VISUAL AND AUDITORY DISTINCTIONS 185 produce the following notes, so that the only difference percepti- ble is that of sound: The individual bells, which constitute a double series, are moveable; they can therefore be mixed together, precisely as are the other objects used in sense education. The bells are handled by the stand and made to vibrate by a small hammer. The first exercise consists in recognizing two bells which produce the same sound and placing them side by side (semi-tones being excluded). Then comes the learning of the notes of the scale, in their order, and in this case it is the mistress who arranges in the desired order one set of bells, leaving the other series mixed up. The exercise is again one of making up pairs, for it consists in sounding one of the bells in the fixed series, and then finding by trial among the mixed group the bell which gives a corresponding note. In this exercise, however, the pairing is .guided in a prescribed order. When the ear is sufficiently accustomed to recognizing and memorizing the succession of simple sounds in the scale, the child may possibly be able, without any guidance, to put the displaced and mixed group of bells into the successive order of the diatonic tones guided solely by his own musical ear; afterwards he may add the semi-tones. As in the case of the other systems of objects, the name is added to the sensation after that has been clearly perceived (smooth, rough, red, blue, etc.). So here the name of the note is made to accompany the sound, after the latter has been distinguished with •certainty. The greatest limit possible for a child six or seven years old is that of recognizing and naming an isolated sound.