732 



FECHNER'S LAW. 



of medium strength.] If the specific stimulus be too intense, it gives rise to 

 peculiar painful sensations, e.g., a feeling of blindness or deafness, as the case may 

 be. The sense organs respond to adequate stimuli, but only within certain limits 

 of the stimulus, e.g., the ear responds only to vibrating bodies emitting a certain 

 range of vibrations per second ; the retina responds only to the vibrations of the 

 ether between red and violet, but not to the so-called heat vibrations or to the 

 chemically active vibrations. 



[It was Weber who worked out the relation between the intensity of stimuli and the changes 



in the quantity of the resulting sensations. He used the method of "least observable differ- 



is applied to sensations of pressure and the measurements of lines by the eye. Hence, 



it is called Weber's Law ; but Fechner expanded it and assumed that all just observable differ- 



.11 :es are equally great, and so the law is sometimes called by his name.] 



[Fechner's Law. Expressed in another way, the result depends on (1) the strength of the 

 stimulus, and (2) the degree of excitability. Supposing the latter to be constant, while the 

 former is varied, it is found that if the stimulus be doubled, tripled, or quadrupled, the sensa- 

 tion increases only as the logarithm of the stimulus. Suppose the stimulus to be increased 10, 

 100, or 1000 times, then the sensation increases only as 1, 2, or 3. Just as there is a lower 

 limit of excitation limiiial intensity (or threshold), so there is an upper limit or maximum of 

 excitation or height ;f sensibility, when any further increase produces no appreciable increase 

 in the sensation. Thus, we do" not notice any difference between the central and peripheral 

 portion of the sun's disc, though the difference of light intensity is enormous (Sully). Between 

 these two is the range of sensibility ( Wundt). There is always a constant ratio between the 

 strength of the stimulus and the intensity of the sensation. The stronger the stimulus already 

 applied, the stronger must be the increase of the stimulus in order to cause a perceptible increase 

 of tin- sensation (Weber's Law). The necessary increment is proportional to the intensity of 

 the stimulus, and it varies for each sense organ. If a weight of 10 grammes be placed in the hand, 

 it is found that 3*3 grammes must be added or removed before a difference in the sensation is 

 perceptible ; if 100 grammes are held, 33*3 grammes must be added or removed to obtain a per- 

 ceptible difference in the sensation. The magnitude of the fraction indicating the increment of 

 stimulus necessary to obtain a perceptible difference of the sensation, is spoken of as the constant 

 pro])ortion or the discriminative sensibility. In the above case it is 1 : 3. The following table 

 gives approximately the constant proportion for each sense : 



Tactile Sensation, . . 1 : 3. Muscular Sensation, . 6 : 100. T tf 



Thermal ,, . . 1 : 3. \ Visual ,, . 1 : 100.^.] 



Auditory ,, . . 1:3.} 



[The application of the law to temperature sensations is beset with great difficulties, while 

 for taste and smell we do not know that it is really applicable. From an experimental point 

 of view, it cannot be said to be proved, and its application is obviously somewhat restricted to 

 certain sensations, and to these, only within a certain range. It certainly does not hold good 

 for sensations of pressure, and muscular sense, near the lower limits for these senses. "At 

 best it is only an approximately correct statement of what holds true of the relative intensity 

 of certain sensations of light and hearing, and less exactly of pressure and the muscular sense, 

 when these sensations are of moderate strength " (Ladd).] 



The term after-sensation is applied to the following phenomenon, viz., that, as 

 a rule, the sensation lasts longer than the stimulus producing it ; thus, there is an 

 after-sensation, after pressure is applied to the skin. Subjective sensations 

 occur when stimuli due to internal somatic causes excite the nervous apparatus of 

 the sense organ. The highest degrees of these, depending mostly upon pathological 

 stimulation of the sensory cortical centres, are characterised as hallucinations, 

 e.g., when a delirious person imagines he sees figures or hears sounds which have no 

 objective reality. In opposition to this condition, the term illusion is applied to 

 modifications by the sensorium of sensations actually caused by external objects, 

 e.g., when the rolling of a waggon is mistaken for thunder. 



In a new-born child, the sense of touch is strongly developed, that of pain slightly, muscular 

 sensations are undoubtedly present, while smell and taste are frequently confounded. Auditory 

 stimuli are heard from the second day onwards, the stimulus of light immediately after birth, 

 but a peripheral field of vision does not yet exist (Cuignet). Towards the fourth to fifth week, 

 the movements of convergence and accommodation are noticable, while after four months, 

 colours are distinguished. The various stimuli are not perceived simultaneously a reflex 

 inhibitory centre is not yet developed (Ocnzmcr). 



