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.
The healthy eye is so constructed that when its muscles are at rest distinct images of distant objects are focussed on the retina. To see near objects muscular effort is required; hence the greater fatigue which follows long gazing at them.
In a hypermetropic eye more effort is needed to see near objects, and this results in muscular fatigue. Hypermetropic persons can often read well for a while, but then complain that they can no longer see distinctly. This kind of weak sight should always lead to examination of the eyes by an oculist, to see if glasses are needed; otherwise severe neuralgic pains about the eyes are apt to come on, and the overstrained organ may be permanently injured.
Children sometimes have hypermetropic eyes, and in that case should be at once provided with suitable spectacles. In old age another kind of long-sightedness (presbyopia) is common: it is due to too great stiffness of the crystalline lens, which does not become convex enough during accommodation to focus on the retina the images of near objects.
Short-sighted eyes appear to be more common now than formerly, especially in those given to indoor pursuits. Myopia is rare among those who cannot read or who live mainly out of doors. It is not so apt to lead to permanent injury of the eye as hypermetropia, but the effort to see distinctly any but near objects is apt to produce headaches and other symptoms of nervous exhaustion. If the myopia becomes gradually worse, the eyes should be rested for several months.
Why is the eye apt to be fatigued by the continued contemplation of near objects ?
How does the hypermetropic eye differ from the normal in the above respect? What should be done at once if a child is found to have hypermetropic eyes? What is presbyopia?
In what classes of persons in myopia most frequent?