Double-Gauss lens objective lens by adding a meniscus lens to its single convex and concave lens design. Alvan Clark and Bausch & Lomb further refined the design in 1888 by taking two of these lenses and placing them back to back making a "double Gauss" with indifferent photographic results. Current double Gauss lenses can be traced back to an 1895 improved design, when Paul Rudolph of Carl Zeiss Jena thickened the interior negative menisci and converted to them to cemented doublets of two elements of equal refraction but differing dispersion in the "Zeiss Planar" of 1896 to correct for chromatic aberration. It
Lens clock clock can be used if a dial thickness gauge is not available. To do this, the contact lens is placed concave side up on a table or other hard surface. The lens clock is then brought down on it such that the center prong contacts the lens as close to its center as possible, and the outer prongs rest on the table. The thickness of the lens is then the sagitta formula_4 in the formula above, and can be calculated from the optical power reading, if the distance between the outer prongs is known. Lens clock A lens clock is