PSYCHO-PHYSICAL METHOD 119 



difference of position and length with great accuracy. 

 In a similar way, in measuring temperature we do not 

 attempt to effect the measurement by a direct compari- 

 son, but by means of the thermometer we cause the 

 changes and differences of temperature to appear as 

 changes and differences of position apprehended by the 

 eye. Almost all refined physical experiment consists in 

 converting small differences of quantity inappreciable by 

 the senses into some kind of difference more easily' 

 perceptible. 



But in attempting to measure psychical quantities 

 these two great resources are denied us. We have no 

 means of procuring at any moment a sensation of any 

 particular intensity that might serve as a standard unit 

 of quantity. And even if we could do this, there would 

 remain the further difficulty that we could not take a 

 number of such units and, by adding them together, make 

 a sum of units equal to the quantity to be measured, as 

 when in physical measurement we take a sum of inches 

 on the foot-rule or a sum of ounces in the pan of the 

 balance. Nor can we convert the intensities of sensa- 

 tion that we wish to measure into any more easily 

 measurable quantities, as we do in the case of physical 

 measurement. 



Fechner, looking round for means of measurement, 

 noted four ways in which measurement can be made : 



(i) The just perceptible sensation a very feeble 

 stimulus excites no sensation, and by then gradually 

 increasing the strength of the stimulus we can note the 

 quantity of stimulus that just suffices to produce a per- 

 ceptible sensation ; that is then the measure of the 

 threshold of sensation. (2) We can compare two sensa- 

 tions together directly, and note their equality or 

 difference. (3) We can directly compare degrees of 



