There is little or nothing of psychology in Mackenzie's book, and yet, like other writers on voice-production, he appears now and then to be groping for it. Thus, when he speaks of the fundamental tone being reinforced by its overtones--by a number of secondary sounds higher in pitch and fainter in intensity--he adds very beautifully that every resonance-cavity has what may be called its elective affinity, or one particular note, to the vibrations of which it responds sympathetically like a lover's heart answering that of his beloved. "As the crude tone issues from the larynx, the mouth, tongue and soft palate, moulding themselves by the most delicately adaptive movements into every conceivable variety of shape, clothe the raw bones of sound with body and living richness of tone. Each of the various resonance-chambers reëchoes its corresponding tone, so that a single well-delivered note is, in reality, a full choir of harmonious sounds." Voice being, like instrumental tone, a commixture of fundamental and overtones, and the manner in which the composite conformation of collective waves strikes the ear being largely determined by the cavities of resonance, the control of these is of great importance to the singer. This control should, by thorough training, be brought to such a degree of efficiency that it becomes subconscious and automatic, so that the resonance-cavities shape themselves instantly to the note that is being produced within the larynx and, vibrating in sympathy with it, sound the overtones. The reciprocal principle of elective affinity between fundamental and overtone, between the shape assumed by the larynx for pitch and the shape assumed by the resonance-cavities for quality, is illustrated by the exciting influence of a sounding instrument upon a silent one tuned to the same pitch which, although not touched by human hand, sounds in sympathy with the one that is being played on. Even a jar standing upon a mantel-shelf, a globe on a lamp, a glass on a table, or some other object in the room, may vibrate and rattle when a certain note is struck on the pianoforte. This is the result of sympathetic vibration. Thus, although vocal tone originates within the larynx, it sets the resonance-cavities into sympathetic vibration, and these produce the harmonics that give the fundamental tone its timbre; the resonance-cavities being to the vocal cords or lips what the body or resonance-box of the violin and the sounding-board of the pianoforte are to their strings, the tube of a cornet or horn to the lips, the body of the clarinet to its reed--the resonating factor which determines the overtones and through these the timbre.