Some years ago I made the following simple experiment: I arranged a rubber bulb, like that used for releasing a photographer's shutter, to connect with a bottle, from which rose a long, vertical glass tube. The bottle contained mercury, and the long tube reached nearly to the bottom. Every part was air-tight, so that when anybody squeezed the bulb the mercury was forced up the vertical tube. It was what is known as a mercury-dynamometer.

During experiments with this dynamometer, what was more natural than to think of trying what would happen if one hand were practiced daily in squeezing the bulb? So one of our graduate students, Miss E. M. Brown, was set to work in the following manner: On the first day she squeezed the bulb as hard as possible with the left hand, while an assistant noted the height of the mercury; this was repeated ten times, and the results were averaged. Immediately thereafter she took ten records with the right hand. Then, on the following days, with some intermissions, she practiced the right hand by squeezing ten times on each occasion. On the last day she again tested the left hand, which had not been practiced in the meantime. The records ran as follows:

DAY
FirstSecondThirdFourthFifthSixthSeventhEightNinth
InchesInchesInchesInchesInchesInchesInchesInchesInches
Right hand28.833.735.636.640.944.747.048.848.6
Left hand29.9............................42.3

Thus the left hand had gained about fifty per cent in strength through practice of the right hand. This peculiar phenomenon of transference of the effects of practice from one side to the other I have ventured to call "cross-education."

The phenomenon was curious enough to suggest other experiments. Another student, Miss T. L. Smith, was set to trying to insert the point of a needle at the end of a rod into a small hole in a drill-gauge without touching the sides. The first experiment consisted of twenty trials with the left hand, with a success of fifty per cent. Immediately thereafter twenty trials were made with the right hand, with a success of sixty per cent. On the following day and on each succeeding day two hundred experiments were made with the right hand, with successes of 61, 64, 65, 75, 74, 75, 82, 79, 78, and 88 per cent. On the last day the left hand, which had not been practiced in the meantime, was again tried, with a success of seventy-six per cent.

These last experiments remind us of certain familiar phenomena. It has frequently been noticed that persons taught to write with the right hand become able to write backward, but not forward, with the left hand. This is the so-called "mirror writing," which appears correct if seen in a mirror. The first published observation of this fact exists in a letter from H. F. Weber to Fechner, the founder of experimental psychology. Fechner, moreover, noticed that with the left hand he could make the figure 9 backward better than in the regular way.

Curiously enough, the principle of cross-education has been put to practical use. A letter (with permission to publish) has been received from Oscar Raif, Professor of Music in the Berlin Hochschule:

"In the spring of 1898 I made an experiment with twenty of my pupils. I began by taking the average speed of each hand with the metronome. The average of the right hand was [j] = 116 (= four times 116 in the minute) [464 beats], and for the left hand 112 [448 beats]. I gave them exercises for the right hand only (finger exercises, scales, and broken accords) to develop rapidity. After one week the average of the right hand was 120 [480]; after two weeks, 126 [504]; three weeks, 132 [528], etc. After two months the right hand yielded 176 [604]. Then I had them try the left hand, which averaged 152 [608], whereas in November the average was only 112 [448]. In two months' time, absolutely without practice, the left hand had risen from 112 [448] to 152 [608]. A few of my pupils had some difficulty in playing the scales in parallel motion, but were able to play them in contrary motion.

"The tenor of my work is that in piano playing the chief requirement is not that each single finger should move rapidly, but that each movement should come at exactly the right time, and we do not work only to get limber fingers, but, more than that, to get perfect control over each finger. The source of what in German is called Fingerfertigkeit is the center of our nervous system—the brain."

**[Transcriber's note: [j] stands for musical symbol of quarter note]**