i CUTANEOUS SENSIBILITY 9 



II. It is only within certain limits of intensity that external 

 agents are effective stimuli. The minimal strength which is 

 necessary to produce a sensation is known as the liminal intensity, 

 or threshold stint ///its. The least perceptible increase of stimula- 

 tion beyond this value is termed the liminal difference, or threshold 

 <>/' difference. Every increment of stimulus up to a certain 

 maximal limit produces an increase of sensation. The maximal 

 sensation is obtained with a comparatively low strength of 

 stimulus. Every increment of stimulus above that point not only 

 fails to increase the sensation, but actually induces fatigue or 

 exhaustion of the peripheral sense-organ, which is the more rapid 

 and complete in proportion as the stimulus is excessive. 



The judgment we are able to form as to the intensity of a given 

 sensation and the quantitative relation between the stimulus and 

 the sensation is necessarily only approximate. We cannot state 

 how much stronger or weaker one sensation is than another ; we 

 can only say whether a sensation is stronger or weaker than, or 

 equal to, another. 



Speaking generally, it may be said that sensation increases 

 within certain limits with the strength of stimulus, but not 

 proportionately to it ; doubling or trebling the stimulus does not 

 double or treble the intensity of the sensation. Common observa- 

 tion shows, in fact, that one and the same stimulus is perceived 

 more, or less, or not at all, according to the conditions under 

 which it takes effect. In the silence of night we perceive the 

 ticking of a watch, while in the noise of day we scarcely hear the 

 voice of any one speaking to us, and the clatter of the railway may 

 prevent us from hearing our own voice. This means that the 

 least stimulus can be perceived when the pre-existent sensation is 

 feeble, and that a much stronger stimulus is required when the 

 organ is excited by a previous strong stimulation. It is therefore 

 obvious that intensity of sensation does not increase proportion- 

 ately to strength of stimulus, but much more slowly. In order to 

 determine the exact quantitative relation between stimulus and 

 sensation it would be necessary to measure the intensity of both 

 by the same methods. And as any such direct measurement of 

 sensation is impossible, the only attempt we can make at solving 

 the problem is to determine the threshold of difference, i.e. how 

 much the strength of stimulus must be increased in order to 

 obtain a perceptible increase in the intensity of the sensation. 



E. H. Weber (1831) first attempted this estimation. While 

 testing the power of discrimination in musculo-cutaneous sensi- 

 bility he met with a surprisingly simple result : the increase of 

 stimulus necessary to produce an appreciable increase in sensation 

 bears a constant ratio to the total stimulus, i.e. is always the same 

 fraction of the total intensity of the stimulus. Thus to appreciate 

 the minimal increase of a weight held in the hand, it is always 



