DISCRIMINATION AND COMPARISON. 545 



The discrimination of lengths by the eye has been found 

 also to obey to a certain extent Weber's law. The figures 

 will all be found in G. E. Miiller, op. cit., part ii, chap, x, 

 to which the reader is referred. Professor Jastrow has 

 published some experiments, made by what may be called 

 a modification of the method of equal-appearing difi"er- 

 ences, on our estimation of the length of sticks, by which it 

 would seem that the estimated intervals and the real ones 

 are directly and not logarithmically proportionate to each 

 other. This resembles Merkel's results by that method 

 for weights, lights, and sounds, and differs from Jastrow's 

 own finding about star-magnitudes.* 



If we look back over these facts as a whole, we see that 

 it is not any fixed amount added to an impression that 

 makes us notice an increase in the latter, but that the 

 amount depends on how large the impression already is. 

 The amount is expressible as a certain fraction of the entire 

 impression to which it is added ; and it is found that the 

 fraction is a well-nigh constant figure throughout an entire 

 region of the scale of intensities of the impression in ques- 

 tion. Above and below this region the fraction increases in 

 value. This is Weber's laiv, which in so far forth expresses 

 an empirical generalization of practical importance, without 

 involving any theory whatever or seeking any absohite 

 measure of the sensations themselves. It is in the 



Theoretic Interpretation of Weber s Law 



that Fechner's originality exclusively consists, in his as- 

 sumptions, namely, 1) that the just-perce23tible increment 

 is the sensation- unit, and is in all parts of the scale the same 

 (mathematically expressed, Js = const.) ; 2) that all our 

 sensations consist of sums of these units ; and finally, 3) that 

 the reason why it takes a constant fractional increase of the 

 stimulus to awaken this unit lies in an ultimate law of the 

 connection of mind with matter, whereby the quantities of 

 our feelings are related logarithmically to the quantities 

 of their objects. Fechner seems to find something in- 

 scrutably sublime in the existence of an ultimate 'psycho- 

 physic ' law of this form. 



* Aniericau J. of Psychology, iii. 44-7. 



