Eyeglasses have a variety of structures to that are used for aiding and correcting defective sight. Generally they consist of a pair of lenses in a frame to hold them in the correct position in front of the eyes. Lenses are in most cases made of ground glass. A lens works by creating an image through the refraction of light. Each lens surface is made as a part of a spherical surface. To each surface there is what is called a center of curvature. This is the point in he middle of the sphere at which every point on the surface is equally spaced around. There are two centers of curvature in a lens and

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