METHODS. 929 



when either very small or very large weights are used ; that is, sensitive- 

 ness to pressure difference is keenest under moderate stimulation. With 

 slower rates of change the threshold difference is lower than with 

 quicker. 1 The average threshold difference, the initial pressure being 

 5 grm. on 20 sq. mm. of the second phalanx of the finger, was found 

 with instantaneous addition to be "35 grm. 



Liminal Difference in Various Skin Regions." 2 



Forehead 

 Lips 



Dorsum of tongue Y^ to j^. 



Cheeks 



Temple J 



Finger nail 



Back of forearm, of leg, and of thigh 



Back of hand 



Back of middle and proximal phalanges of fingers 



1 tn 1 



I 



more than y^. 



Methods — The "liminal difference" for touch has been estimated in 

 various ways — (1) As by Weber, referred to in the above paragraph. Of two 

 stimuli, easily judged to be of different intensity, one is diminished or in- 

 creased to the extinction of perceptible difference ; or, of two indistinguishable 

 stimuli, one is increased or diminished (in the above case by addition to or 

 .subtraction of weight from it), so that a difference between the two stimuli can 

 just be noticed. When the test starts from equality, the difference remains 

 longer unnoticed than when tests go towards equality. The direction of 

 change, i.e. whether decrease or increase, may be known to the subject or not 

 known. The former is the " conscious " method, the latter the " half- 

 conscious." The direction being unknown, the changes may be of regular 

 order or utterly irregular, the latter giving the " unconscious " method. The 

 values of liminal difference found are different for these different methods. 

 These are quick methods, but less precise than others to be mentioned. 

 The two modifications of the method can be used together, and the 

 mean liminal differences ascertained by each compared together. This 

 combination constitutes the method of smallest difference. 3 The method as 

 used by Weber is known as method of just noticeable difference. (2) Method of 

 right and wrong answers. 4 — Two equal stimuli are, under favourable conditions, 

 judged as a rule to be equal, although in certain proportion of instances to be 

 slightly different. Two unequal stimuli are judged to be unequal with greater 



1 Hall and Motora, Am. Jonrn. Psychol., 1887, vol. i. p. 72 ; Stratton, loc. cit.; Seashore, 

 Stud. Yale Psychol. Lab., New Haven, 1896, vol. iv. ; Heinzmann, Arch. f. d. ges. Physiol., 

 Bonn, 1872, Bd. vi. S. 222 ; Fratschner, Jenaische Ztschr. f. Med. u. Naturw., Leipzig, 

 Bd.ii. S. 130. 



2 A. Eulenberg, Ztschr. f. rat. Med., 1861, Bd. x. 



3 G. E. Midler, "Zur Grundlehre d. Psychophysik, " S. 11. 



4 Fechner, " Eleni. d. Psychophysik," Bd. i. S. 71. Fechner elaborated this method, 

 but it appears to have originated in Vierordt's laboratory, see Hegelmayer, also Renz and 

 Wolff in Arch. f. physlol. Hcilk., Stuttgart, 1856, Bd. xiv. ; and Ann. d. P/u/s. u. Chcm., 

 Leipzig, Bd. xcviii. S. 600. 



VOL. II. — 59 



