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.