930 CUTANEOUS SENSATIONS. 



certainty the more they are unequal. When the difference between them 

 is above a certain degree, they are constantly judged to be unequal. As 

 the inequality between the stimuli is reduced more and more, less and less 

 correctly is judgment made as to their inequality. The distinction between 

 the stimuli is in each case either right or wrong. Occasionally no answer can 

 be given, the judgment remains indeterminate. Such instances are dealt with 

 by being added half to right answers, half to the wrong. For each value of the 

 difference between the stimuli the ratio between the number of right answers 



and the total number of answers is determined . The delicacy of percep- 



T 



tion of difference is inversely proportional to the differences of stimulus which 



m 



produce equal ratios . Thus if in two equal sets of experiments, e.g. on two 



individuals, ^ and -f^ of difference of external stimulus elicited the same ratio 

 of right to total answers, e.g. 80 per cent., the liminal difference of stimulus is 

 half as great, and the delicacy of judgment of difference is twice as great in 

 the second as in the first. (3) Method of mean error. 1 It is sought to make 

 the two stimuli equal, that is indistinguishable, by use of the single sense 

 under examination, in this instance touch. At the end of each attempt the 

 objective stimuli judged equal are physically measured, e.g. on the balance. 

 The smaller the difference between the stimuli so measured, the greater the 

 delicacy of judgment of difference, the lower the liminal difference ; always 

 supposing the number of observations made is sufficiently great. (4) Method 

 of apparently equal intervals? Three stimuli are arranged in series by 

 the subject, so that the difference between the first and second appears to 

 him equal to that between the second and third. (5) Method of apparently 

 doubling the stimulus (Merkel). The subject tries to make one stimulus seem 

 just double another. The two stimuli chosen are then physically compared. 

 In regard to the two last methods, it has been objected that considerable intervals 

 are compared one with another by a different law from that applying to barely 

 noticeable intervals. 



It was during his study of touch that E. H. Weber discovered and first 

 investigated the law that bears his name. The visual is perhaps the sense best 

 suited for its accurate observation. For that reason its full discussion must be 

 left to "Vision," but a brief sketch of the law and its significance, and 

 of the superstructure of theory built on it, seems demanded here. Our sense 

 is provided with no exact measure of itself. Yet we can compare sensations 

 of the same quality, and express opinions as to their differences. Thus, of two 

 touches, one may feel "heavy," the other "light." The difference may be 

 wide or not, e.g. the light touch may be little lighter than the heavy, or much 

 lighter. It is usual to consider this kind of difference between easily com- 

 parable sensations to be a difference of quantity. The heavier touch is con- 

 sidered a stronger sensation than the lighter. When the thus assumed quantity 

 of sensation is on the same hypothesis supposed to be increased, and the assumed 

 increase is obtained without increase of the area or duration of application of the 

 stimulus to the sensifacient surface, the increase of sensation is considered an 

 increase of intensity of sensation. The difference between the sensation of a 

 light touch implicating an area of skin and of a heavy touch implicating the 

 same area, is therefore considered to be a difference of intensity. How much 

 more intense one sensation is than another cannot be known. Whether a 

 rifle's report be five or twenty times louder than a pistol's we cannot that way 

 know. We can only roughly judge the degrees of difference. All we know is 

 that a different sensation results when the physical stimulus is intense from 

 what results when the physical stimulus, otherwise similar, is less intense. That 



1 Fechner, "Elem. d. Psycliophysik, " Bd. i. S. 120. For calculation, see Brims, 

 Phil. Stud., Leipzig, 1893, Bd. ix. S. 1. 



2 Plateau, Bull. Acad. roy. d. sc. de JBelg., Bruxelles, 1873, tome xxxiii. p. 376. 



