1124 DURATION OF PSYCHICAL PROCESSES. [BOOK HI. 



the unaccustomed left hand than in the case of the usual right 

 hand. In this way we obtain a measure of so to speak the 

 volitional side of psychical processes. 



In a somewhat similar way we may obtain a measure of the 

 time required for perception. A strong sensation following too 

 closely upon a weak one will prevent the psychical recognition of 

 the weaker one. If, for instance, two or three letters in white on 

 a black background be presented to the eye, and a large white 

 surface be presented afterwards at an interval which is made 

 successively shorter and shorter, it is found that when the interval 

 is made very brief indeed the letters cannot be perceived at all. 

 In proportion as the interval is prolonged, the recognition of the 

 letters increases, until at an interval of about '05 sec. they are 

 fully and clearly recognized. That is to say, the time required for 

 perception is in such a case of about that length. 



The duration of all these psychical processes, as of the simple 

 reaction period itself, varies of course under different circum- 

 stances, and the discrimination period may be conveniently used 

 for measurements of the varying effects of circumstances. Practice 

 shortens the discrimination period as it does the simple reaction 

 period. One of the most powerful influences is that of attention. 

 And it is stated that the shortening of the period is greater when 

 the attention is concentrated on the making of the signal than 

 when it is more especially directed to recognition of the stimulus ; 

 in other words, the volitional processes are more amenable than 

 are the perceptive processes to the psychical action which we call 

 attention. On the other hand, the period is distinctly prolonged 

 if the observer be distracted by concomitant sensations. For 

 example, the period for discriminating between two visual sen- 

 sations is prolonged if powerful auditory sensations be excited at 

 the same time. 



The same method of measurement may be used in other ways 

 and under other circumstances with reference to psychical pro- 

 cesses. It must be remembered, however, that all such obser- 

 vations are open to many fallacies and need particular caution. 

 It not unfrequently happens that false results are obtained; for 

 instance, the subject, expecting the stimulus to be brought to bear 

 upon him and straining his attention, makes the signal before the 

 stimulus actually comes off. And the interpretation of the results 

 obtained are in many cases very difficult ; but it would be out of 

 place to dwell upon these matters any further here. 



