What now, it may be asked, is the relation between the two scales, the one of stimulus differences, the other of intensity differences? Do equal differences in intensity of sensation correspond with equal differences of intensity of stimulus? In seeking the answer to this problem we may employ either of our two units of sensation. We have only to determine the physical intensity of every point on the supra liminal scale, and then see whether the physical differences are or are not equal to the sensation differences; or, employing the other unit, to take a number of DL's from some portion of the scale and then compare the differences be tween their magnitudes. Since these magnitudes are expressed in terms of the stimulus, the rela tion of their differences will furnish an im mediate answer to our question.
We find, whichever unit we employ, that the two scales do not correspond point for point, that equal differences in intensity of sensation correspond rather with relatively equal dif ferences in the intensity of stimulus. Let us suppose, for example, that the points on the sensation scale form an arithmetical series like 1, 2, 3, 4, etc., then the points on the stimulus scale to which these will correspond will form a geometrical series like 2, 4, 8 16, etc., or like 3, 9, 27,81, etc.; the exponent of the geometrical series varies with the sense department — for noise it is 4/3, for light 101/100. This law,
which is known as Weber's Law (q.v.), ex plains many phenomena of everyday life which, however, are so common that they often fail to excite our notice. The relation of black print to the white page seems always the same, yet the intensity of stinlight changes with every hour of the day; the painter is able to simulate a natural scene, yet he cannot begin to reproduce the actual intensities of light and shadow by means of his pigments; the same musical com position may be rendered by orchestra and again by piano without loss of dynamic effect, yet the intensity of the former is much greater than that of the piano. All these result from the fact that the sense impressions depend not upon their absolute but upon their relative like ness. Consult Delboeuf, J., 'Elements de psychophysique' (Paris 1883) ; Ebbinghaus, H., Grundziige der Psychologie (Leipzig 1905) ; Fechner, G: T., 'Elemente der Psychophysik' (Leipzig 1899) ; Kiilpe, O., of Psy (New York 1910) ; Myers, C. S.. 'Textbook of Experimental Psycholpgy (Lon don 1910) ; Nagel, W., 'Handbuch der Physio logie des Menschen, (Braunschweig 1905) ; Titchener, E. B., Psychology, II' (New York 1905) ; 'Textbook of Psychology (New York 1910) : Wundt. W.. der physiologischen Psychologie) (Leipzig 1908).