PRACTICAL PHASES OF MENTAL FATIGUE. 515 



it was thought best not to permit him to make the entire fifteen 

 trials. 



Another pupil, "W. R., two years younger, illustrates a different 

 type. In the morning trials he was no better than A. M., but he, 

 too, was subjected to five dift'erent tests at half past eleven, with the 

 result that he could in every instance complete the task without any 

 apparent fatigue. There was no constraint apparent in the face or 

 hands, no unusual effort to co-ordinate the muscles of the body, and 

 no twitchings of any kind. :N"ow, it seems probable that in the case 

 of "W. K. the brain was able to adjust effort in right degree to the 

 needs of the occasion, while with A. M. there was such prodigalitv 

 in the expenditure of energy in various irrelevant motor tensions 

 and activities that it not only defeated its purpose, but it was soon 

 largely spent. A. M. showed this tendency to nervous extrava- 

 gance in all the work of the school. "While an unusually bright boy, 

 he yet became fatigued in the performance of duties that W. R. 

 could discharge with no evidence of overstrain; indeed, the latter 

 boy seemed never to reach a point beyond which he could not go 

 with safety if he chose. 



Further illustrations of this principle of individual differences 

 in the conservation of nervous energy were afforded by another sim- 

 ple experiment. The apparatus employed consisted of a plate of 

 smoked glass set in a frame so that it could be moved horizontally. 

 Just touching the glass, and adjusted to it by a delicate spring, was 

 a fine metal point which could be maintained at any height by a silk 

 thread to be held in the fingers of the subject to be experimented 

 upon, who stood with closed eyes endeavoring to keep his hand per- 

 fectly quiet for one half minute. During the test the glass was 

 moved slowly in the frame, the metal point thus tracing a line which 

 was a faithful index to most of the movements, at any rate, of the 

 subject's hand. Five sets of experiments were made upon a number 

 of pupils in the morning soon after the opening of school, and again 

 just before the noon recess. The accompanying tracings are re- 

 productions of those gained at one of the tests, and are typical ex- 

 amples. The first two were secured from a girl, M. L. R., eleven 

 years of age. The one made at half past eleven, after two and a 

 quarter hours' work in school, shows a significant phenomenon which 

 could be easily witnessed during the experiment. She had be- 

 come so fatigued that all muscular expressions were unusually con- 

 strained. During the short period while the experiment continued 

 one could observe the arm and fingers contracting, which accounts 

 for the upward direction of the tracing. The body swayed almost 

 to the point of falling, the fingers of the hand not employed were 

 clinched, and all the expressions indicated great tension. The 



