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.

  [Illustration:  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.

    [Illustration: 
      * * * * * * *
      A B C
    ]

For the same reason, of two squares absolutely identical in size, one marked with alternately clear and dark cross-bands, and the other with alternately clear and dark upright markings, the former will appear broader and the latter higher than the other.

Experiment 183.  Make on a white card two squares of equal size.  Across the one draw horizontal lines at equal distances, and in the other make similar vertical lines.  Hold them at some distance.  The one with horizontal lines appears higher than it really is, while the one with vertical lines appears broader, i.e., both appear oblong.

  Experiment 184.  Look at the row of letters (S) and figures (8).  To

    [Illustration: 
      S S S S S S S S 8 8 8 8 8 8 8 8
    ]

some the upper halves of the letters and figures may appear to be of the same size as the lower halves, to others the lower halves may appear larger.  Hold the figure upside down, and observe that there is a considerable difference between the two, the lower halves being considerably larger.

Experiment 185. To illustrate imperfect visual judgment.  The length of a line appears to vary according to the angle and direction of certain other lines in relation to it (Fig. 147).  The length of the two vertical lines is the same, yet B appears much longer than A.

[Illustration:  Fig. 147.—­To show False Estimate of Size.