3o8 REPORTS ON THE STATE OF SCIENCE, ETC. 



used to predict equality of colour (under conditions of equality of bright- 

 ness) for various combinations of stimuli. They cannot be used to obtain 

 a quantitative measure of the difference between two different colour 

 sensations. The sets of numerals assigned to any stimulus as a measure of 

 its colour are in no sense a measure of the colour sensation evoked by that 

 stimulus : they are simply a measure of the relative quantities of three 

 standard stimuli which, if combined, would evoke the same colour 

 sensation. 



The sensory properties which can be quantitatively described by the 

 results of photometric or colorimetric measurements are typical of all those 

 in which controllable stimuli are compared with respect to the relative 

 amounts of them which evoke the same intensity or quality of sensation, or 

 which, more generally, produce the same psychological effect. The psycho- 

 logical criterion of equality which we impose defines, qualitatively, some 

 psycho-physical magnitude, a B magnitude, defined completely by the ratio 

 of two or more stimulus quantities which fulfil the psycho-physical criterion 

 of equality under prescribed conditions. Such magnitudes can be measured, 

 and their measurement gives important information about ' sensory events,' 

 but such measurements do not serve to place dissimilar sensations on a 

 quantitative scale either of intensity or quality. 



This of course has always been recognised by leading psychologists. 

 Thus Fechner, as quoted by Titchener (Experimental Psychology, Vol. II, 

 i905)P-xxiii) says, ' . . . the measure of sensitivity, as a measure of mere 

 capacity of sensation, is not to be confused with a measure of sensation 

 itself. Nor does it presuppose any such measure, but only the observation 

 of instances of equal sensations, under like or different conditions of 

 stimulation.' 



A type of measurement which has been supposed to provide a measure 

 of sensation intensity is that based on the determination of just noticeable 

 differences of stimulus intensity, usually designated j.n.ds. for brevity. 

 The principle of such measurements is sufficiently familiar to anyone who 

 can be interested in this Report that it is needless to describe it here. The 

 relation between j.n.ds. and stimulus intensity has been determined for 

 various senses and in itself provides valuable information about the opera- 

 tion of the various sensory mechanisms. The ratio of the j.n.ds. at any 

 intensity to the intensity tells us the minimum fractional change in that 

 intensity which is perceptible. We may term it the fractional sensitivity, 

 and its determination under properly specified conditions is not only of 

 interest to the psycho-physicist, but is of great practical importance. 

 Fractional sensitivity is a 5 magnitude defined as the ratio of two stimulus 

 quantities associated in a specified manner — namely, that the smaller of 

 them is just noticeable when added to the larger. Its evaluation involves 

 only measurement of stimulus intensities, and the association between 

 phenomena and numbers if entirely provided by the scales of measurement 

 established for stimulus intensities. 



Fechner, however, believed that a scale of sensation intensity could be 

 based on j.n.d. measurements. His contention has received much criticism 

 frorn the beginning, both from psychologists and physicists, and I find it im- 

 possible to say that psychologists on the whole have been more or less ready 

 to accept its implications than physicists. Despite the criticism which has 

 been lavished on it, however, Fechner's general principle is still believed 

 by many to afford a basis for measurement of sensation intensities. This, 

 is probably due to the fact that much of the criticism has been on questions 

 of detail or on questions of principle which are not really relevant and the 

 real objections have been lost like a needle in a haystack of discussion. I 



