Nevertheless, purely psychical measurement is not entirely impossible. Within any one series of sensation-intensities, e.g. a series of greys, the contrasts or “distances” separating different pairs of intensities are perfectly homogeneous with one another and can be measured in terms of one another or in terms of an arbitrarily chosen unit of “ sensedistance.” Given two brightness-intensities a and b, it is quite possible to find, within limits of error, a brightness-intensity c which is as much higher than b in the scale of intensities as 6 is than a, i.e. such that the sense-distance bc = the sense-distance ab; or, again, it is quite possible, theoretically, to find a brightness-intensity d which bisects the sensedistance ab, i.e. which is such that it is as far removed from a in the scale of intensities as b is from it—in symbols, ad = db.