CHAP, ii.] THE BRAIN. 821 



mers. It was early found that when two observers were watch- 

 ing the appearance of the same star, a considerable discrepancy 

 existed between their respective reaction periods, and that the 

 difference, forming the basis of the so-called ' personal equation,' 

 varied from time to time according to the personal conditions of 

 the observers. 



516. The events taking place in the central stage are of 

 course complex, and this stage may be subdivided into several 

 stages. Without attempting to enter into psychological ques- 

 tions, we may at least recognize certain elementary distinctions. 

 The afferent impulses started by the stimulus, whatever be their 

 nature, when they reach the central nervous system undergo 

 changes, and as we have seen, probably complex changes before 

 they become sensations ; and further changes, now of a more 

 distinctly psychical character, are necessary before the mind can 

 duly appreciate the characters of these sensations and act accord- 

 ingly. Then come the psychical processes through which these 

 appreciated sensations, or perceptions, or apperceptions as they 

 are sometimes called, determine an act of volition. Lastly, there 

 are the executive processes of volition, the processes which, 

 psychical to begin with, end in the issue of coordinate motor 

 impulses, or, in other words, start the distinctly physiological 

 processes of the efferent stage. We may thus speak of the time 

 required for the perception of the stimulation, of the time re- 

 quired for the action of the will, and of the time required for 

 the complex psychical processes which link these two together. 

 Accepting this elementary analysis, it is obvious that the total 

 length of the central stage may be varied by differences in the 

 length of each of these parts; and a more complete analysis 

 would of course open the way for further distinctions. Hence, 

 by studying the variations of the whole reaction time under 

 varying forms of psychical activity, we may form an estimate of 

 time taken up by various psychical processes. 



We may take as an instance the case in which the subject 

 of the experiment has to exercise discrimination. The mode 

 of making the signal being the same, and the stimulus being of 

 the same order in each trial, that is to say, visual, or cutaneous, 

 or auditory, &c., and general circumstances remaining the same, 

 two different stimuli are employed, and the subject is required 

 to make a signal in response to the one stimulus, but not to 

 the other ; the subject has to discriminate between the psychi- 

 cal effects of the two stimuli. Suppose, for example, the 

 stimulus is the sound of a spoken or sung vowel, and the sub- 

 ject is required to make a signal when a is spoken or sung, but 

 not when o is spoken or sung. If the subject's whole reaction 

 period be determined (i) in the usual way, with either a or o 

 spoken (and the result will be found not to differ materially 

 whether a or o be used), the subject knowing that only a or 



