Like the practical “schools,” the theoretical have also become innumerable. I take as a few of the most important works the following: Adolph Kullak’s “Aesthetic of Piano-playing,” re-edited by Bischoff, a unique and profound work on the theory of the art, as a hundred years’ experience and the careful observations of the author have enlarged it; Hugo Riemann’s “Comparative Theoretical and Practical Piano School, presenting system, method, and materials, in a historic and organic connection”; and, among the innumerable “schools” and volumes of exercises, the various thoughtful works of Germer. Two main principles have come to occupy the first place in the newest school-practice. First, the systematic carrying out--not merely of a musical mechanic of the hand, but its thorough gymnastic. This was the natural advance, which carried yet further the teaching of Czerny. The hand is adapted for piano-playing--as in the systems of Thilo, Virgil, and others--by gymnastics of the fingers, and stretching of the muscles. Thus a great part of the gymnastic cultivation is got over before actual musical practice begins. In this work the dumb keyboards, which to-day are constructed with great delicacy, have borne a great part. They now admit of legato playing and of different degrees of strength in touch. [Illustration: Hand of Eugene d’Albert. Röntgen ray photograph by Spies.] The second great principle is to take into account in instruction the peculiarities of the pupil’s hand. It stands to reason that the same exercises will not do for all hands. One hand demands this, the other that. This method is carried out with the utmost precision by the greatest of teachers, Leschetizky. A similar principle prevails in modern teaching of singing. We no longer endeavour to base voice-cultivation on the universal vowel A, but on that vowel which comes most natural to the organ of the pupil. * * * * *