HISTORY OF THE CORRECTIVE LENS

And how they correct vision

11/16/2012

We generally take for granted one of the world’s most important inventions – spectacles. Imagine what life would be like not being able to see images clearly or sharply.

The early history of image magnification is shrouded with mystery. In ancient times, someone noticed that convex-shaped glass magnified images. Around the year 1000 A.D. crude technology began to develop reading stones which were nothing more than simple magnifiers. English Franciscan Friar Roger Bacon discovered that letters could be seen better and larger when viewed through less than half a sphere of glass. Bacon's experiments confirmed the principle of the converging lens. Bacon recognized that this could assist weak eyes or the vision of aged persons. Early recorded evidence demonstrates that glasses first appeared in Pisa, Italy about the year 1286. Each crude lens or crystal was surrounded by a frame and given a handle. They were worn on the face like glasses you see today but held in front of the eyes while reading. Essentially someone took two existing mounted stones and connected them with a rivet. Benjamin Franklin in the 1780's developed the bifocal. Later he wrote, "I therefore had formerly two pairs of spectacles, which I shifted occasionally, as in traveling I sometimes read, and often wanted to regard the prospects. Finding this change troublesome, and not always sufficiently ready, I had the glasses cut and a half of each kind associated in the same circle. By this means, as I wear my own spectacles constantly, I have only to move my eyes up or down, as I want to see distinctly far or near, the proper glasses being always ready." Evidently the idea of bifocals had already been experimented with in London as early as 1760 though never used extensively.

Bifocals progressed little in the first half of the 19th century. The terms bifocal and trifocal were introduced in London by John Isaac Hawkins, whose trifocals were patented in 1827. In 1884 B. M. Hanna was granted patents on two forms of bifocals which become commercially standardized as the "cemented" and "perfection" bifocals. Both had the serious faults of ugly appearance, fragility, and dirt-collection at the dividing line. At the end of the 19th century the two sections of the lens were fused instead of cemented, an idea originated by de Wecker in Paris and patented in 1908 by Borsch. At the turn of the 20th century, there was a considerable increase in the use of bifocals.
Nearsightedness or myopia is a common vision condition in which you can see objects near to you clearly, but objects farther away are blurry. The degree of your nearsightedness affects your ability to focus on distant objects. People with severe nearsightedness can see clearly only objects just a few inches away, while those with mild nearsightedness may clearly see objects up to several yards away.
Nearsightedness may develop gradually or rapidly, often worsening during childhood and adolescence. Nearsightedness tends to run in families. A basic eye exam can confirm nearsightedness. You can easily correct the condition with eyeglasses or contact lenses. Another treatment option for nearsightedness is surgery. Hyperopia, also known as farsightedness, is a defect of vision caused by an imperfection in the eye often when the eyeball is too short or the lens cannot become round enough, causing difficulty focusing on near objects, and in extreme cases causing a sufferer to be unable to focus on objects at any distance. As an object moves toward the eye, the eye must increase its optical power to keep the image in focus on the retina. If the power of the cornea and lens is insufficient, as in hyperopia, the image will appear blurred. People with hyperopia can experience blurred vision, asthenopia, accommodative dysfunction, binocular dysfunction, amblyopia, and strabismus. Hyperopia is often confused with presbyopia, another condition that frequently causes blurry near vision. Presbyopes who report good far vision typically experience blurry near vision because of a reduced accommodative amplitude brought about by natural aging changes with the crystalline lens. It is also sometimes referred to as farsightedness, since in otherwise normally-sighted persons it makes it more difficult to focus on near objects than on far objects.
The causes of hyperopia are typically genetic and involve an eye that is too short or a cornea that is too flat, so that images focus at a point behind the retina. Eye glasses work in the same manner whether they are for nearsightedness or farsightedness.
As early as 1845 Sir John Herschel suggested the idea of contact lenses, though he evidently did nothing about it. The practical application of a lens to the eyeball did not occur until late in the century, when F. E. Muller, a German maker of glass eyes, blew a protective lens to place over the eyeball of a man whose lid had been destroyed by cancer. The patient wore the lens until his death, twenty years later, without losing his vision. The term contact lens originated with Dr. A. Eugen Fick, a Swiss physician, who in 1887 published the results of independent experiments with contact lenses. In 1889 August Muller, a German medical student described his own experimentation with contact lenses. Although his attempts to use ground lenses were not successful, he did help lay the groundwork for further experimentation. In 1892 other doctors and optical firms in Europe cooperated in developing practical contact lenses; before long several firms began specializing in manufacturing them. By the early 40's a variety of contact lenses was available: blown glass, ground glass, molded glass, plastic and glass, and all plastic. All were still comparatively large and could not normally be tolerated for long periods of time. Improvements in manufacturing, material, and fitting of contact lenses lead to increased numbers of Americans wearing them. By 1964 over 6 million people in the United States were wearing contact lenses, 65% of them female.
The eye glass lens is curved so that it bends the light rays that hit your eyes so you can see images clearly. Understanding how glasses correct your vision begins with a basic understanding of how the eye works. At the back of the eye is the retina, a layer of cells that react to light. The reaction is sent to the brain, and the brain translates the activity of the cells into an image, or the thing that you see.
When your eye looks at something, the light rays come together, or focus, inside your eye. In someone with perfect vision, the rays focus directly on the surface of the retina. The image also must shrink, and it needs to be curved, because the retina is curved. The pupil and cornea are responsible for shrinking, focusing, and curving the image. If they have any irregularities, your vision will be blurry.
Nearsighted individuals cannot clearly see things that are distant. This happens because the light rays come into focus in front of the retina. Farsighted individuals have the opposite problem. The shape of their eyes causes the light rays to come into focus behind the retina, causing things that are near to them to be out of focus. Some people have blurry vision due to an astigmatism, or a condition where the curvature of the eye is irregular, creating a second focal point within the eye. All three of these common problems can be corrected with glasses.
Eyeglass lenses are curved pieces of glass. The curvature of the lens bends the light rays as they approach your eye. This helps the rays focus on your retina, instead of behind or in front of it. There are two main types of lenses used in eye glasses or contacts. Convex lenses, curve in slightly, are used for nearsighted people. This bends the light towards the bottom and top of the lens, thus pushing the focal point back towards the retina. Farsighted individuals need a concave lens. The light that passes through a concave lens is bent towards the center, pushing the focal point forward. The goal of the correction is to have the focal point hit the retina exactly where it should for ideal vision. The degree of the lens’s curve changes with the strength of the prescription.
I have always taken the invention of glasses for granted because frankly I have never needed them. I have always had amazing vision and could not imagine having to use contacts or glasses to achieve what my eyes already can do. When I was younger I loved the idea of wearing glasses so much that in second grade I pretended to be blind. But now upon reflection I love my eyesight and could not imagine having to use eye glasses to correct my vision.
Sources:
-Drewery, Richard D., Jr. "What Man Devised That He Might See  By Richard D. Drewry, Jr., M.D." Optometrists|Valencia|Glendale|Teagle Optometry|History. Teagle Opt, 2007. Web. 16 Nov. 2012. .
-Staff, Mayo Clinic. "Definition." Mayo Clinic. Mayo Foundation for Medical Education and Research, 03 Mar. 2012. Web. 16 Nov. 2012. .
-"How Glasses Correct Your Vision." How Glasses Correct Your Vision. Glass Crafters, n.d. Web. 16 Nov. 2012. .
-Bellis, Mary. "The Inventions and Scientific Achievements of Benjamin Franklin." About.com Inventors. About, n.d. Web. 16 Nov. 2012. .