140 REfORT — 1876. 



any estimation of the amount of sensational effect following a given stimulus would 

 probably be erroneous, because our percei^tions are usually qualitative and only 

 rarel}^, and never absolutely, quantitative. Fechner recognized this fact, and lie 

 employed for the solution of the problem various methods by which he measm'ed 

 not sensations themselves, but the amount of discriminati^•e sensibility between two 

 sensations produced by stimuli of unequal magnitudes, and he studied the ratio 

 between the difference of weight and the absolute quantity of the stimulation. By 

 varj'ing the amount of the stimulus in e\ery possible way, he eliminates the chances 

 of error, and arrived at definite results. These results he formulated into a general 

 " psychophysical law," Avhich may be expressed in \ arious ways. Mathematically 

 it may be put, that " sensation increases in proportion to the logarithm of the 

 stimulus." Now "logarithms increase in equal degrees when the numbers so in- 

 crease that the increment has always the same ratio to the magnitude of the num- 

 ber." It may be put in another way by saying that " the more intense a sensation 

 the greater must be the added or diminished force of stimulation in order that this 

 sensation undergo an appreciable change of intensity." The mode of arriving at 

 some of Fechner's residts may be better understood by an experiment which any 

 one can repeat. In the case of muscular sensation, suppose two weights, A and B : 

 we wish to ascertain the least difference between these perceptible by the muscular 

 sense, say wlien we lift them in the hand. Let it be so arranged that both weiglits 

 are composed of difl'erent pieces, so that the one may be made less or more than 

 the other at pleasure. If A and B be nearly equal in absolute weight, the person 

 on whom the experiment is made sviW judge them to be of equal weight. Let 

 weights be now added to B until the difference between A and B becomes percep- 

 tible, and as a test, let the weights be again removed from B until, in sensational 

 effect, A becomes again equal to B. Let the same experiment be repeated with 

 weights of different absolute amount, and it ■\\'ill be found that there is a distinct 

 ratio between the abs.dute weight and tlie weight that had to be added to it or 

 taken from it to produce tlie least perceptible difference of impression, whatever 

 these weights may be, up tj the limits, of cour.se, which I have already noticed. It 

 will always be found that the additional or subtracted weight is one third that of 

 the absolute weight, — a fraction which indicates the degree of intensity of the 

 stimulus required to produce the least perceptible feeling of difference of sensation, 

 and which may be termed the " constant proportional " of that kind of sensation. 

 This fraction, in the case of sensibility to temperature, Fechner found to be one 

 third ; Rentz, WoK, and Volkmann arrived at the same fraction -with regard to 

 auditory impressions ; and various observers have found that in visu.al impressions 

 it is one hundi'edth. 



Now the intensity of sensation depends on two conditions : — (1) the intensity of 

 the excitation ; and (2) the degree of excitability of the seusory organ at the mo- 

 ment of excitation. But suppose the excitability of the organ equal on two occa- 

 sions, the intensity of tlie sensation does not increase proportionately to the increase 

 of the excitation. That is to say, suppose we bring into a dark chamber a lumi- 

 nous body such as a candle — it produces a certain luminous sensation ; then intro- 

 duce a second, third, and fourth — the excitation is double, triple, or quadruple ; but 

 experiment shows that the increase in the amount of the sensation is much less ; 

 in others words, let the .stimulus increase from 10 to 100 time', and from 100 to 

 1000 times, the sensation will be oidy one, two, and three times stronger. The 

 impLirtance of the discovery of this remarkable law is, that it shows a distinct 

 matliematical relationship between stimulation and sensation. Possibly it may be 

 found to have applications to other psychological phenomena. .May it not vary in 

 different animals, and even in different iiidi\ iduals ^ 



Criticism of Fechner's Method. 



It is quite noticeable, however, that, in the case of each sense, the law did not 

 hold good throughout the whole range of variations in intensity of stimidus ; and 

 it is not surprising, when we consider the complexity of the onditions, that such 

 should be the case. All of these experiments were made in the case of visual im- 

 pressions, for example, on the li^ing eye, cjiinected by the optic nerve with the 



