146 THE DISCOVERY OF THE CHILD the child makes of it. These two extremes, when brought together, demonstrate the most striking difference which exists in the series, and thus establish the most outstanding contrast which the material renders possible. The contrast, being a striking one, makes the difference very evident and even before he has used the things the child is interested in them. ISOLATION OF A SINGLE QUALITY IN THE MATERIAL Whatever object we wish to use for the education of the senses, it, of necessity, presents many diverse qualities like weight, rough- ness, colour, form, size, etc. How then ought we to proceed so that the series will bring one quality only into prominence? We must isokte, from among the many, one single quality. This difficulty is overcome by the series itself and its gradations; we must prepare objects identical among themselves in all respects except the variable quality. If we want objects suitable for teaching colour differences, we must have them made of the same substance, form and size and differing in colour only. Or, if we want tp prepare objects with a view to teaching the various tones of the musical scale, it is necessary that they be perfectly alike in appearance, as are the bells which we use in our system; these are of the same shape and size and are mounted on identical supports, but when struck with a small mallet, they give out different sounds and these sounds constitute the only difference, perceptible to the senses. For this reason the little instruments which are put into the hands of children as musical toys, which have longer or shorter rods or tubes of different heights arranged like organ pipes, do not lead themselves to a real exercise in musical sense, tending to differentiate sounds; for the eye is able to help in distinguishing them, being guided by the different dimensions, whereas the ear ought to be the sole receiver and the sole judge. This method is successful in differentiating between things very clearly; it is evident that clearness constitutes the principal factor for raising interest in making distinctions.