It is a remarkable fact that every object we see has its picture formed upon the back wall of our eyes. The eye is a darkened chamber, and the whole of the front part of it acts as a lens to bring the rays of light coming from objects we wish to see to a focus on its back wall, thus forming a picture there as distinct as the picture formed in the camera obscura of the photographer. This has not only been proved by the laws of optics, but has been actually demonstrated in the eyes of rabbits and other animals. Experimenters have held an object before the eye of a rabbit for a few moments, and have then killed the animal and removed the eye as quickly as possible, and laid its back wall bare, and have distinctly seen there the picture of the object upon which the eye had been fixed. It is a truly wonderful fact that these pictures upon the back wall of the eye can be changed so rapidly that the picture of the object last looked at disappears in an instant and makes way for the picture of the next. We know that the picture formed on the back wall of the eye is carried back to the brain by the optic nerve, but there our knowledge stops. Science cannot tell us how the brain, and through it the mind, completes the act of seeing. It is there that the finite and the infinite touch, and, as our minds are finite, we cannot comprehend the infinite.
But there is enough that we can understand, and it shall be my endeavor in this paper to make some plain statements that will help as a guide in the preservation of those wonderful and useful organs.
FAR AND NEAR SIGHTEDNESS.
We have to use our eyes for near and far distant vision. In gathering pictures of distant objects the normally shaped eye puts forth little or no effort. It is the near work, such as reading, sewing, or drawing, that puts a real muscular strain upon the eyes. There are certain rules that apply to the use of the eyes for such near work regardless of the age of the person.
READING.
1. In reading, a book or newspaper should be held at a distance of from ten to fifteen inches from the eyes. It is hardly necessary to caution anybody not to hold the print further away than fifteen inches. The only objection to holding ordinary print too far away is that in so doing the pictures formed on the back wall of the eye are too small to be readily and easily perceived, and the close attention consequently necessary causes both the eyes and the brain to tire. Most persons quickly find this out themselves, and the tendency is rather to hold the book too near, for the nearer the object to the eye, the larger its picture upon the retina, or back eye wall. But here we encounter another danger. The nearer the object the eyes are concentrated upon, the greater the muscular effort necessary; so that by holding the book too near, the labor of reading is greatly increased, and the long persistence in such a habit is likely to produce weak eyes, and may, in some instances, lead to real near-sightedness. When children are observed to have acquired this habit and cannot be persuaded out of it, they should always be taken to a physician skilled in the treatment of the eye for examination and advice. A little attention at such a time may save them from a whole lifetime of trouble with their eyes. Of course, the larger the print, the farther it may be held from the eyes.
POSITION.
2. The position of the person with regard to the light should be so that the latter will fall upon the page he is reading, and not upon his eyes. It is generally considered most convenient to have the light shine over the left shoulder, so that in turning the leaves of the book, the shadow of the hand upon the page is avoided. It is not always possible to do this, however, and, at the same time, to get plenty of light upon the page. When one finds himself compelled to face the light in reading, or in standing at a desk bookkeeping, he should always contrive to shade his eyes from a direct light. This may be done with a large eye shade projecting from the brow. A friend of mine, a physician, is very fond of reading by a kerosene lamp, the lamp being placed on a table by his side, and the direct light kept from his eyes by means of a piece of cardboard stuck up by the lamp chimney.
PROPER LIGHT.
3. The illumination should always be sufficient. Nothing is more injurious to the eyes than reading by a poor light. Many persons strain their eyes by reading on into the twilight as long as they possibly can. They become interested and do not like to leave off. Some read in the evening at too great a distance from the source of light, forgetting that the quantity of light diminishes as the square of the distance from the source of light increases. Thus, at four feet, one gets only one-sixteenth part of the light upon his page that he would at one foot. It is the duty of parents and others who have charge of children to see to it that they do not injure their eyes by reading by insufficient light, either daylight or artificial light. There is a common notion that electric light is bad for the eyes. The only foundation I can think of for such a notion is that it is trying to the eyes to gaze directly at the bright electric light. It is bad to gaze long at any source of light, and the brighter the source of light gazed at, the worse for the eyes, the sun being the worst of all. I have seen more than one person whose eyes were permanently injured by gazing at the sun, during an eclipse or otherwise. As a matter of fact, nothing short of sunlight is better than the incandescent electric light to read by or to work by.