When the eye is too long from before backwards, or the lens is too curved, it can readily be seen from the drawings that distant objects are brought to a focus in front of the retina instead of directly upon it. Thus, short-sighted people see distant objects in a blurred, indistinct way. Accommodation is a great effort, and the strain is considerable unless suitable glasses are obtained to correct the error of refraction. The glasses ought to be concave, made so that the glass is thicker at the edges than at the centre. Now, this glass counteracts the effect of the too convex eye, so that rays of light are focussed not in front of the retina but directly upon it.
It is tremendously important to get glasses to exactly counteract the error of refraction in near sight, and no person should ever attempt this without the prescription of a competent oculist. The very least exaggeration of the lens of the glass will convert the natural short sight into the opposite condition, and the poor eye would be strained in two directions. The great danger in short sight is that it increases, and neglected myopia may lead to blindness. All cases of myopia in children should be under the direct care of an oculist, as, when it is not properly attended to, short sight is progressive and sight may be lost in one or both eyes.
This is the opposite condition. The lens is too flat, the eyeball too short from front to back, and objects are focussed behind the retina (see diagram). In these cases, in reading a book or newspaper it has to be held at a considerable distance from the eye in order that the printed matter may be focussed. This kind of eye is requiring constant muscular effort to focus, with the result that there is pain and fatigue after reading or writing, and often headache as well. Doubly convex glasses, to increase the focussing power of the eye, will put the matter right.
This is the condition when accommodation is difficult or painful, and it may or may not be due to some error of refraction. Sometimes the muscles of the accommodative apparatus are disorganised, perhaps, from inferior health, and most people know that after serious illness their eyesight is affected for a time. In all cases of weak sight the eyes should be tested, and glasses obtained, if necessary. Astigmatism, for example, is a cause of poor vision, which can be corrected by glasses. In such cases there is an irregularity of the surface of the cornea or the lens, either from above downwards, or from left to right, and the eye cannot see horizontal and vertical lines with equal clearness at the same time. Even when glasses are not necessary the following rules should be followed by anyone whose sight can be characterised as "weak."
Long sight. The eye really is too short; the light rays do not meet before striking the retina
The rays of light (A) enter the eye, and are focussed on the retina by the lens
Short sight. In this case the eye is too long from front to back, or the lens is too curved ways read in sufficient light so that it falls upon the work from above and behind. The eyes should never be used at twilight. Flickering gas-jets and lamps are extremely injurious to the eyes. If the eyes are tro uble-some, it is best to use them only in daylight.
Never read whilst travelling in 'bus, train, carriage, or any vehicle which causes vibration and makes it difficult to keep the eyes fixed on the type. The business man who devours his evening paper in a poorly lighted railway carriage or 'bus on his way from the city, is tempting Providence in respect of his eyesight. The woman who does fine sewing and intricate embroidery for hours at a time may produce work which satisfies her sense of art and utility, but her eyesight is more valuable than anything she will gain.
Reading in bed is a dangerous practice, because the eyes cannot accommodate so well when the head is lying down, and the people who cannot do without their half hour's mental dissipation at ii p.m. should read propped up by pillows with a good light coming over the left shoulder from behind.
In illness or convalescence the eyes should be used sparingly, as the muscles are lowered in tone and cannot be overworked without danger. Those who are troubled with weak sight will find the condition very much improved if they will attend to their general health. The eyes, with many people, supply a health indicator. When they are well and in good condition, their sight gives them no t r o u b 1 e . When they get run down, eye-strain, headache, and fatigue very quickly appear.
Anyone with weak sight should rest the eyes as much as possible and get plenty of sleep. It is only during sleep that the eyes are in a state of per-feet repose. When working, rest the eyes every now and again deliberately. Look away from reading, writing, or sewing for a time into the distance. Better still, close the eyes, and, when they feel hot, painful, and tired, accept it as a warning that work should be stopped.
Lastly, let no considerations of economy, laziness, or pressure of work prevent you from consulting an oculist if your eyes seem weak, easily tired, painful after use. The "stitch in time" policy is specially applicable to the eyesight, as eye trouble is progressive, and neglect may have serious results.