56 GENERAL VIEW AND BASIS OF SCIENTIFIC RESEARCH. 



graph, for registering the time occupied in psychical pro- 

 cesses. 1 By means of this instrument he has found that 

 the amount of time required by a man of middle age to 

 perform a single act of thought is about the twenty-fifth of 

 a second. 2 



6 The department of physiological psychology has been 

 largely carried on by help of electric stimulation, a mode of 

 experiment introduced by Ritter, improved on by Purkinje 

 and others, greatly elucidated by the celebrated researches, 

 of Du Bois Reymond and his followers into the electric 

 phenomena of nerve, and giving promise recently of throw- 

 ing light not only on the actions of the senses, but also on 

 those of the central organs. It is impossible to review in 

 detail the long series of investigations relating to the 

 dimensions of sensations which have been carried out by 

 German physiologists. They date back to a period ante- 

 cedent to that of Miiller, though they have only recently 

 been carried out in a systematic way by a kind of scientific 

 concert. The results thus obtained are very abundant, and 

 must be considered as a valuable addition to the physio- 

 logical basis of psychology. They include, among other 

 points, approximate determinations of the degree or force, 

 and also the duration of stimulation necessary to the least 

 possible sensation, of the changes in a sensation consequent 

 on the prolongation of a given stimulus, and of the precise 

 duration of a sensation after the stimulation has ceased. 

 This quantitative determination of sensation was naturally 

 carried out in the first instance in the department of visual 

 impression. Ehrenberg, Johannes Miiller himself, and 

 Plateau may be mentioned among those who first assisted 



1 Catalogue of Special Loan Collection of Scientific Apparatus, 3rd 

 edit., South Kensington Museum, 1877, p. 968. 



2 Science Conference, South Kensington Museum, 1876, vol. ii. 

 p. 228. 



