Bifocals From Wikipedia, the free encyclopedia Jump to: navigation, search Bifocals are eyeglasses whose corrective lenses each contain regions with two distinct optical powers. Bifocals are most commonly prescribed to people with presbyopia who also require a correction for myopia, hyperopia, and/or astigmatism. Contents 1 History 2 Construction 3 Problems 4 Future 5 References 6 See also History Benjamin Franklin is usually credited with the creation of the first pair of bifocals in the early 1760's, though the first indication of his double spectacles comes from a political cartoon printed in 1764. A great number of letters and publications from that time period refer to Dr. Franklin's double spectacles, including his first reference to them in a letter dated August 21, 1784. John Isaac Hawkins, the inventor of trifocals, coined the term bifocals in 1824 and credited Dr. Franklin with their invention. Construction Franklin's original bifocals were designed with the most convex lenses (for close viewing) in the lower half of the frame and the least convex lenses on the upper. Up until the beginning of the 20th century two separate lenses were cut in half and combined together in the rim of the frame. The mounting of two half lenses into a single frame led to a number of early complications and rendered such spectacles quite fragile. A method for fusing the sections of the lenses together was developed by Louis de Wecker at the end of the 19th century and patented by Dr. John L. Borsch, Jr. in 1908. Today most bifocals are created by molding a reading s
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A day (symbol: d) is a unit of time equivalent to 24 hours. It is not an SI unit but it is accepted for use with SI. The SI unit of time is the second. The term comes from the Old English dg, with similar terms common in all other Indo-European languages, such as dies in Latin and dive in Sanskrit. Contents 1 Definitions 1.1 International System of Units (SI) 1.2 Astronomy 1.3 Colloquial 2 Introduction 3 Civil day 4 Leap seconds 5 Astronomy 6 Boundaries of the day 7 Metaphorical days
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