934 CUTANEOUS SENSATIONS. 



produces by summation some of the effects of variation of stimulus in intensity. 

 In examining Weber's " law," and Fechner's corollary thereto, the variation of 

 the value of the stimulus should be ascertained to refer either purely to its 

 intensiveness or purely to its extensiveness. The examination of Weber's law- 

 began as regards tactual sense exclusively as an examination of intensity. 1 



The spatial quality of touch. — Simultaneous tangible stimuli, alike 

 except as to site of incidence, have in many cases the property of 

 evoking sensations distinct one from another. With this property is 

 intimately bound up much of that perception of the spatial relations 

 between environment and organism which the latter possesses. By 

 some it has been argued that all the numerous sensations contribu- 

 tory to space perception possess, by associative experience or as a 

 native attribute, a cpuality of locality, a "local sign." 2 The degree 

 to which " locality " attaches to different sensations is various. Touch 

 and sight possess it in high degree. In contradistinction to the organs 

 of muscular sense and the semicircular canals, the skin and eye may 

 be considered the external organs for space perception. From this 

 point of view the two offer similarities of arrangement — 1. The nerve- 

 endings are spread out in a single plane for the reception of stimuli which 

 likewise are of plane extension. 2. (a) Excitation of each point of the 

 sensifacient surface is colligate with a sensation different from that 

 ensuing on simultaneous excitation of any other point of the surface, (j3) 

 provided the second point be more than a minimal distance from the former 

 point. 3. An object impresses the skin over an area the extension of 

 which in the plane of contact is identical with its own ; in the retina, 

 the area, although of smaller extension, owing to the ocular dioptric 

 mechanisms, similarly conforms to that offered by the object. 4. The 

 spatial extension of the object generates, therefore, a set of simultaneous 

 sensations ; these can be compounded and, thanks to 2 /?, as a continuum. 

 5. For each position assumed by these two sense organs, the percep- 

 tions attaching to each distinct point of their sensifacient surfaces come 

 by experience to correspond with a definite place in the environment. 



The localising delicacy of touch has been measured in two ways — (1) The 

 skin of a blindfolded person is touched with a blunt needle, — the person 

 touched has then to indicate the position of the place touched. (2) The 

 blunted points of a pair of compasses 3 are laid lightly and simultaneously on 

 the skin, and the smallest distance of the points apart at which they are 

 perceptibly separate is measured (Weber). 4 If the points are sufficiently close 

 together, a single sensation results. The liminal distance can be determined, 

 either by the method of just noticeable difference, or by the method 5 of right 

 and wrong answers. 6 For the latter about thirty pairs of blunt needles are 

 set, each pair in a little holder at a certain fixed distance apart. The distances 

 should range from about 3 mm. to 80 mm. The answers obtained are 

 "single," "double," "uncertain." The last are treated as half "single," 



1 For fuller discussion of the Weber-Fechner "law " in regard to touch, reference can be 

 made to Wundt's "Grundzuge d. physiol. Psychol.," Leipzig, 1893, Bd. i. ; " Vorlesungen 

 ueber die Menschen- u. Thier-seele," Leipzig, 1892, translated by Creighton and Titchener, 

 London, 1894. Also, cf. Helmholtz, " Handbuch d. physiol. Optik," Berlin, 1897. 



2 Lotze, "Medizin. Psychol.," Leipzig, 1852. 



3 Compasses used for thus testing the skin are often termed sesthesiometers. 



4 Arch. f. Anat., Physiol, u. wissensch. Med., Leipzig, 1835, S. 152 ; and in Wagner's 

 " Handworterbuch," loc. cit. 



5 Vierordt, Ztschr. f. Biol., 1870, Bd. vi. S. 58 ; G. E. Midler, Arch. f. d. ges. Physiol., 

 Bonn, 1879, Bd. xix. S. 191. 



6 Not necessarily as a step toward the examination of the Weber-Fechner psycho- 

 physical "law." 



