Another defect of the eye, known as astigmatism, is due to the fact that the eye does not always have a perfectly spherical front (cornea). The curvature in one direction is different from that in others. For example, the vertical curvature may be more convex than the horizontal. Such a condition produces a serious defect of vision. It can be corrected by means of cylindrical lenses of the proper strength so placed before the eye as to correct the defect in curvature.

Still another defect of vision is known as presbyopia or farsightedness due to old age. It has the following explanation: In early life, when we look at near objects, the crystalline lens automatically becomes thicker, more convex. This adjustment brings the rays to a focus on the retina, which is required for good vision. As we get old, the crystalline lens loses its power to change its adjustment for near objects, although the eye may see at a distance as well as ever. The old person, therefore, must wear convex glasses when looking at near objects, as in reading and sewing.

Another visual defect of a different nature is known as partial color blindness. The defects described above are due to misshapen eyes. Partial color blindness is due to a defect of the retina which makes it unable to be affected by light waves producing red and green. A person with this defect confuses red and green. While only a small percentage of the population has this defect, it is nevertheless very important that those having it be detected. People having the defect should not be allowed to enter occupations in which the seeing of red and green is important. It was recently brought to the author’s attention that a partially color-blind man was selling stamps in a post office. Since two denominations of stamps are distinguished by red and green colors, this man made frequent mistakes. He was doing one of the things for which he was specially unfitted. It is easy to detect color blindness by simple tests.

So great is the importance of good vision in school work and the later work of life, that every teacher should know how to make simple tests to determine visual defects. Children showing any symptoms of eyestrain should be required to have their visual defects corrected by a competent oculist, and should be warned not to have the correction made by a quack. There is great popular ignorance and even prejudice concerning visual defects, and it is very important that teachers have a clear understanding of the facts.

Defects of Hearing. Hearing defects are only about half as frequent as those of sight. They are nearly all due to catarrhal infection of the middle ear through the Eustachian tube. The careful and frequent medical examination of school children cannot, therefore, be too strongly emphasized. The deafness or partial deafness that comes from this catarrhal infection can seldom be cured; it must be prevented by the early treatment of the troubles which cause it.

Summary. The mind is closely related to the body. Especially is it dependent upon the brain, nerves, and sense organs. The sense organs are special adaptations of the nerve-ends for receiving impressions. Each sense organ receives only its particular type of impression.

The main visual phenomena are those of color-mixture, after-images, adaptation, and contrast. Since sensation is the basis of mental life, defects of the sense organs are serious handicaps and should be corrected if possible. Visual defects are usually due to a misshapen eyeball and can be corrected by proper glasses, which should be fitted by an oculist. Hearing defects usually arise from catarrhal trouble in the middle ear.

CLASS EXERCISES

  1. Make a study of the relation of the mind to the body. Enumerate the different lines of evidence which you may find indicating their close relationship.
  2. Can you find any evidence tending to show that the mind is independent of the body?
  3. Color-Mixture. Colored disks can be procured from C. H. Stoelting Company, Chicago. If a small motor is available, the disks can be rotated on the motor and the colors mixed. Mix pairs of complementary colors, also pairs of non-complementary colors, and note the result. A simple device can be made for mixing colors, as follows: On a board stand a pane of glass. On one side of the glass put a colored paper and on the other side of the glass put a different color. By looking through the glass you can see one color through transmitted light and the other color through reflected light. By inclining the glass at different angles you can get different proportions of the mixture, now more of one color, now more of the other.
  4. Negative After-Images. Cut out pieces of colored paper a half inch square. Put one of these on a white background on the table. With elbows on the table, hold the head in the hands and gaze at the colored paper for about a half-minute, then blow the paper away and continue to gaze at the white background. Note the color that appears. Use different colors and tabulate the results. Try projecting the after-images at different distances. Project the after-images on different colored papers. Do the after-images mix with the colors of the papers?
  5. An interesting experiment with positive after-images can be performed as follows: Shut yourself in a dark closet for fifteen or twenty minutes to remove all trace of stimulation of the retina. With the eyes covered with several folds of thick black cloth go to a window, uncover the eyes and take a momentary look at the landscape, immediately covering the eyes again. The landscape will appear as a positive after-image, with the positive colors and lights and shades. The experiment is best performed on a bright day.
  6. Adaptation. Put on colored glasses or hold before the eyes a large piece of colored glass. Note that at first everything takes on the color of the glass. What change comes over objects after the glasses have been worn for fifteen or twenty minutes? Describe your experience after removing the glasses. Plan and perform other experiments showing adaptation. For illustration, go from a very bright room into a dark room. Go from a very dark room to a light one. Describe your experience.
  7. Contrast. Take a medium gray paper and lay it on white and various shades of gray and black paper. Describe and explain what you find.
  8. Color Contrast. Darken a room by covering all the windows except one window pane. Cover it with cardboard. In the cardboard cut two windows six inches long and one inch wide. Over one window put colored glass or any other colored material through which some light will pass. By holding up a pencil you can cast two shadows on a piece of paper. What color are the shadows? One is a contrast color induced by the other; which one? Explain the results.
  9. Make a study of the way in which women dress. What do you learn about color effects?
  10. From the Stoelting Company you can obtain the Holmgren worsteds for studying color blindness.
  11. Defective Vision. Procure a Snellen’s test chart and determine the visual acuity of the members of the class. Seat the subject twenty feet 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.”
  16. By pricking the skin with the point of a needle, you can stimulate the “pain spots.”
  17. The sense of taste is sensitive only to solutions that are sweet, sour, salt, or bitter. Plan experiments to verify this point. What we call the “taste” of many things is due chiefly to odor. Therefore in experiments with taste, the nostrils should be stopped up with cotton. It will be found, for example, that quinine and coffee are indistinguishable if their odors be eliminated by stopping the nose. The student should compare the taste of many substances put into the mouth with the nostrils open with the taste of the same substances with the nostrils closed.

REFERENCES FOR CLASS READING