ANTONIO ALIOTTA, La Misura in Psicologia Sperimentale. 561 commonly adduced objection that sensations vary discontinuously, whereas the corresponding stimuli are variable continuously. " Infinite gradations of stimuli exist to which there are no corre- sponding changes in sensations " (p. 53). I hold that except at the threshold, this statement is demonstrably untrue. Supposing that an individual is just able to appreciate the difference between weights of 300 and 310 grams, but is unable to distinguish between 300 and 309 grams. According to the above view, the stimulus is capabb of being continuously changed from 300 to 309 grams without any change being produced in the corresponding weight- sensation ; it is only when 310 grams is reached that there is a sudden (discontinous) change in the sensation. Now if the sensation remains unchanged for the interval between 300 and 309 grams, it also remains unchanged when these two weights are re- placed by two of 309 and 318 grams. Consequently the sensations produced by weights of 300 and 318 should be indistinguishable, whereas they can be easily discriminated. To my mind the only escape from this difficulty is by admitting that the intensity of a supraliminal sensation varies pari passu with the magnitude of the stimulus, and by recognising that the denial of this relation is the result of confusion between sensation and sensation difference. It is the physiological basis of the sensation difference which has to reach a certain value before the sensations can be discriminated. While the sensations vary con- tinuously, the sensation difference has to pass beyond a threshold before it can be experienced. Indeed, nothing could be more apt in this connexion than Prof. Aliotta's own statement : " The differ- ence between two sensations, as revealed to our consciousness, is a psychical fact sui generis, not less simple than the sensations themselves to which it refers, although of different nature " (p. 74). It is only below the threshold of differential (and below the threshold of absolute) sensibility, that sensation differences (or sensations) fail to change continuously with change of magni- tude of stimulus ; under which conditions, of course, they cease to have experiential existence. Thus Weber's law is simply an expression of the constant relation between these thresholds and the magnitudes of the stimuli employed. It applies to the absolute threshold of a sensation, to the threshold of a sensation difference, and finally since we may compare not only slight differences or thresholds but also equalities of experiences to the equality between two sensation differences. Now Prof. Aliotta consistently maintains that it is wrong to regard Weber's law as expressing the relation between physical and psychical magnitudes (p. 91). If his object here is to avoid the implication that intensities are magnitudes directly capable of measurement, I am in agreement with him. But I am strongly opposed to his interpretation of the law that it correlates "two different measurements of the same physical magnitudes ". " When
