820 DURATION OF PSYCHICAL PROCESSES. [BOOK m. 



and chiefly based upon calculations of the whole reaction time, 

 are very uncertain. Hence all attempts to estimate the length 

 of the " central " stage, the " reduced reaction period " as it is 

 sometimes called, by subtracting the efferent and afferent stages, 

 must be subject to much error. But a good deal may be learnt 

 by studying the variations under different circumstances of the 

 reaction period as a whole. 



Taking first of all the cases in which the events of the central 

 stage are simple, such as those where the subject has merely to 

 make a signal upon feeling a sensation, we find that the length 

 of the reaction period is dependent on the intensity of the stim- 

 ulus, being shorter with the stronger stimulus. But variations 

 in the strength of the stimulus, especially in the case of minimal 

 stimuli, have a much more striking effect in determining the 

 certainty of the reaction than in affecting the length of the 

 period. Thus when the signal is made in response to some 

 visual sensation, upon seeing an electric spark for instance, if 

 the spark be a very weak one the subject of the experiment often 

 fails to make the signal at all, though he may rarely fail if the 

 spark be a strong one. 



Some of the most marked variations in the length of the 

 reaction period are determined by the individuality of the sub- 

 ject. Thus with the same stimulus applied under the same cir- 

 cumstances the reaction period of one person will be found very 

 different from that of another. 



The length of the reaction period varies also according to the 

 nature and disposition of the peripheral organs stimulated. In 

 general it may be said that cutaneous sensations produced by 

 the stimulus of an electric shock applied to the skin (the signal 

 for instance being made by the right hand when the shock is felt 

 by the left hand) 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 said to 

 be that of visual sensations produced by direct electrical stimu- 

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

 reaction period is for cutaneous sensations jth, for hearing |th, 

 and for sight --th of a second. 



Practice materially shortens the reaction period ; indeed, after 

 long practice, making the signal, at first a distinct effort of the 

 will, takes on the characters of a reflex act, with a correspond- 

 ingly shortened interval. Lastly, we may add that in the same 

 individual and with the same stimulus, the length of the period 

 will vary according to circumstances, such as the time of year, 

 the weather, and the like, as well as according to the condition 

 of the individual, whether fresh or fatigued, fasting or replete, 

 having taken more or less alcohol, and the like. 



The reaction period of vision has long been known to astrono- 



