"The purpose of this experiment was to determine the relation of the length and distribution of periods of work to economy in learning. The learning consisted in associating numbers with letters. These associations were formed while transcribing prose into numbers.
"One group of persons worked 10 minutes at a time twice a day for six days. The second group worked 20 minutes at a time once a day for six days. The third group worked 40 minutes at a time every other day for six days.
"The records show that the 10-minute group improved more rapidly than the 20-minute group and the latter improved much more rapidly than the 40-minute group. The 20-minute group transcribed on the average 31 more letters in every five minutes than the 40-minute group and the 10-minute group transcribed on the average ten more letters in every five minutes than the 20-minute group."
Thus we see that ideas "stick" best when they are impressed in periods of only thirty minutes or less, two or three times a day. We may use this as a rule for thinking—it means the importance of keeping the brain always rested. This is far more essential than the saving of mere time. Muscles, especially in gross masses, may be fatigued without nervous harm, (in fact this kind of fatigue makes for sound, restful sleep) but never the nerves. There are nine thousand million neurons or nerve-units, weighing only a
