The investigations of the intensive sensible dis crimination are included under Weber's law, a proposition NN hid; may be stated thus: the ratio of the increment of stimulus, necessary to give a noticeably different sensation, to the existing stimulus is constant. The problem is, then, to discover whether this principle of the constancy of the relative difference limen is applicable to each sense department, and to ascertain the nu merical value of the constant in those depart ments in which its applicability is established. E. H. Weber himself determined the constants for pressure as tollows: with simultaneous ap plication of the weights to both hands, 1/3; with successive applications to the same hand, 1/14 to 1/30; with simultaneously lifted weights (involv ing both pressure and strain sensations) 1/15 to 1/20; with weights lifted successively with one hand, 1/40. These figures have been modified by subsequent investigators, notably G. T. recliner, J. Merkel, and recently L. Martin and G. E. Mul ler (in an extended qualitative analysis of the condition, of such tests). The diseriminability of temperature intensities varies with the yrart of the body le,ted, and especially, it seems, with the thickness of the epidermis. recliner's constant i- 1.8° C., E. H. Weber's 1.4° to 1.5° C. The region of finest discrimination seems to be that between 26° and 33° C. Cooling is noted sooner than warming, in the proportion of three to two. Of taste, little is known save that Weber's law hold, approximately for bitter and salt. The phenomenon of 'taste contrast' is a source of un avoidable disturbance in this field. A careful survey of the olfaetory sense has confirmed the applicability of Weber's law to smell, and es tablished the constant at 1/3 with 36 per cent. of the substances tested, and at 1/4 for 26 per cent. Most successful results are those obtained
in audition. The relative sensible discrimination for noise, tested by the fall phonometer or the sound pendulum, remains constant at 1/3 within a wide range of intensities. leelmieal difficulties impede the observation of tones, but enough has been done to show that, at least within the central part of the scale, variations in tonal in tensity follw the law. Work in brightness is subject to many sources of error—difficulties in securing stanch rd conditions of adaptation. aceom modation, etc. The many investigations which haw. been completed do not. therefore, concur as to the constant. taieh is variously estimated at 1/60. 1/100.1/120 to 1/230. The value 1/110 may he assumed as approximately correct. Of interest in this connection is the apparatus which can he employed to demonstrate the existence of the con stancy or to indicate its numerical value. `Mas son's disks' are white disks, on one radius of which a number of heavy black section, are marked. Upon rapid revolution there is pro duced a series: of concentric gray rings decreas ing in darkness toward a periphery. The same ring remains, within certain limits, just visibly gi ay when the illumination is changed.
BIBLIOGRAPHY. Ebbinghaus, Urundziige der Bibliography. Ebbinghaus, Urundziige der Psycholoyie (Leipzig, ; Gamble, American Journal of Psychology, vol. x. (Worcester, 1898) ; hiesow, Philosophische Mudien, vol. N. ( Leipzig, 1894) ; Kuelpe, Outlines of Psychology (New York, 1895) ; Martinand Mueller, Zur Analyseder Unterschiedsempfindlichkeit xperimentclle Bei trage (Leipzig. 1899) Stumpf, Toupsycho/ogic, vol. i. (ib., 1833) ; Wundt, Grundt-age der physio logischen Psychologie, vol. i. (ib., 1893i. See PSYCHOLOGY, EXPERIMENTAL; FECHNER; PSYCHO PHYSICS; PSYCHOLOGICAL APPARATUS.