VISUAL AND AUDITORY DISTINCTIONS 171' The stand together with the cylinders belonging to it, looks, rather like the ordinary receptacle for weights belonging to a balance. Within the cylinders embedded in their supports there exists a regularly graduated difference: 1. In the first stand, the cylinders are all of the same dia- meter, but differ in height. The shortest is 4 cm. high, and others- increase each by half a centimetre, up to the tenth, which is 5 cms. high. 2. In the second stand, the cylinders are all of equal height, but the circular section decreases regularly. Whilst the diameter of the section of the smallest cylinder is i cm., the diameters of the other sections increase by half a centimetre up to a diameter of 5 cms. 3. In the third stand, the cylinders diminish in all three* dimensions, combining the differences met with in the two other sets. 4. Finally, in the fourth stand, the cylinders differ in three dimensions, but height and section in opposite directions. At first, the children take only one of the stands, hence four children can find occupation with them at the same tone. The exercise is the same with all four insets. After being placed on the table, they are used by removing all the pieces, mixing them up and then replacing them, fitting each piece into its appropriate hole. In this exact correspondence between the cylinder and the hole in the stand there exists the * control of error'. If, for example, in the case of the first inset the child makes- a mistake in putting it back, one cylinder will disappear within a hole which is too deep, and another will project because one is not deep enough. The irregularity which results, apparent to sight and touch, affords an absolute, material control of the mistake made. It follows that the objects must be put back into their places with care, that the replacement of them must be repeat- edly tested, so that all may be in place at the same level in the stand.