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II. Short summary of recent Cambridge experimental work. 

 Introduction by Prof. F. C. Bartlett, F.R.S. 



The following is a brief memorandum written by Mr. Craik on work 

 relevant to problems of sensory measurement done mainly in the Psycho- 

 logical Laboratory, Cambridge, during the last few years. Nearly all the 

 methods involved rest on a use of the principle, or method, of just percept- 

 ible differences. They are consequently subject to whatever assumptions 

 as to measurability may be involved in that method. A considerable amount 

 of work on supraliminal differences has also been carried out, but the 

 difficulties encountered have not been satisfactorily surmounted, and in 

 consequence this work is not dealt with in the present report. 



The general upshot of the whole of the work has been to show as clearly 

 and definitely as possible that all the formulae which have ever been pro- 

 posed correlating physical measurements of intensity of stimuli with just 

 perceptible difference series are valid only within wide limits. Some of 

 the limits have now been thoroughly studied : these are, 



(a) rate of application of the stimulus ; 

 (6) degree of practice of the subject ; 



(c) knowledge by the subject of the accuracy or inaccuracy of his 



reactions ; 



(d) state of adaptation 



(i) of the peripheral organs, 

 (ii) of the central nervous system mechanism concerned. 



