THE PSYCHOLOGICAL BASIS 19 



sensation into adequate and inadequate. The former are those which 

 arouse the sensation under ordinary circumstances, the latter are all 

 other effectual stimuli. Thus light is an adequate stimulus for the eye, 

 pressure on the eyeball an inadequate. 



The next great step in the advance of our knowledge of the relation- 

 ships subsisting between physical stimuli and sensations was made by 

 E. H. Weber, the founder of modern psycho-physical methods. In 

 general terms it may be stated that a stimulus must attain a certain 

 intensity in order to excite a sensation and that stimuli of greater 

 intensity excite stronger sensations. There is therefore a quantitative 

 relationship between the stimulus and the sensation. The minimum 

 effectual intensity of stimulus is called the general threshold or the general 

 liminal value. A higher value may arouse a sensation differing in 

 quality from the other ; this value is called the specific threshold or the 

 specific liminal value. Thus a coloured light of low intensity may excite 

 a colourless sensation ; when of a higher intensity it may excite a 

 sensation of colour. 



The intensity of the stimulus has to be increased by a definite amount 

 before a difference in the amount of the sensation becomes appreciable. 

 This amount may be called the differential threshold or the liminal 

 discrimination value. Equal increases of physical intensity do not give 

 equal increases of sensation. There are three chief psycho-physical 

 methods whereby estimates can be obtained. (1) The method of least 

 perceptible differences or the limiting method : one of two identical 

 stimuli is regularly increased by small increments until a just perceptible 

 difference between the constant and the variable stimulus is experienced. 

 (2) The method of right and wrong answers or the constant method : 

 the observer is asked which is the greater in the case of such pairs of 

 stimuli (constant and variable), the variables here being fewer in number 

 and presented with 'the constant in irregular order, and the average of a 

 large number of such estimates being taken. (3) The method of mean 

 error or the method of production : the observer picks out what he 

 regards as just appreciably different stimuli ; the sum of the differences 

 from the standard divided by the number of trials gives a lower limit 

 of the threshold value. These results are accurate only when submitted 

 to strict statistical processes 1 . 



Weber stated his conclusions in the form of a law : The just appreci- 

 able increase of stimulus bears a constant ratio to the original stimulus ; 

 i.e. two stimuli in order to be discriminated must be in a constant ratio, 

 1 See Myers, Text-book of Exp Psychology, Chaps, x. and xv. London. I'.tll. 



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