A series of experiments was arranged, in which one of two unequal lines of white on a black background being fixed in an upright position a certain distance from the centre, the other was shifted until the arrangement was felt to be pleasing. It was found that when two lines of different sizes were opposed, their relative positions corresponded to the relation of the arms of a balance, that is, a small line far from the centre was opposed by a large one near the centre. A line pointing out from the centre fitted this formula if taken as "heavy," and pointing in, if taken as "light." Similarly, objects of intrinsic interest and objects suggesting depth in the third dimension were "heavy" in the same interpretation. All this, however, did not go beyond the proof that all pleasing space- arrangements can be described in terms of mechanical balance. But what was this mechanical balance? A metaphor explains nothing, and no one will maintain that the visual representation of a long line weights more than a short one. Moreover, the elements in the balance were so far heterogeneous. The movement suggested by an idea had been treated as if equivalent to the movement actually made by the eye in following a long line; the intrinsic interest--that is, the ideal interest--of an object insignificant in form was equated to the attractive power of a perspective, which has, presumably, a merely physiological effect on the visual mechanism. I believe, however, that the justification of this apparent heterogeneity, and the basis for explanation, is given in the reduction of all elements to their lowest term,--as objects for the expenditure of attention. A large object and an "interesting" object are "heavy" for the same reason, because they call out the attention. And expenditure of effort is expenditure of attention; thus, if an object on the outskirts of the field of vision requires a wide sweep of the eye to take it in, it demands the expenditure of attention, and so is felt as "heavy." But what is "the expenditure of attention" in physiological terms? It is nothing more than the measure of the motor impulses directed to the object of attention. And whether the motor impulse appears as the tendency to follow out the suggestions of motion in the object, all reduces to the same physiological basis. It may here be objected that our motor impulses are, nevertheless, still heterogeneous, inasmuch as some are toward the object of interest, and some along the line of movement. But it must be said, first, that these are not felt in the body, but transferred as values of weight to points in the picture,--it is the amount and not the direction of excitement that is counted; and secondly, that even if it were not so, the suggested movement along a line is felt as "weight" at a particular point.