The eye is one of the most complex and, at the same time, one of the most efficient developments of creation. It encloses all optical regularities, which the scientists and the inventors of the first cameras and telescopes had to work out with difficulties through generations.
This confirms that nature once again is a better and quicker inventor than man. The eye consists of a ball-shaped hull built in layers. Therefore the latin name for the eyeball: Bulbus Oculi. The so-called leather skin is the outer layer.
A tough protection which marks off the eye against the surrounding tissue, except the the area of the pupil. So the eye is a closed system, only connected with the body by the optic nerve and its blood vessels. As in a camera, the objective, the whole optical system, is placed before the vision level. In the eye this is the light intercepting cornea, the iris, and last not least the lens.
To be able to see perfectly, two things have to be right: the brightness and the focus. The iris is responsible for the regulation of the brightness. It is placed directly in front of the lens and is connected with tiny muscles. When it gets bright outside, the iris contracts and lets in less light. Similarly, when the brightness is reduced, the diameter of the iris enlarges. A truly intelligent system comparable with the automatic shutter of modern cameras, only that the eye performs a lot better.
The retina lens is responsible for focus. The light is deflected by passing the transparent cornea, and the lens turns the incoming beams into a sharp image.Depending if there is an object close to the eye or a view into the distance, the lens changes its power of interception exactly so that the beams get focused onto the retina.
The lens itself is soft and flexible and that makes for the power of vision. The more it arches and so becomes more ball-like, the more the light gets focused like with a strong optical glass.The more it flattens, the less the light's way is changed. Again this is made possible by muscles which pull more or less strongly at the edges of the lens and so get it into the desired shape.
The comparison with a camera can be extended to the topic of care too. Who doesn't regularly clean the objective of his camera, eventually will see nothing. The same with the eye: in the rear section, between lens and iris, constantly a nutrition and cleaning fluid is produced, which, among other things, provides for the lens. A delicate balance is needed: there is always only that quantity of fluid produced, which can flow off the front section. The best camera is useless without an appropriate film. This, in the eye, is the retina. Between it and the leather skin there are minuscule veins along the so-called vein skin, which transport oxygen and nutriments even to the furthest corners. Before the light reaches the retina, it has to pass first through the “glass body", a gelatinous liquid which occupies the biggest space inside the eye.
The retina transforms the incoming light impulses into nerve signals and leads them along the optic nerve to the brain.But the retina is capable of much more. It can emphasize movements of things or enhance contrasts. This work is done by about 130 million optic cells. Most of them are tightly packed at the so-called “Macula", the centre of focused vision. Only when the images from the outside arrive here focused, and are properly transformed into nerve signals, is it possible for us to see.
The retina is no more than half a millimeter thick and consists of nine layers.
A gigantic power of vision in the smallest space.
Many people suffering from sight defects wish to be able to see well without their “optical crutches". For many decades, glasses and contact lenses were the only means to compensate sight defects but, for some years now, there is a new possibility besides these instruments to correct impaired vision: the so-called Refractive Surgery. It is the surgical correction of short or farsightedness or astigmatism. The aim of this operation is to change the eye's power of interception in a way that seeing properly without glasses or contact lenses becomes possible again. The correction of sight defects is done at the cornea or, inside the eye, at the lens.
There are four different kinds of sight defects.
- Shortsightedness ( Myopie ) - Farsightedness ( Hyperopie )
- Astygmatism ( Astigmatismus ) - Old age sight defects ( Presbyopie )
The short-sighted person sees well nearby, but badly at a distance. The cause of this interception fault is either a too long eyeball or a too great power of interception of the cornea. The focus of the image isn't exactly on the retina but in front of it. The afflicted squeezes his eyes when looking into the distance. He meets acquaintances without recognizing them. Shortsightedness is mostly corrected by a dispersive lens (glasses or contact lenses).
The far-sighted person has either a too short eyeball or not enough interception power of the cornea. The focus of images lies behind the retina. If he still sees focussed at close range, then only with great effort. This often leads to headaches and fatigue, usually in the evening or after work. Far-sightedness is corrected by a corrective lens. Often the curve of the cornea is cause of unfocussed vision. Its surface is not, as it is supposed to, ball-shaped, but egg-shaped. Horizontal, vertical or diagonal lines cannot be seen clearly.
Astigmatism can be corrected by special lenses whose curvature balance the cornea's curve.
Old age sight defects is a natural development of vision, which occurs for everybody from the fourth decade onwards.The eye's lens loses its elasticity, which reduces its ability to adapt to different distances. This is corrected by reading glasses, bifocal or multifocal glasses. The latter give the ability to see focused in medium distances as well. Nowadays one can obtain multifocal glasses at the opthalmic optician's. These are always individually adapted, but there are restrictions which the opthalmic optician has to observe. Unfortunately even in our age of progress people who have an astigmatism of more than one diopter and at the same time suffer from old age sight defects still cannot be helped as efficiently as desired.
- UNFOCUSED VISION AND ITS TREATMENT.
In general corrections of sight defects are done by changing the eye's power of interception. The focus of the incoming light beams is corrected so that it lies again on the retina. Glasses are placed in front of the eye as an additional lens. Contact lenses swim on the cornea. In the surgical correction the eye's power of interception is changed. The surgeon can do that by working on the cornea's curvature or by changing the lens power of interception so, that the light beams give a focused image onto the retina. These surgical methods are known as REFRACTIVE CORNEA OR LENS SURGERY. If you have any further questions I, Georg Seidel, opthalmic optician, am at your disposal.
Furthermore I highly recommend the optometrist here in Sta. Eulalia, Mrs. Petra Rohde, because foresight is better than hindsight.