INVESTIGATION OF THE PKOCESS OF THOUGHT. 57 



in building up this part of the science of the senses. It 

 is, however, by the labours of more recent investigators, 

 including Volkmann, E. N. Weber, Fechner, Wundt, and 

 Helmholtz, that the quantitative appreciation of sensation 

 has been mainly accomplished. Weber's researches into 

 the limits of discriminative sensibility, directed in the first 

 instance to the impressions of the tactile surface, and ex- 

 tended by himself and others, Helmholtz, Forster, Aubert, 

 to retinal impressions, mark an important step in the pro- 

 gress of this method of study, while the yet more remark- 

 able generalisations on the facts thus collected reached by 

 Fechner, and formulated by him in his famous psycho- 

 physical law, have served to reduce this department of 

 observation to something like a distinct and complete 

 branch of the science of physiological psychology. 

 Fechner's employment of the least recognisable sensation 

 and of the least recognisable difference of sensation as 

 constant units, the same for all orders of impression, must 

 be regarded as a most fruitful extension of the scope of 

 subjective observation, by the addition of an objective 

 method acquired in the region of physical research.' 1 



' These experiments aim at determining the duration 

 of the processes involved in recognising a momentary ex- 

 ternal impression, and in recording this recognition by a 

 simple voluntary movement; and they aim further at dis- 

 covering what variations in this duration are brought about 

 by variations in the impression and its attendant circum- 

 stances.' ' The several steps of the process here studied 

 are thus marked off by Wundt : (i.) the transition from 

 the organ of sense to the brain ; (ii.) the entrance into the 

 field of view of consciousness or perception ; (iii.) the en- 

 trance into the field of view of attention or apperception ; 



1 Mind, vol. i. p. 25. 



