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 optic nerve.  On bringing the card nearer, the circle reappears, the cross, of course, being visible all the time (see Experiment 180, p. 355).

Experiment 211. To map out the field of vision.  A crude method is to place the person with his back to a window, ask him to close one eye, stand in front of him about two feet distant, hold up the forefingers of both hands in front of and in the plane of your own face.  Ask the person to look steadily at your nose, and as he does so observe to what extent the fingers can be separated horizontally, vertically, and in oblique directions before they disappear from his field of vision.

Experiment 212. To illustrate imperfect judgment of distance.  Close one eye and hold the left forefinger vertically in front of the other eye, at arm’s length, and try to strike it with the right forefinger.

On the first trial one will probably fall short of the mark, and fail to touch it.  Close one eye, and rapidly try to dip a pen into an inkstand, or put a finger into the mouth of a bottle placed at a convenient distance.  In both cases one will not succeed at first.

  In these cases one loses the impressions produced by the convergence of
  the optic axes, which are important factors in judging of distance.