Page:Mind (Old Series) Volume 12.djvu/348

 THE PERCEPTION OF SPACE, (ill.) 335 being impressed, both eyes see their object in the same direction, and the two objects consequently coalesce into one. The same thing may be shown in still another way. With fixed head converge the eyes upon some conspicuous objective point behind a pane of glass ; then close either eye alternately and make a little ink-mark on the glass ' covering ' the object as seen by the eye which is momen- tarily open. On looking now with both eyes the ink-marks will seem single, and in the same direction as the objective point. Conversely let the eyes converge on a single ink-spot on the glass, and then by alternate shutting of them let it be noted what objects behind the glass the spot covers to the right and left eye respectively. Now with both eyes open, both these objects and the spot will appear in the same place, one or other of the three becoming more distinct according to the fluctuations of retinal attention. 1 Now what is the direction of this common place ? The only way of defining the direction of an object is by pointing to it. Most people, if asked to look at an object over the horizontal edge of a sheet of paper which conceals their hand and arm, and then to point their finger at it, raising the hand gradually so that at last a finger-tip will appear above the sheet of paper, are found to place the finger not between either eye and the object, but between the latter and the root of the nose, and this, whether both eyes or either alone be used. Hering and Helmholtz express this by saying that we judge of the direction of objects as they would appear to an imaginary cyclopean eye, situated between our two real eyes, and with its optical axis bisect- ing the angle of convergence of the latter. Our two retinae act, according to Hering, as if they were superposed in the place of this imaginary double-eye ; we see by the corres- ponding points of each, situated far asunder as they really are, just as we should see if they were superposed and could both be excited together. either be parallel or converge upon an imaginary point some distance behind the plane of the pictures, according to the size and distance apart of the pictures. The accommodation, however, has to be made for the plane of the pictures itself, and a near accommodation with a far-off convergence is something that the ordinary use of our eyes never teaches us to effect. 1 These two observations prove the law of identical direction only for objects which excite the fovere or lie in the line of direct looking. Ob- servers skilled in indirect vision can, however, more or less easily verify the law for outlying retinal points.