This section is from the book "The Human Body: An Elementary Text-Book Of Anatomy, Physiology, And Hygiene", by H. Newell Martin. Also available from Amazon: The Human Body.
In the normal eye the range of accommodation is very great, allowing light from objects infinitely distant up to that proceeding from those only about eight inches in front of the eye to be brought to a focus on the retina. In the natural healthy eye parallel rays of light meet on the retina when the muscles controlling the crystalline lens are at rest and the lens is at its flattest (A, Fig. 94). Such eyes are emmetropic. In other eyes the eyeball is too long from before back; in the resting state parallel rays meet in front of the retina (B). Persons with such eyes cannot see distant objects distinctly without the aid of diverging (concave) spectacles; they are short-sighted or myopic. Or the eyeball may be too short from before back; then, in its resting state, parallel rays are brought to a focus behind the retina (C). To see even distant objects, such persons must therefore use muscular effort to increase the converging power of the lens; and when objects are near they cannot, with the greatest effort, bring the rays proceeding from them to a focus soon enough. To get distinct retinal images of near objects, they therefore need converging (convex) spectacles. Such eyes are called hypermetropic, or in common language long-sighted.
What is meant by the "accommodation" of the eyeball?
Where do parallel rays of light which have entered a healthy eye meet in a focus? Where do such rays meet when the eyeball is too long from back to front? What is the result as regards vision? What form of spectacle lenses do short-sighted persons require? Explain what is meant by a hypermetropic or long-sighted eye. What sort of spectacles do long-sighted persons require ?