798 OF THE ORGANS OF THE SENSES, AND THEIR FUNCTIONS. 



to be felt, it must be proportionately more foeble as the excitation to which it is 

 added is feeble, and stronger in proportion to the strength of the original or primary 

 excitation. Hence the conclusion may be drawn that the intensity of a sensation 

 increases, not directly in proportion to the intensity of the excitation that produces 

 it, but in some other ratio. Now it may be asked, what proportion does the increase 

 in the intensity of the sensation bear to the increase in the intensity of the stimulus ? 

 And in order to acquire this knowledge, three methods of experimentation have been 

 adopted, named by Fechner respectively the method of the smallest perceptible differ- 

 ences, the method" of true and false estimates, and the method of menu error. The 

 first method consists in this: Two weights, A and B, are to be compared. If they 

 are nearly alike, they may be judged to be equal ; if the difference be considerable, 

 it will easily be recognized. If the difference d between A and B be gradually in- 

 creased or gradually diminished, a point will be arrived at when the difference is only 

 just discernible, and the smaller the difference perceptible, the more acute is the sen- 

 sibility of the subject. The second method consists in taking two weights, the differ- 

 ence between which is so small that an error in judgment is possible. In proportion 

 as the difference increases, the number of accurate conclusions will augment at the 

 expense of the inaccurate ones. If the total number of experiments be 100, and the 

 number of true estimates be 70, we have the proportion T 7 5 g established by the com- 

 parison of the two weights A and B. Now suppose a weight a to be given, we seek 

 to determine the weight b, which compared with a will give the same proportion of 

 7_o ff . Cases of indecision must be placed half to the account of the true and half to 

 the account of the false estimate. The third method consists in taking a weight, A, 

 exactly determined by balance, and it is sought by the judgment alone to fix on 

 another weight which is judged equal to the first. In general the second weight 

 differs from the first by a quantity, d, which is small in proportion to the acuteness 

 of the sensibility of the subject. This experiment is frequently repeated, the positive 

 and negative errors added, the signs being neglected. The total is divided by the 

 number of trials, and the mean error is thus obtained. By the employment of one 

 or other of these methods the following results are obtained : 



Sensation of Weight. The hand is extended and supported, the eyes are bandaged, 

 and successive weights, as a drachm, an ounce, a pound, etc., are placed upon it. On 

 each weight various small additional weights are placed till a distinct difference is 

 perceived. The results of numerous experiments by the first method have shown 

 that a very simple relation is maintained, viz., that whatever may be the weight 

 originally placed on the hand, it requires an addition of one-third of that weight to 

 enable a distinct difference to be perceived. 



Sensation of Muscular Effort. This is a much more delicate means of estimating 

 differences of weight than the foregoing, since not only does the sense of pressure, but 

 the sense of muscular effort guide the judgment; and experiments have shown that 

 an additional weight of jf^ths can be detected, i.e., a weight of 6 grains added to 

 100 grains, or of 60 grains to 1000, is perceptible. 



The Sensation of Temperature seems to follow the same law as that of simple pres- 

 sure without muscular exertion, an addition of one-third being requisite to be per- 

 ceived. In regard to light, photometrical experiments have shown that the intensity 

 of a candle or other source of light must be increased r jpth in order that the increase 

 should be perceptible. For the sensation of sound, the increase or diminution must 

 be equal to one-third Kenz and Wolff found that for two sounds to be clearly dis- 

 tinguished they must be in the relation of 100: 72. The proportion given by Volk- 

 mann is 4 : 3, which equals 100 : 75. 



From all this it would appear that if S represent the value of the minimum per- 

 ceptible sensation, the second possible sensation would be for tactile impressions S + 

 ; for muscular effort, S + T J 7 ; for temperature, S + J; for light, S + T ^ 5 ; for 

 sound, $ -f- i J and the results of numerous experiments are as follow : 





Constant proportionals. Perceptible minimum. 



Touch J, ..... Pressure of 0002 grin, to 0.05 grm., according to the region 



of the body to be experimented on. (Aubert.) 

 Muscular effort J 7 , . Contraction of the internal rectus of the eye to the extent of 



0.004 mm. 

 Temperature J,. . . (The heat of the skin being taken at 18.4 Cent.) Cent. 



(? fa Fechner.) 

 Sound J, ..... Ball of cork weighing 0.001 grm. falling 0.001 m. on a plnte 



of glass, the ear being at a distance of 91 millimetres, 3| 



inches. (Schafhautl.) 

 Light T J 5 , .... Illumination of black velvet by a candle at a distance of 8 



feet 7 inches. (Volkmann.) 



