2. The pupils must be mentally active while the concrete object or illustrative material is being used, and not merely gaze in a passive way upon the objects. It requires mental activity to grasp the abstract facts that the objects or illustrations typify. A tellurion will not teach the changes of the seasons; bundles of splints, notation; nor black-board examples, the law of agreement; unless these are brought under the child's mental apprehension. The sole purpose of such materials is, therefore, to start a flow of imagery or ideas which bear upon the presented problem. 3. The objects should not be so intrinsically interesting that they distract the attention from what they are intended to illustrate. It would be injudicious to use candies or other inherently attractive objects to illustrate number facts in primary arithmetic. The objects, not the number facts, would be of supreme interest. The teacher who used a heap of sand and some gunpowder to teach what a volcano is, found his pupils anxious for "fireworks" in subsequent geography classes. The science teacher may make his experiments so interesting that his students neglect to grasp what the experiments illustrate. The preacher who uses a large number of anecdotes to illustrate the points of his sermon, would be probably disappointed to know that the only part of his discourse remembered by the majority of his hearers was these very anecdotes. In his enthusiasm for objective teaching, the teacher may easily make the objects so attractive that the pupils fail altogether to grasp what they signify. 4. In the case of pictures, maps, and sketches, it is well to present those that are not too detailed. A map drawn on the black-board by the teacher is usually better for purposes of illustration than a printed wall map. The latter shows so many details that it is often difficult for the pupil to single out those required in the lesson. The black-board map, on the other hand, will emphasize just those details that are necessary. For the same reason the sketch is often better than the printed picture or photograph. Any one who can sketch rapidly and accurately has at his disposal a valuable means of communicating knowledge, and every teacher should strive to cultivate this power. MODES OF PRESENTATION COMPARED The relative clearness of different modes of presenting knowledge may be seen from the following: