BLIND SPOT. The place of entry of the optic nerve in each retina is insensitive to light. Mariotte in 1668 first demonstrated the existence of the blind spot. Its existence may be easily shown as follows: Pin a large sheet of paper against the wall so that a cross marked thereon is:) may be at the level of the eyes; fix the sition of the head by means of a headrest a ruler about 50 centimetres in length, held by e teeth at one end, the other end resting against the i paper, is a convenient headrest) ; with one eye closed or covered, look steadily at the cross with the other eye; move a pencil, that has been covered with paper so that the point only shows black, from the cross toward the temporal side of the field of vision; mark on the paper the position at which the pencil point disappears; move the pencil farther and mark the Rosition at which the point reappears. By moving the point in various directions near this place on the paper, and marking the positions where the pencil point disappears, and reappears, a series of marks may be made which furnish an outline figure of the form of the blind spot. The di ameter of the blind spot (1.5 mm.) corresponds to a visual angle varying from 39' to 9° 47' .
The average is about 6'. An image of light sufficiently small, thrown upon the optic nerve by means of the ophthalmoscope, gives rise to no sensations. These experiments show that at
the blind spot we see nothing, yet, as we look at this page with one eye only the surface ap pears to be covered with letters in the regular forms i there is no blank space corresponding to the blind spot. In binocular vision the blind spot of one retina is covered with a sensitive portion of the other retina. Why we should not be aware of our inability to see a continuous field with one eye is a problem for which there are two proposed explanations. The blind spot may be filled out by association, whose nature is determined by the character of the surround ing field, or by eye movements which serve as retinal local signs for the insensitive region. Probably the two processes are necessary and aid each other in presenting to the mind the continuous visual field. Consult Ladd, 'Ele ments of Physiological Psychology' ; Helmholtz, H. Von, 'Physiologische Optik) (Hamburg 1911) ; Sandford, 'Course in Experimental Psychology' (ex. 113, 114) ; Wundt, logische Psychologie) (Leipzig 1910); Titche ner, E. B., 'Experimental (New York 1901).