s 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