The conditions of fatigue are similar to those of practice. Fatigue occurs when mental functions are repeated too many times in immediate succession. But the result is not perfection, but deterioration of the performance. The sensitivity for weak stimuli or small differences of stimuli disappears. Attention is decreased, that is, fewer mental states are vivid, and they are also less vivid. New ideas do not easily enter consciousness. Reproduction, as in the processes of reading and arithmetic, is slow and inaccurate. Action becomes slow and awkward, and may cease altogether. Fatigue is obviously a protective measure. When the continued performance of a task threatens to exhaust the organs, their resistance to the call for action increases, and finally they completely refuse to respond. Because of the continuity of all organic processes, this refusal in extreme cases is impossible without a lesser degree of refusal before the extreme is reached. The first indications of fatigue thus appear soon after a prolonged mental activity has begun, as a diminution of the effects of practice. This leads often to the astonishing consequence that a certain performance is executed better at the beginning of a practice period than at the end of the preceding period. The acquired practice is then still effective, while the effect of fatigue is absent. This experience does not justify the conclusion that skill has increased during the time of intermission. Because of the great importance of fatigue for mental and bodily health, numerous investigators have in recent years undertaken to study it more closely by experimental methods. Especially fatigue caused by school work has been much under discussion in scientific and popular periodicals and even in the daily press. Little progress, however, has been made in our knowledge of fatigue. It has proved difficult to find reliable methods of measuring it, and the great complexity of the conditions has interfered with the interpretation of the experimental results. The attempt has been made to measure mental fatigue indirectly by measuring the muscular fatigue caused by repeatedly lifting a weight; or by measuring the minimum distance of two touches on the skin recognizable as two. Although there are probably relations of cutaneous sensitivity and of muscular fatigue to mental fatigue, they are not definitely known, and by some their very existence is doubted. Other tests used for the measurement of fatigue are adding numbers of several digits, adding a long series of digits, and taking dictation. In these tests the mental work is very one-sided and too simple to permit conclusions with regard to fatigue under ordinary conditions of mental activity. A disturbing element in these tests is the rapid perfection of the work under the influence of practice. If we choose more complicated tasks such as translation into another language, mathematical problems,