Thus, the line suggesting movement out from the center fitted the formula if taken as ‘heavy’ and vice versa, the line suggesting movement in, if taken as ‘light.’  Similarly, objects of interest and objects suggesting movement in the third dimension were ‘heavy’ in the same interpretation.  But this interpretation, in its baldest form, fitted only a majority of the pleasing arrangements; the minority, in which the consistent carrying out of the lever principle would have left a large unoccupied space in the center, exactly reversed it, bringing the ‘light’ element to the center and the ‘heavy’ to the outer edge.  Later experiments showed that this choice implied a power in the ‘lighter’ objects, owing to their central position, to cover or infuse with vitality the empty space about them, so that the principle of balance seemed to maintain itself in one form or another.

All this does not go beyond the proof that all pleasing space arrangements can be described in terms of mechanical balance.  But what is this mechanical balance?  A metaphor, no matter how consistently carried out, explains nothing.  The fact that a small object far from the center is usually opposed by a large object near the center tells us nothing of the real forces involved.  Physical balance can be explained by principles of mechanics, but no one will maintain that the visual representation of a long line weighs more than that of a short one.  Moreover, the elements in the balance seem utterly heterogeneous.  The movement suggested by an idea—­the picture of a man running—­has 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 has been equated to the attractive power of a perspective which has, presumably, a merely physiological effect on the visual mechanism.  What justification can be given either of this heterogeneous collection of elements or of the more or less arbitrary and external metaphor by which they have been interpreted?

I believe that the required justification of both points of view 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; a deep perspective, because the eye rests in it;—­why, is another question.  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.’  It may be said that involuntary attention is given to the object of intrinsic interest, while the uninteresting object far on the outskirts needs a voluntary effort to perceive it, and that the two attitudes cannot be treated as identical.  To this it may be answered that an object on the outskirts of a field of view