Cowles[41] observes that the first prominent and serious mental concomitant of nervous depletion is revealed in the inability to direct the attention continuously upon any given subject; and James has said that when one is fatigued the mind wanders in various directions, snatching at everything which promises relief from the object of immediate attention. Experiments in the laboratory upon the keenness of sense discrimination of data appealing to sight, hearing, touch, and the other senses, show that there is lessened ability in conditions of fatigue;[42] and this is accounted for probably by the waning power of attention. The mind can be held to one thing, excluding irrelevant matters. This phenomenon is further illustrated in the following simple experiment: The pupils in a large graded school in Buffalo, N. Y., were required upon three successive days, at half past nine o'clock and again at half past eleven in the morning, to trisect a line three inches long. The results, calculated for one hundred and fifty children, show that on the average they were several millimetres nearer correct in the morning trisections than in those just before the midday recess.[43] It seems that this test measured the degree of attention which pupils were able to exert at different hours during the day, and it confirmed what must in a way be known to every one--that a day's work in school reduces the energy of attention. Doubtless every instructor has remarked how much more difficult it is at half past eleven than at ten to hold the thoughts of students to the subject in hand, and if recitations in intricate studies occur late in the forenoon, progress will be slower and more errors will be made, simply because pupils are unable to attend so critically. [Footnote 41: _Op. cit._, p. 47.] [Footnote 42: See Educational Review, _op. cit._; Galton, Journal of the Anthropological Institute, 1888, pp. 153 _et seq._] [Footnote 43: Since this article was written extensive investigations on school-room fatigue have been made in the schools of Madison, Wis., under the writer's direction, and the general principles here mentioned have been corroborated.]