The boy begins by having before him a copy of the letters he is to write. Under the guidance of the eye the hand traces these letters. At each instant the eye points out to the hand the direction in which to move. As the hand occasionally wanders from the prescribed direction the eye immediately notes the deviation and bids the hand to correct it. The hand responds to the demands of the eye, immediately, without thought on the boy's part of nerve impulse or of muscular contraction. By repeated efforts the boy improves upon his first clumsy attempts; with each repetition he approaches nearer to the model. In the course of this progress the muscular sense gradually comes to the assistance of the eye as a sort of supplementary guidance. But at no time is the eye relieved of the responsibility of guiding the hand in writing. To sum this up, the movements of the hand in writing are guided, so far as the consciousness is aware, directly by the sense of sight. We have here the law of voluntary muscular guidance. In all voluntary movements the muscles are guided in their contractions, through some instinctive process, by the sense or senses which observe the movements themselves, and more especially, the results of the movements. In most actions the two senses concerned are sight and muscular sense. The more an action becomes habitual the more it tends to be performed under the guidance of muscular sense, and to be free from the necessity of the guidance of the eye. But muscular sense does not usually rise so high into consciousness as sight, in the guidance of muscular activities. Many oft-repeated movements, especially those of walking, become thoroughly habitual and even automatic; that is, the muscular contractions are performed as purely reflex actions, without conscious guidance of any kind. But even in walking, the necessity may at any instant arise for conscious guidance. In such a case the sense of sight immediately comes into service; from reflex the movements become voluntary, and consciously guided. In the case of most complex actions the sense of sight furnishes the most important guidance. If the muscular operations of singing are subject to the general laws of psychological control, the guidance of the vocal organs must be furnished by the sense which observes the results of the movements involved. This is the sense of hearing. Just as in writing the hand is guided by the eye, so in singing the voice is guided by the ear. There can be no other means of guiding the voice. Muscular sense may under certain conditions supplement the sense of hearing, but under no circumstances can muscular sense assume full command. The net result of the application of psychological principles to the problem of tone-production is simply this, that the voice is guided directly by the ear.