from the chart, which should be placed in a good light. While testing
one eye, cover the other with a piece of cardboard. Above each row of
letters on the chart is a number which indicates the distance at which
it can be read by a normal eye. If the subject can read only the
thirty-foot line, his vision is said to be 20/30; if only the forty-foot
line, the vision is 20/40. If the subject can read above the twenty-foot
line and complains of headache from reading, farsightedness is
indicated. If the subject cannot read up to the twenty-foot line,
nearsightedness or astigmatism is indicated.

12. _Hearing._ By consultation with the teacher of physics, plan an
experiment to show that the pitch of tones depends on vibration
frequency. Such an experiment can be very simply performed by rotating a
wheel having spokes. Hold a light stick against the spokes so that it
strikes each spoke. If the wheel is rotated so as to give twenty or
thirty strokes a second, a very low tone will be heard. By rotating the
wheel faster you get a higher tone. Other similar experiments can be
performed.

13. Acuity of hearing can be tested by finding the distance at which the
various members of the class can hear a watch-tick. The teacher can plan
an experiment using whispering instead of the watch-tick. (See the
author's _Examination of School Children_.)

14. By using the point of a nail, one can find the "cold spots" on the
skin. Warm the nail to about 40 degrees Centigrade and you can find the
"warm spots."

15. By touching the hairs on the back of the hand, you can stimulate the
"pressure spots."