532

W. JAMES ! position out of an infinity does a broken line look straight. Accordingly, it is impossible by projecting the after-image of a straight line upon two surfaces which make a solid angle with each other to give the line itself a sensible 'kink'. Look with it at the corner of your room : the after-image, which may overlap all three surfaces of the corner, still con- tinues straight. Volkmann constructed a complicated surface of projection like that drawn in Fig. 22, but he found it impos- sible so to throw a straight after-image upon it as to alter its visible form. Fig. 22. D One of the situations in which we oftenest see things is spread out on the ground before us. We are incessantly drilled in making allowance for this perspective, and reducing things to their real form in spite of optical foreshortening. Hence if the preceding explanations are true, we ought to find this habit inveterate. The lower half of the retina, which habitually sees the farther half of things spread out on the ground, ought to have acquired a habit of enlarging its pictures by imagination, so as to make them more than equal to those which fall on the upper retinal surface ; and this habit ought to be hard to escape from, even when both halves of the object are equi-distant from the eye, as in a vertical line on paper. Delboeuf has found accordingly, that if we