CLASSIFICATION OF THE SENSE ORGANS 733 



Weber's Law. — While it is true that the intensity of a sensation 

 increases with the strength of the stimulus, the former does not pre- 

 serve a direct relationship to the latter. Clearly, the different sense- 

 organs are adjusted in such a way that they can receive their specific 

 stimuli with the greatest possible ease; in other words, their threshold 

 value is low. This implies that the maximum value of a sensation is 

 attained very quickly and that stimuli of greater intensity do not aug- 

 ment the primary impression in any appreciable measure; in fact, their 

 tendency is to produce fatigue. Now, while it is a relatively simple 

 matter to determine the strength of the stimulus which is required to 

 evoke a certain sensation, it is difficult to obtain comparative values 

 of sensations, whether of the same or of different character. This in- 

 ability on our part of rating sensations in an absolute quantitative man- 

 ner forces us to rely solely upon our power of perceiving slight differences 

 in them. One way of doing this is to ascertain how greatly a stimulus 

 must be increased in order to give rise to an appreciable sensation of 

 difference. Consequently, the only two means at our disposal for rating 

 sensations in a quantitative way are first, the determination of the 

 threshold value of the stimulus required to elicit a certain sensation, and 

 secondly, the determination of the increase in the strength of the 

 stimulus necessary to produce a distinct difference in the sensation. 



E. H. Weber, 1 who first attempted to obtain such values, made the 

 observation that the stimulus necessary to cause a sensation of differ- 

 ence, is proportional to the intensity of the stimulus then acting, ^.e., 

 it forms a fractional increment of the latter. Subsequent determina- 

 tions, however, have shown that this law holds true only for stimuli of 

 moderate intensity. Thus, if a weight of 30 grams is placed upon 

 the tip of the index finger of a hand supported at the wrist, and is 

 properly adjudged by means of the muscle-sense, an additional weight 

 of 1 gram must be added to or subtracted from these original 30 grams 

 before a distinct difference in this sensation will be obtained. If this 

 test is now repeated with a primary weight of 60 grams, it will be found 

 that 2 grams are required in order to produce a sensation of difference, 

 and with 90 grams an additional weight of 3 grams, and so on, until 

 the physiological limit is reached at about 1000 grams. In this par- 

 ticular case, therefore, the increment is lio of the original stimulus. 

 Naturally, this law holds true for any quality of stimulus, provided 

 that the latter is of moderate intensity. For sounds the appreciation 

 difference is 3^, and for light Jf20 of the original stimulus. Conse- 

 quently, the eye is the most sensitive organ. 



Fechner's Psychophysical Law. — The attempt has been made by 

 Fechner^ to generalize the preceding law and to place it upon an ac- 

 curate quantitative basis. It is stated that the sensations show the 

 same relationship to the stimuli as the logarithms to their numerals. 

 Fechner's law, therefore, may be expressed by the formula S = C 



^ Wagner's Handworterbuch der Physiol., iii, 1846, 481. 

 ^Elemente der Psychophysik, Leipzig, 1860. 



