BLIND SPOT. A roundish oval spot in each retina, which is blind. It corresponds to the place of entry of the optic nerve. The spot is some 1.8 millimeters in width, i.e. subtends an angle of some 6° (the limits given by Helmholtz are 18° 55' and 12° 25'). That it is totally blind, whether to brightness or color. was first proved by the French physicist Mariotte in 1668. and is easily demonstrable. Its form differs somewhat in different eyes. With careful obser vation, it can be shown to possess certain irregu lar prolongations, which represent the course of the large blood-vessels that enter the eye along with the nerve.
A striking experiment may be performed as follows: Close or bandage the right eye. and with the left gaze steadily at a point upon a wall or screen some 7 feet distant. Let another person seat himself directly before the wall, on your left-hand side, in such a position that his forehead is on a level with the point of gaze, and his nearer cheek some 50 centimeters distant.
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 on the wall, and noting the points at which a colored pencil, moved to and fro by an experimenter. disappears and reappears. you can make an accurate map of the blind spot. including the vascular pro longations mentioned above.
This experiment raises a difficulty. fail to see the face, and yet you see something, i.e. the unbroken 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 surprising, if it were gained in monocular vision. but under 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? Let us look at the experimental data: (1) There seems to he no doubt that the spatial value of the blind spot, in visual perception, 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 indifferent 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 paper 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. Ion see a solid disk, of the color of the ring. (3) lf the surface is checkered or variously col ored, there is ( for practiced observers) uo tilling 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 you 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 spot 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 stimulus. We must rather suppose that the local signs attaching to retinal points along the edge of the spot are extensible signs, carrying with them the sense-values and space-valnes 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 (t?mard the centre of the spot) is limited by the structure and function of the rest of the retina. If this ex planation appear far-fetched. we may have re course (2) to a central theory; that is to say, we may assume that the blind spot is tilled out by 'association' or 'imagination,' by centrally aroused sensations (see SENSATION ) , while the periphery is and remains blind. There is no in herent difficulty in this view, and it accords well with the fact that the unpracticed observer sees the spot as filled, while the more practiced ob server is aware that he does not see with it at all. We nmst 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, suspect that their development has been influ enced, in each eye, by the normal space-values of the corresponding area of the other eye.