BLIND SPOT. 179 BLIND SPOT. from it. If your gaze remains constant, his face will entirely disappear, and the wall will seem to extend continuously away from you on the left. By spreading a sheet of paper ou tho wall, and noting the points at which a colored pencil, moved to and fro hy an experimenter, disappears and reappears, you can make an accurate map of the lilind sjiot. including the vascular pro- longations mentioned above. This experiment raises a difficulty. You fail to see the face, and yet jou see something, i.e. the unbrol^en color of the wall. The blind spot is thus filled out with color sensation. Now in binocular vision this result might have been ex- pected. For the optic nerve enters the eyes on the nasal side of each retina : so that when we are using the two eyes together for purposes of visual perception, superposing (as it were) the one upon the other, and laying temporal half over nasal and nasal over temporal, the area that is blind in the one eye will correspond to an area in the other that is endowed with normal vision ; the two eyes supplement each other. Further, the result would not be sur])rising. if it were gained in monocular vision, but xmder the ordi- nary conditions of seeing: for the eye is intrin- sically a moving organ, and we instinctively turn the spot of clearest vision (see Eye) upon any object in the visual field that we wish to observe. But what of the result, in monocular vision, when we have taken especial pains to restrain the eyeball from movement? b a FBOJECTION OF BLI.VD SPOT OF BIOBT EYE. Cc, optic axis ; ba, arpa of optic nerve ; A B, area of blind spot an projected on the screen CAB. Let us look at the experimental data: (1) There seems to he no doubt that the spatial value of the blind spot, in visual [)prcpption, is entirely normal ; points seen on the inner and outer edges of the spot do not approximate or run together, as they would if the spot were wholly indifTerent for purposes of space-perception. Paste nine large letters, in three vertical columns of three letters each, upon a sheet of paper. Hold the paper at such a distance and in such a position before the eye that the central letter of the square falls within the blind spot, while the sur- rounding eight letters are still visible. You will find that the surrounding letters still form a square for perception: whereas, if the blind spot had no spatial function, the two letters to right and left of the central would approach each other, and the whole figure would take on an hour-glass form. (2) If the surface at which we are looking is uniformly colored, the blind spot is filled out by this color. This law was demonstrated by our first experiment. It may also be proved as follows: Paste on a sheet of jinper a colored ring, large enough to contain the blind spot within it. Let the ring be so wide that the spot slightly overlaps its inner edge. You see a solid disk, of the color of the ring. (3) If the surface is checkered or variously col- ored, there is (for practiced observers) no filling of the spot. Paste upon a sheet of paper a rec- tangular cross, the two vertical arms of red and the two horizontal of blue, with the four arms mitred at the centre. Hold the paper in such a way that the centre, with its two red and two- blue triangles, falls at the centre of the blind spot. At first you will think that j-ou see the two red or the two blue arms as a continuous band of red or blue ; with practice, you will be able to convince yourself that at the central point of the figure you really see nothing. The last of these three facts is intelligible enough: if the spoi is blind, it stands to reason that one does not see with it. The first two facts, however, require explanation. We may ( 1 ) attempt a peripheral theory of the phenom- ena : that is to say, we may assume that the retinal sensations set up in the near neighbor- hood of the blind spot somehow 'irradiate' over the spot itself. The 'irradiation can hardly be thought of as physical, an irradiation of the stinuilus. We must rather suppose that the local signs attaching to retinal ppints along the edge of the spot are e.tensil)le signs, carrying with them the sense-values and space-values of the whole blind area. When one is required to localize a pressure upon the skin of the back, one may make a mistake of as much as 5 centi- meters in any direction; the local sign is extend- ed so far in all directions. The local signs of the particular ring of retinal tissue under considera- tion would be similarly extended, only that the direction of their extension (toward the centre of the spot) is limited by the structure and functi(m of the rest of the retina. If this ex- planation appear farfetched, we may have re- course (i) to a central theory; that is to say, we may assume that the blind spot is filled out by 'association' or 'imagination,' by centrally aroused sensations (see Sensation), while the periphery is and remains blind. There is no in- herent difliculty in this view, and it accords well with the fact that the unpracticed observer sec3 the spot as filled, while the more practiced ob- server is aware that lie does not see with it at all. We must then suppose that the local signs of the rim of the spot have taken shape under the same general laws that have conditioned vis- ual local signs at large; and we may, perhaps.
