We may regard the pressure spot as the unit of cutaneous extent, as the retinal cone is that of visual extent. The limen of pressure separation, falsely assumed to be identical with the liminal extent. was first investigated by E. II. \Veber. Weber's results show clearly the dependence of the limen of separation upon the place stim ulated. The following values, taken from his classical table, illustrate this point : Tip of tongue. 1 nun.; tip of finger. 2 nun.; cheek, 11 nun.; forehead. 23 nun.; middle of back. GS nun. By the stimulation of individual pressure spots Goblscheider found much lower values: Chin. .3 cheek, nun.; forehead, .7 min.; back. 5.0 MM. !**4111requent work upon :esthesiometry, or 'Weber's sensory circles,' as these experiments are called. has emphasized the law of Vierordt that the space-Innen at any point in the of a limb is inversely proportional to the distance of the stimulated part from the axis of rotation, and have called attention to the rapid increase of the limen of sep9 rnlion with fatigue an increase so characteristic as to be urged by (:riesbach as a practical test for degree of general fatigue.
'flue third problem. diseriminability, is termed in the sphere of vision 'eye measurement."I'he results of the many invest mad( are often different, since they depend upon many factors ditlieult of isolation I'Vc11 under PX1WrillICIlta con (iitions. These factors are eye movement. the cnclitt of the compared extents (see under 11.4.usmx). their absolute length, their distance from the eye, their direction ( vertical. horizontal, etc.), the use of monocular or binocular vision, etc. Running the eyes along the lines, i.e. the introduction of the strain sensations set up in the eye muscles, appears to aid discrimination of linear extents. When we estimate extent in this way, i.e. partly in terms of intensity, the relative difference limen is constantly at 1/50; two lines seem different if one is one-fiftieth longer or shorter than the otter. Lately Sehoute has demonstrated that, with the resting eye, one can discriminate at least four different extents, all of which fall within the limits of a single retinal cone, although the actual basis of this discrimina tion is the quantity of light which falls upon the cone. The discrimination of tactual, like that of visual, extents, is complicated by extraneous fac tors; arm-movement measurements involve the factors of the duration and intensity of strain sensations, while in cutaneous experiments proper it is difficult to exclude judgments based upon visual terms. Two circular surfaces applied to the tip of the tongue may he recognized as dif ferent when their diameters are no more than 0.5 mm. and 1 min.; but on the back they must
be 2 mm. and 25 mm. respectively. Cold sur faces appear larger than warm surfaces of equal size.
Our provisional assumption posited extent as an ultimate property of certain sensation sys tems, as irreducible and unanalyzable as quality or intensity. This view regards the perception of depth and all other spatial relations as derivative products of associations formed by experience. Thus the quasi-spatial nature of cer tain sense-qualities, e.g. the seeming differences in the 'bigness' of sounds, does not demand the as sumption of any elementary spatial attribute in these sensations. It remains to be pointed out that this view of extent has not gained universal acceptance. At the one extreme certain psychol ogists, notably Wundt. seek to derive all spatial determinations of mental processes from other non-spatial contents of consciousness. (See Fusmx.) They do not, therefore, postulate a spatial attribute for any sensation system. At the other extreme certain psychologists, of whom James may be considered typical, ascribe an ele mentary spatial attribute not only to visual and cutaneous, but to all sensations. James prefers the term 'voluminousness.' *This element, discern ible in every sensation, though more developed in some than in others, is the original sensation ol space. . . . This 'vastness' is as great in one direction as another. its dimensions are so vague that in it there is no question as yet. of surface as opposed to depth; 'vol me' being the best short name for the sensation in question." From the psychological standpoint the merit of these three positions can he adjudicated only upon the verdict of trained introspection as aided by the experimental method. The principle of parsimony must incline us to the simplest con sistent theory. From this point of view the merit appears to lie with those who maintain the mid• die position, though from the genetic side the Wundtian explanation is most satisfactory. In other words. we may consider that the adult human is unable by introspection to get behind extent or `spread-out-ness' as an nIti mate datum of experience. although we may construct a theory of its genesis from other, sim pler non-spatial processes.
Consult : KU ipe, of Psychology, translated by Titchener ( London, 1895) Principles of Psychology (London, 1390) ; Outline of Psychology (New York, 1899) ; ‘Vundt, Orundsiigc der physiologiseh en Psyrhulo yic (Leipzig, 1893) ; id., Outlines of Psychology (Leipzig, 1898).