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 gla.s.ses, which should be fitted by an oculist. Hearing defects usually arise from catarrhal trouble in the middle ear.

CLa.s.s 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 gla.s.s. On one side of the gla.s.s put a colored paper and on the other side of the gla.s.s put a different color. By looking through the gla.s.s you can see one color through transmitted light and the other color through reflected light. By inclining the gla.s.s 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 gla.s.ses or hold before the eyes a large piece of colored gla.s.s. Note that at first everything takes on the color of the gla.s.s. What change comes over objects after the gla.s.ses have been worn for fifteen or twenty minutes? Describe your experience after removing the gla.s.ses. Plan and perform other experiments showing adaptation. For ill.u.s.tration, 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 gla.s.s or any other colored material through which some light will pa.s.s.

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 cla.s.s. 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 cla.s.s 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 p.r.i.c.king 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 CLa.s.s READING

COLVIN AND BAGLEY: _Human Behavior_, Chapters VII and XII.

MuNSTERBERG: _Psychology, General and Applied_, Chapters III, IV, VI, and VII.

PILLSBURY: _Essentials of Psychology_, Chapters II, III, and IV.

PYLE: _The Outlines of Educational Psychology_, Chapter II.

t.i.tCHENER: _A Beginner's Psychology_, Chapter I, par. 3; also Chapter II.

CHAPTER IV

INHERITED TENDENCIES

=Stimulus and Response.= We have learned something about the sense organs and their functions. We have seen that it is through the sense organs that the world affects us, stimulates us. And we have said that we are stimulated in order that we may respond.

We must now inquire into the nature of our responses. We are moving, active beings. But how do we move, how do we act when stimulated? Why do we do one thing rather than another? Why do we do one thing at one time and a different thing at another time?

Before we answer these questions it will be necessary for us to get a more definite and complete idea of the nature of stimulus and response.

We have already used these terms, but we must now give a more definite account of them. It was said in the preceding chapter that when a muscle contracts, it must first receive a nerve-impulse. Now, anything which starts this nerve-impulse is called the stimulus. The muscular movement which follows is, of course, the response. The nervous system forms the connection between the stimulus and response.

The stimulus which brings about a response may be very simple. Or, on the other hand, it may be very complex. If one blows upon the eyelids of a baby, the lids automatically close. The blowing is the stimulus and the closing of the lids is the response. Both stimulus and response are here very simple.

But sometimes the stimulus is more complex, not merely the simple excitation of one sense organ, but a complicated stimulation of an organ, or the simultaneous stimulation of several organs. In playing ball, the stimulus for the batter is the on-coming ball. The response is the stroke. This case is much more complex than the reflex closing of the eyelids. The ball may be pitched in many different ways and the response changes with these variations.

In piano playing, the stimulus is the notes written in their particular places on the staff. Not only must the position of the notes on the staff be taken into account, but also many other things, such as sharps and flats, and various characters which give directions as to the manner in which the music is to be played. The striking of the notes in the proper order, in the proper time, and with the proper force, is the response.

In typewriting, the stimulus is the copy, or the idea of what is to be written, and the response is the striking of the keys in the proper order. Speaking generally, we may say that the stimulus is the force or forces which excite the sense organs, and thereby, through the nervous system, bring about a muscular response.

This is the ordinary type of action, but we have already indicated a different type. In speaking of typewriting we said the stimulus might be either the copy or ideas. One can write from copy or dictation, in which the stimulus is the written or spoken word, but one can also write as one thinks of what one wishes to write. The latter is known as _centrally initiated action_. That is to say, the stimulus comes from within, in the brain, rather than from without.

Let us explain this kind of stimulation a little further. Suppose I am sitting in my chair reading. I finish a chapter and look at my watch. I notice that it is three o'clock, and recall that I was to meet a friend at that time. The stimulus in this case is in the brain itself; it is the nervous activity which corresponds to the idea of meeting my friend.

If we disregard the distinction between mind and body, we may say that the stimulus for a response may be an idea as well as a perception, the perception arising from the immediate stimulation of a sense organ, and the idea arising from an excitation of the brain not caused by an immediate stimulation of a sense organ.

=Instincts and Habits.= In human action it is evident that there is always a stimulus to start the nerve-impulse which causes the action. If we make inquiry concerning the connection between the stimulus and response; if we ask how it has come about that a particular stimulus causes a particular response rather than some other possible response, we find two kinds of causes. In one case the causal connection is established through heredity; in the other, the causal connection is established during a person's lifetime through training.

A chicken, for example, hides under some cover the first time it hears the cry of a hawk; it scratches the first time its feet touch sand or gravel; it pecks the first time it sees an insect near by. An infant closes its eyes the first time it feels cold wind blow upon them; it cries the first time it feels pain; it clasps its fingers together the first time a touch is felt inside them. The child's nervous system is so organized that, in each of the cases named, the stimulus brings forth the particular, definite response. These acts do not have to be learned.

But it is quite different in typewriting and piano playing. One _must learn_ what keys on the piano to strike in response to the various situations of the notes as written in the music. One must also learn the keys on the typewriter before he can operate a typewriter. And in the case of other habits, we find, for example, that one does not respond by saying "81" for 9 times 9; nor "13" for 6 plus 7; nor "8" for 15 minus 7; nor "8" for the square root of 64; nor "144" for the square of 12, etc., until one has learned in each case.