Page:The New International Encyclopædia 1st ed. v. 10.djvu/780

* INTENSITY OF SENSATION. intensity, but we Uo not interpret the alteration simply as a clianye in intuiisily, because there is always in operation a centrally excited sensation ot gray. (See Visual Sensatio.v.) If we could change the intensity of a eulur lone without liav- ing this central process in operation, the change would be correctly noted as intensive only. The main problems of intensity are two: (1) That of the minimal intensity of sensation, and (2) that of the change in intensity of stimulus which is just noticeable in sensation. There must be, accordingly, a cletirniinalion for each sense department of the intensive limen (q.v.), and of the intensive sensible discrimination. Tiie im- portance of these problems is attested by the fact that all the classics of the science of psycho- physics (q.v.) have been concerned with the re- lation obtaining between intensity of stimulus and intensity of sensation rather than with the more intricate lU'pendencies between the other attributes of stimulus and sensation. The liminal intensity of pnsstirc depends upon the place stimulated. With cork pellets as veights, it has Ix-en found to be 2 mg. upon the forehead, nose, and cheeks; 5 nig. upon the lips, upper arm. and nape of the neck; 10 mg. u|)on the fingers: and 1 g. on the nails. By stimulat- ing individual pressure spots with fine hairs, Von Frey obtained much lower values, averaging, for the calf 1.44 gr. mm., and for the wrist 1.28 gr. mm. (where gr. denotes the pressure exerted by the hair, and mm. denotes the radius of its cross- section) . The pniii limen. as recorded by the hair Piethod. gives the following values in gr/mmS 692 INTENSITY OF SENSATION. than that for warmth ; that the sensitivity is greater over the lattrral surfaces of the body than in the median plane; and that, as a rule, it in- creases from the periphery toward the trunk. The liminal intensity for tusli depends not only on the concentration of the solution, but also on the area stimulated, the amount of movement vithin the buccal cavity, and the general condi- tion of excitability of the taste nerves prevailing at the time. Kurthermore, the tip of the tongue is most sensitive to sweet, the base to bitter, the sides to sour, and the body to salt. The average limina have been found to be represented by the Fia, 1. FORM OF HTRlPTrRE's APPARATUS FOR TE8TIS0 HKS8ITIVITV TO TOCCH. The rack bears rode with cork pellets* of different welKhta depending from- thcni. The weight or the llKhtcst o( tnese cannot be telt on the hand by the normal subject. where mm' denotes the area of the cross-section of the hair: over the epicondyle of the humerus, oO; over the radius, 20; over the knee-pan, 40; over the olecranon of the ulna. 40 ; on the con- junctiva of the eye. 2 to 7. On account of the variation of the actial temperature of the skin, or the 'physiological zero-point.' the examination of the intensive sensibility of trmperaturc sensa- tions efxhibits peculiar difficulties. We can mere- ly say that the sensitivity for cold is greater Fl(i. J. I.KHMANN'H APPARATl'8 FOR nETERMISISO THE LIUEN OP HOrNb INTENSITY. The forcepe Z containing a leaden bull In plncml on the Ici'nd o( a mllUnieter serew. The bnll Is released b.v ppesnure of the forceps and falls. hIdiik tin- dotted line, on the Hliint- iiiic Hui-fare. whirli is covered with cloth and on which Is laid a glass or copper plntc. The height of the fall regu- lates a<<urat«Iy the objective Intensity of the Bound stimulus. following figures, based upon a solution in 100 parts of distilled water: salt, 0.24; saccharine, (1.49; hydrochloric acid, O.OOfi.'t; sulphate of quinine, 0.00005. The intensity of olfactory sen- sations is tikewi.se dependent upon the rate and manner of breathing, the rate of dilfusion of the odorous vapor, and the general coialitions of the smell org.ms, as well as u|)on the amount of saturation of the air with the minute particles of the odorous substance. Current figures show- that the limen is very low for some qualities; e.g. musk, 1/200000 mg. : siil]iliurctcd hydrogen, 1/5000 mg. ; bromine, 1/000 mg. In iiudition. the existence of absolute silence is to be doubted. Most authorities agree that we can never get free from the noises caused by the pumping of (he blood through the ears. Closing the ears, or standing with open cars near any reflecting sur- face, only intensifies the sound. The liminal in- tensity for noise may, then, be regarded as that intensity which is just perceptible above this internal sound. A cork pellet weighing 1 mg. and falling through 1 mm. upon a glass plate can just be heard at 01 mm. from the ear. A lone from a pipe civing 181 vs. per second is audible when the air-particles set in motion by it pos- sess an anijilitude of excursion of only 0.00004 mm., and the mechanical work done by it upon the ear is one three-billionth kilogram-meter. Other investigations have placed the latter value as low as 0.0022 mg.-mm. The sensitivity for high tones is in general greater than that for Irw tones. The investigation of the brifihtnr.in limen reveals a difficulty analogous to that cited in connection with the auditory limen. The existence of the 'idio-retinal light' precludes abso-