THE EXERCISES 159 How TO INITIATE THE CHEJ> INTO THE EXERCISES WITH THE SENSE MATERIAL, CONTRASTS, IDENTITIES, GRADATIONS One ought to begin with very few contrasting stimuli, for which purpose is collected a number of objects similar in Icind but showing gradation, growing finer and less perceptible. For example, when it is a matter of recognizing tactile differences, we begin with only two surfaces, one perfectly smooth and the other very rough; if we are experimenting with the weight of things, first will be presented tablets which are the lightest of the series and afterwards the heaviest; for sounds, the two extremes of the graduated series are offered; for colours, the brightest and most highly contrasting tints like red and yellow are chosen; for shapes, a circle and a triangle, and so on. In order to make the differences still clearer, it is well to mix together with the greatest contrasts the identities (in con- trast to the great differences), offering a double series of objects; in a mixture of pairs, in which all are mixed in con- fusion, would be sought similar things two by two—two sounds equally loud and two equally faint, two things having the same yellow colour and two of an identical red. The exercise of searching for similarities among contrasts marks the differences strongly, by making them prominent. The final exercise, that of gradation, consists in placing in graded order a system of similar objects mixed up confusedly; for example, a series of cubes of the same colour but of different dimensions, the difference being systematically graduated (for example, having a difference of 1 cm. in the length of the sides). Of a similar character will be the presentation of a series of yellow objects, the shades of which will grow gradually paler, from dark to light; or a series of rectangles having one pair of equal sides fixed, and the other decreasing systematically, Such objects must be arranged side by side in the positions which they should occupy in a graduated series.