When everything is in position, the tension from the cardboard flattens the paper lens, while pulling the strings releases this tension and permits the lens to become more rounded. With this simple device the changes in the curvature of the lens for near and distant vision are easily shown.
CHAPTER XXIII - THE GENERAL PROBLEM OF KEEPING WELL
"To cure was the voice of the Past: to prevent is the divine whispering of To-day."
As stated in the introduction to our study, the fundamental law of hygiene is the law of harmony: Habits of living must harmonize with the plan of the body. Having acquainted ourselves with the plan of the body, we may now review briefly those conditions that help or hinder its various activities. The hygiene already presented in connection with the study of the various organs may be condensed into general rules, or laws, as follows:
1. Of exercise: Exercise daily the important groups of muscles.
2. Of form: Preserve the natural form of the body.
3. Of energy: Observe regular periods of rest and exercise and avoid exhaustion.
4. Of nutriment: Eat moderately of a well-cooked and well-balanced diet and drink freely of pure water.