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 i-n 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 
= 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 [604]; 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
I 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."
These facts, however, require further investigation, for it is evident that we must begin with the fact of cross-education and proceed to more complicated cases. Indeed, cross-education has shown itself to be one step of a ladder up which we must climb even if there were no other motive except that of curiosity as to what we could find at the top. If practice of one hand educates the other hand, will it not also educate the foot? Again, if practice of one
