en. LIL] SPECIFIC NERVOUS ENERGY 763 



The remaining characters ascribable to sensation are protensity 

 on which our perception of duration is based and affective tone, 

 which give us our experience of pleasure, indifference, or displeasure. 

 But these we will not discuss ; they are more suitably studied in 

 works on psychology. 



It is of interest to note how intimately the various characters of 

 sensation are bound up with one another. If we attempt experi- 

 mentally to change one character, it is difficult to avoid simultane- 

 ously changing another. For example, when we increase the 

 extensity of a warm sensation by putting more of our arm into hot 

 water, we at once increase the intensity of the sensation. If we 

 increase the area of a very distant colour stimulus, we alter its hue. 

 The hue of a colour is also apparently altered by increasing the 

 intensity of the stimulus. To many people the pitch of a sound 

 appears altered by increasing its loudness. 



It is likewise important to remember that the characters of a 

 sensation depend not only on the strength, vibration-rate, duration, 

 etc., of the stimulus, but also upon the condition of the sensory 

 apparatus which is stimulated and upon the temporary condition of 

 neighbouring sensory areas; nay, the characters of a sensation 

 depend upon the state of the nervous system generally, that is to 

 say, upon the total mental state at the moment of application of the 

 stimulus. 



The strength of a stimulus must not fall below a certain 

 minimum in order that a sensation may result. Too light a touch, 

 too faint a sound, will produce no effect on consciousness. That 

 strength of stimulus which just suffices to evoke a sensation is called 

 the liminal (from limen, a threshold) * value of the stimulus, or its 

 absolute threshold. 



Similarly, the difference between two stimuli must not fall below 

 a certain minimum in order that that difference may be appreciated. 

 If two musical tones are of too nearly identical pitch, if two colours 

 are of too nearly identical hue, the difference may be imperceptible. 

 There is, hence, a liminal value for a stimulus difference. This is 

 known as the differential threshold of the stimulus. 



Weber's law states that the just appreciable difference between 

 two stimuli depends on the ratio of that difference to their magni- 

 tudes, and not on the absolute difference between their magnitudes. 

 Fechner, after bringing forward further evidence in favour of the 

 law, endeavoured to deduce from it the conclusion that the strength 

 of a sensation is proportional to the logarithm of its stimulus ; in 

 other words, that the stimulus must increase in geometrical pro- 

 portion for the sensation to increase in arithmetical proportion. 



* Strictly speaking, the liminal value is that strength of stimulus which in a 

 series of trials as often just fails as it just succeeds in evoking a sensation. 



