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<A HREF="http://www.triopticaonline.com/gafas-online/&quot;> Gafasonline</A> , also known as eyeglass lenses , eyeglasses or spectacles , are an optical instrument consisting of a pair of glasses fastened to a frame that rests on the nose by a bow and two pins that help hold them in your ears. They are a combination of two health products : lenses and optical mount that fits professional optometrist or a particular patient based on the correction required for each eye, the pupillary distance and the selected frame . It is mainly used to compensate defects of vision , such as nearsightedness , farsightedness or astigmatism , although there are special glasses to protect eyes in activities where there is a risk for these ( in this case if no optical correction they are not medical devices ) . Work by focusing light through the lenses to compensate for visual defects. There are also bifocal glasses you combine two different lenses graduation in one , suitable for presbyopic patients allowing the user to see far and close , and trifocals see also allow an intermediate distance . The latter no longer used while bifocals are used less and less in favor of progressive lenses . The earliest historical reference to the increase of view goes back to Egyptian hieroglyphics of V century BC, which represented simple glass lenses . The earliest written record of magnification view dates from the first century BC, when Seneca, a tutor of Emperor Nero of Rome, wrote: " Letters, however small and blurry , are seen more widely and clearly through a globe or glass full of water ' .1 Nero also said he saw the gladiatorial games using an emerald as a lens corrector.2 using a convex lens for a wider image is discussed in Alhazen optics book ( 1021). His translation from Arabic into Latin in the twelfth century was instrumental to the invention of eyeglasses in Italy XIII.1 century The treatise irride ("In the rainbow ") Grosseteste Robert English , written between 1220 and 1235, mentions using optics to "read the smallest letters from incredible distances ." A few years later , Roger Bacon also would be released for writing about the properties of lenses extending in 1262.3 [url=http://www.triopticaonline.com/gafas-online/]Gafasonline[/url] , in the form of flat crystals of smoky quartz , were used in China during the twelfth century . Similarly, the Inuit have used snow lenses for eye protection. However, while corrective offered no benefit , 4 really improved visual acuity narrowing the field visión.5 The use of the term " sunglasses " by historians is anachronistic before the twentieth century.