DOCKERAY : PHYSICAL FATIGUE AND MENTAL EFFICIENCY. 231 



interval between learning and recall was usually only twenty 

 minutes, and it might be possible in these experiments for 

 fatigue to affect recall as well as to exert a retroactive effect 

 upon learning. However, a part of the series were tested 

 twenty-four hours after learning. In these, as well as in the 

 series tested twenty minutes after learning, he found very 

 little evidence of a retroactive influence when mental work 

 was used. In the tests in which physical work was used after 

 learning the tests showed a lower per cent of recall, but, as 

 the physical work was always used in the short intervals, it is 

 probable that it affected the results through the influence of 

 physical fatigue on the recall, rather than that it affected the 

 learning process. This is Mr. De Camp's interpretation of the 

 results. If we accept his conclusions as applicable to our 

 sounder test, then we find that physical fatigue has affected 

 our test either by decreasing the power of discrimination or by 

 interfering with the practice effects (recall) of the first test. 



That interference with the practice effects is not the only in- 

 fluence of fatigue is demonstrated by the results of Do. Though 

 after rest there was an average of only 4.2 per cent decrease 

 in omissions, which was counterbalanced by 5.9 per cent in- 

 crease in errors, he shows an average increase of 0.5 per cent 

 in omissions and 14.1 per cent increase in errors after walk- 

 ing for two hours. Though S. improved greatly after rest, he 

 shows as decided decrease in discrimination after the physical 

 fatigue. Inasmuch as fatigue could influence the practice ef- 

 fects only to the point of rendering no improvement, these 

 results must be due, in part, at least, to the direct influence 

 of fatigue upon attention and discrimination. 



The introspections of the subjects throw further light upon 

 the nature of the processes with which the sounder test deals. 

 In every case it was reported that the sound to be reacted to 

 was held in mind and the other three sounds were dropped out. 

 Confusion always resulted when the other sounds were not 

 inhibited. In this the results and introspections uniformly 

 agreed. To test the degree to which this inhibition process 

 was effective, an extra minute was occasionally added to the 

 second test without the knowledge of the subject, and only 

 two sounders used, the one to which the subject was to react, 

 and one other. In no case did the subject know what had 

 occurred, though he usually realized near the end of the minute 



