c 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.

Experiment 213. Hold a pencil vertically about twelve inches from
the nose, fix it with both eyes, close the left eye, and then hold the
right index finger vertically, so as to cover the lower part of the
pencil. With a sudden move, try to strike the pencil with the finger. In
every case one misses the pencil and sweeps to the right of it.

Experiment 214. _To illustrate imperfect judgment of direction_. As
the retina is spherical, a line beyond a certain length when looked at
always shows an appreciable curvature.

Hold a straight edge just below the level of the eyes. Its upper margin
shows a slight concavity.

Surface Anatomy and Landmarks.

In all of our leading medical colleges the students are carefully and
thoroughly drilled on a study of certain persons selected as models. The
object is to master by observation and manipulation the details of what is
known as surface anatomy and landmarks. Now while detailed work of this
kind is not necessary in secondary schools, yet a limited amount of study
along these lines is deeply interesting and profitable. The habit of
looking at the living body with anatomical eyes and with