Although we do not know for certain when prescription eyeglasses were invented, we do have proof that they did exist during 13th Century.Prescription-Eyeglasses.com They were referred to as spectacles rather than glasses. The need for prescription glasses increased in 15th Century. The first reading glasses were held by one hand only during the times that they were needed for reading. It was later in the 17th century when people actually wore the glasses regularly, after an optometrist created them with both earpieces. Today most prescription eye glasses are made from a plastic material that can be polished and tinted like glass lenses. The advantage to the plastic lens is the lighter weight of the prescription eyeglasses as well as the durability is greatly increased.
When you look at your prescription for glasses, you will see numbers listed under the headings of OS and OD. They are Latin abbreviations; OS (oculus sinister) means the left eye and OD (oculus dextrus) means the right eye. For people who have astigmatism, there will be three numbers in your prescription. The general form for writing these numbers is S x C x Axis
The S refers to the spherical portion of the prescription, which is the degree of nearsightedness or farsightedness discussed above.
The C refers to the cylinder or astigmatism, and can be a negative or a positive number. It measures in diopters the degree of astigmatism that you have. The bigger this number, the more astigmatism you have.
The Axis is a number anywhere between 0 and 180 degrees. It reveals the orientation of the astigmatism It is not enough to specify how much astigmatism there is; you have to know where the difference in curvature is taking place.
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