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An engraving depicting a lighthouse optics, showing the light source surrounded by eight Fresnel echelon or lighthouse lenses. P is the weight which drives the mechanism for revolving the lens, M, which includes a governor for regulating the speed, 19th century
An engraving depicting a Fresnel echelon or lighthouse lens from the front and side. At A is a plano-convex lens a foot in diameter surrounded by glass rings whose curvature is calculated so that they have the same focus as A. M, N are mirrors which turn light which escapes the lens into the same path as the primary beam, 19th century
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