in. Fasten a long piece of
thread to it, so it can be pulled down in a moment. Gaze steadily at the
red paper. Have it removed while looking at it intently, and a greenish
spot takes its place.

Experiment 206. Lay on different parts of the skin a small, square
piece of paper with a small central hole in it. Let the person close his
eyes, while another person gently touches the uncovered piece of skin
with cotton wool, or brings near it a hot body. In each case ask the
observed person to distinguish between them. He will always succeed on
the volar side of the hand, but occasionally fail on the dorsal surface
of the hand, the extensor surface of the arm, and very frequently on the
skin of the back.

Experiment 207. _Wheatstone's fluttering hearts_. Make a drawing of
a red heart on a bright blue ground. In a dark room lighted by a candle
hold the picture below the level of the eyes and give it a gentle
to-and-fro motion. On continuing to look at the heart it will appear to
move or flutter over the blue background.

Experiment 208. At a distance of six inches from the eyes hold a
veil or thin gauze in front of some printed matter placed at a distance
of about two feet. Close one eye, and with the other we soon see either
the letters distinctly or the fine threads of the veil, but we cannot
see both equally distinct at the same time. The eye, therefore, can form
a distinct image of a near or distant object, but not of both at the
same time; hence the necessity for accommodation.

Experiment 209. Place a person in front of a bright light opposite
a window, and let him look at the light; or place one's self opposite a
well-illuminated mirror. Close one eye with the hand and observe the
diameter of the other pupil. Then suddenly remove the hand from the
closed eye: light falls upon it; at the same time the pupil of the other
eye contracts.

Experiment 210. _To illustrate the blind spot. Marriott's
experiment_. On a white card make a cross and a large dot, either black
or colored. Hold the card vertically about ten inches from the right
eye, the left being closed. Look steadily at the cross with the right
eye, when both the cross and the circle will be seen. Gradually approach
the card toward the eye, keeping the axis of vision fixed on the cross.
At a certain distance the circle will disappear, i.e., when its image
falls on the entrance of the opti