644 RAPIDITY OF CEREBRAL OPERATIONS. [BOOK in. 



The reaction period, both reduced and unreduced, varies 

 according to the nature and disposition of the peripheral organs 

 stimulated. The reaction period of vision has long been known 

 to astronomers. It was early found that when two observers 

 were watching 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. 



In general it may be said that tactile sensations produced by 

 the stimulus of an electric shock applied to the skin, are followed 

 by a shorter reaction period than are auditory sensations, while the 

 period of these is in turn shorter than that of visual sensations 

 produced by luminous objects; on the other hand, the shortest 

 period of all is that of visual sensations produced by direct electrical 

 stimulation of the retina. Roughly speaking we may say that the 

 reaction period or physiological time is for feeling nj-th, for hearing 

 Jth, and for sight Jth of a second. But even with the same 

 stimulus, the reaction period will vary according to circumstances, 

 such as the time of year, weather, &c., and according to the 

 condition of the individual, previous practice, fatigue, and the like. 



The calculations involved in ' reducing ' the reaction period are 

 obviously open to much error; in general the reduced reaction 

 period may be said to be less than -j-^th of a second, that is to say 

 an intelligent person takes about this time to perceive and to will. 



The reaction period just given belongs to cases where a 

 single stimulus is used, and all that the person experimented 

 on has to do is to perceive the stimulus, and to make an 

 effort in accordance. If, however, the stimulus, instead of being 

 applied to a part of the body determined by previous arrange- 

 ment, as for instance to the left foot, were applied either 

 to the left or the right foot, without the person being told 

 which it was to be, and it was arranged that he should make 

 a signal when the left foot, but not when the right foot was 

 stimulated, additional mental exertions would be necessary; and 

 it is found that in such a case the reaction period is considerably 

 prolonged. The difference between a simple reaction period, and 

 one in which a mental decision has to be carried out before the 

 voluntary effort to make the signal is initiated, gives the time 

 required for a person to ' make up his mind ' in accordance with 

 the nature of the sensation which he receives ; this is found to be, 

 roughly speaking, from to -$ of a second. 



