DISCRIMINATION AND COMPARISON. 541 



true judgments about the difference between the two sen- 

 sations which we are comparing. 



" But the larger this difference is, the more the number of the true 

 judgments will increase at the expense of the false ones ; or, otherwise 

 expressed, the nearer to unity will be the fraction whose denominator 

 represents the whole number of judgments, and whose numerator rep- 

 resents those which are true. If m is a ratio of this nature, obtained 

 by comparison of two stimuli, A and B, we may seek another couple 

 of stimuli, a and 6, which when compared will give the same ratio of 

 true to false cases." * 



If this were done, and the ratio of a to 6 then proved 

 to be equal to that of A to B, that would prove that pairs 

 of small stimuli and pairs of large stimuli may affect our 

 discriminative sensibility similarly so long as the ratio of 

 the components to each other within each pair is the same. 

 In other words, it would in so far forth prove the Weberian 

 law. Fechner made use of this method to ascertain his 

 own power of discriminating differences of weight, record- 

 ing no less than 24,576 sejDarate judgments, and computing 

 as a result that his discrimination for the same relative 

 increase of weight was less good in the neighborhood of 

 500 than of 300 grams, but that after 500 grams it improved 

 up to 3000, which was the highest weight he experimented 

 with. 



(3) Tlie Method of Average Errors consists in taking a 

 standard stimulus and then trying to make another one of 

 the same sort exactly equal to it. There will in general be 

 an error whose amount is large when the discriminative 

 sensibility called in play is small, and vice versa. The 

 sum of the errors, no matter whether they be positive or 

 negative, divided by their number, gives the average error. 

 This, wdien certain corrections are made, is assumed by 

 Fechner to be the 'reciprocal' of the discriminative sensi- 

 bility in question. It should bear a constant proportion 

 to the stimulus, no matter what the absolute size of the 

 latter may be, if Weber's law hold true. 



These methods deal with just perceptible differences. 

 Delboeuf and Wundt have experimented with larger differ- 



* J. Delba>uf, Elements de Psychophysiquf (1883). p. 9. 



