H A P T E R IX VISUAL AND AUDITORY DISTINCTIONS Material: Solid insets and blocks. Recognition of Dimensions, by Visual Means only. The various series demonstrate differences in dimensions. In one series, the differences are concerned with one dimension only (height); in another, there is a graduated difference in two- dimensions (area); in another, there is a graduated difference in all three dimensions increasing harmoniously; in another, the difference also concerns the three dimensions, but in the inverse sense. SOLID INSETS There are four strong blocks of natural coloured wood, bright- ly polished; all four have the same shape and dimensions (55 cms. long, 5 cms. high, 8 cms. wide). Each of one of these contains ten insets, which are cylindrical in shape, smooth and slippery and which are handled by means of a knob placed on the top. They can be taken out of, and replaced easily, in holes which, hollowed out in the stand, correspond perfectly and exclusively to each cylinder.