Fig. 145.
Experiment 179. To show the blind spot. The left eye being shut, let the right eye be fixed upon the cross as in Fig. 145. When the book is held at arm’s length, both cross and round spot will be visible; but if the book be brought to about 8 inches from the eye, the gaze being kept steadily upon the cross, the round spot will at first disappear, but as the book, is brought still nearer both cross and round spot will again be seen.
Experiment 180. To illustrate the duration of retinal impressions. On a circular white disk, about halfway between the center and circumference, fix a small, black, oblong disk, and rapidly rotate it by means of a rotating wheel. There appears a ring of gray on the black, showing that the impression on the retina lasts a certain time.
Fig. 146.—Optic Disks.
The disk A, having black and white sectors, when rotated rapidly gives an even gray tint as in B.
Experiment 181. Mark off a round piece of cardboard into black and white sectors as in A ([Fig. 146]). Attach it so as to rotate it rapidly, as on a sewing machine. An even gray tint will be produced as in B.
Experiment 182.To illustrate imperfect visual judgments. Make three round black dots, A, B, C, of the same size, in the same line, and let A and C be equidistant from B. Between A and B make several more dots of the same size. A and B will then appear to be farther apart than B and C.