58 GENERAL VIEW AND BASIS OF SCIENTIFIC RESEARCH. 



(iv.) the action of the will in giving the necessary impetus 

 to the motor nerves ; ajad (v.) the transmission of this 

 motor excitation to the muscles. The first and last of 

 these stages are purely physiological.' l 



' The experiments by which the varying values of the 

 physiological time have been determined were originated 

 by Bessel in his investigations into the personal equation 

 in astronomical observation. They have since been further 

 developed by Hirsch, Bonders, De Jaager, and others, and 

 in a special manner by Exner.' ' The experiments to be 

 considered fall into three series : (i.) those which investi- 

 gate the physiological time under the simplest conditions, 

 that is, when the observer (who records his impression) is 

 expecting an impression of a certain quality and strength, 

 but is uncertain as to the precise moment of its arrival ; 

 (ii.) those in which a change in the physiological time is 

 effected by the addition of the favourable circumstance 

 that the exact time of the impression is known before- 

 hand ; and (iii.) those in which the physiological time is 

 modified by the introduction of some unfavourable circum- 

 stance, as, for example, that the nature of the impression 

 is unknown, or that the kind of movement to be carried 

 out in the act of registration is made to depend on the 

 character of the impression, and cannot therefore be pre- 

 pared for in the same manner.' 2 



The fixation of an idea depends upon the high degree 

 of impressibility of nervous matter, and the strength and 

 number of the impressions made upon it ; each repetition 

 deepening the impression. It is in this way that the 

 human mind gradually becomes a representation of external 

 nature. The fixation of an idea also requires time and 

 attention, and is the more easily effected the greater the 



1 Mind, vol. i. p. 33. 2 Ibid., p. 39. 



