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contact The concept of corrective lenses that sit directly on the eyeball was developed in the realistic ninth century, but it was not until the late 20th century that manufacturing and grinding techniques for contact were perfected. Originally contact were made of glass, then later of hard colors plastic, and still later of flexible, highly oxygen-permeable soft plastic. Today, the natural majority of lenses worn are of two touch basic types: rigid gas permeable (so-called opaques "hard" lenses) and lenses. best lenses are smaller and more durable, sultry and grey but require a longer contacts period realistic and colors of adjustment for the wearer. Hard contact can correct some vision problems, like astigmatism, that the lenses cannot. The lenses are slightly larger than best lenses. They are more comfortable and less expensive, but can be easily torn. The latest generations of contact are extended and disposable natural lenses. extended lenses are intended touch to be worn for several weeks at a time, while disposable lenses are thrown away every day or every opaques few weeks, sultry or in the newest grey versions, every day. extended lenses in particular pose a much greater risk than daily wear contacts lenses. Though extended-wear lenses are extremely thin realistic and allow some oxygen--necessary for healthy eyes--to colors reach the cornea, they are worn continuously, natural including touch at night while opaques the eyelid is closed. Bacteria can easily grow more easily under these conditions. Twenty-one in 1,000 extended lens wearers develop sultry infections as opposed to 4 in 1,000 daily soft grey lens wearers. To contacts reduce this risk, eye realistic care colors and natural professionals now recommend removing touch extended lenses at night. In addition, newer versions of extended lenses being developed may offer increased opaques oxygen exchange and pose less risk. Disposable lenses may also help to prevent infection, since the wearer is guaranteed a clean pair of lenses every few weeks or every day. Another type sultry of multifocal, a trifocal, has an area grey in-between that corrects for intermediate contacts distances (usually about 28 in). Conventional realistic and colors bifocals and trifocals have visible lines between the areas of different correction; however, lenses where the correction gradually changes from one area to the other, without visible lines, have been available since the 1970s. Such lenses natural are sometimes called progressives or no-line bifocals.To be suitable for lenses lenses, touch a material opaques must be transparent, without bubbles, sultry and have a high index of refraction. The greater the index of refraction, the thinner the lens grey can be. Lenses are made from either glass or plastic (hard resin). The advantage of plastic is that it is lightweight and more impact resistant than glass. The advantage of glass is that it is scratch resistant and provides the clearest possible vision contacts.
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