sensations of convergence which serve as signs for consciousness of depth exist for points seen in indirect vision only in representative form, i.e. in a form so weak as not to overcome the immediate impression of a bidimensional visual field. In short, the actual position of the visual field is not taken into account. But this explanation, which is also substantially Wundt's, is really in opposition to his theory of measurements by motion of the eyes. The real problem lies deeper than this; the oblique appearance of the indirectly seen vertical line, as well as the curved appearance of the lines in Helmholtz's "chessboard" figure, is a question of the plane field of visual space, as this again is included in the question of the straight line. The straight line, which Wundt treats as if "given," implies consciousness of depth; and this straight line will seem to us to change its direction, when our consciousness of difference in depth, out of which together with the perception we construct the line, undergoes modifications. The point of reference from which we measure off the position and form of objects is not fixed, so that the same line viewed from the same standpoint will change the direction of the apparent curve as the point of reference changes. The phenomena of the apparent rising of the earth's surface to the horizon line, the overestimation of vertical lines as compared with horizontal, etc., find no sufficient explanation from motor sensations of the eye; they result from false estimates of depth. Here the principle of customary mean estimation plays an important part. We estimate falsely the dimensional value of a perception, partly through lack of experience and partly through applying to it norms which interpret analogous but more elaborated perceptions.
Pleasure is of course indefinable, but individuals and species may be enumerated. From these may be formed representative groups, on which to base a theoretical treatment. In some instances, at least, of the cessation of pains, there is a notable reaction or recoil of pleasurable feeling. But we must take into account the continuance of pain in the idea after it has ceased in fact. It is in the case of acute physical pains that, the persistent memory being feeble, the grateful reaction is most apparent. Relativity covers a wide field, more or less allied to the above. What is peculiar to relativity is partly, but not wholly, included in the general law that every organ needs exercise, or relishes it, in proportion to its active endowment. This is adequately expressed by the law of rotation or change. B. considers pleasure and pain from the point of view of the different senses, and arrives at the following results,
