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Using chicken eyeglasses was still practiced in 1973, evidenced by Illinois' The Hawk-Eye newspaper that a farmer had 8,000 chickens fitted with the rose-colored variety. [20] One inventor of a form of the glasses proposed legislation in Kansas to require all chickens in the state to be fitted with glasses, but his campaign was unsuccessful. [21]
Other additives used in crown glasses include zinc oxide, phosphorus pentoxide, barium oxide, fluorite and lanthanum oxide. The crown/flint distinction is so important to optical glass technology that many glass names, notably Schott glasses, incorporate it. A K in a Schott name indicates a crown glass (Krone in German).
Prism projector, 640×360 pixels (equivalent of a 64 cm/25 in screen from 2.4 m/8 ft away) ... is a discontinued brand of smart glasses developed by Google's X ...
8×42 roof prism binoculars with rainguard and opened tethered lens caps. Binoculars or field glasses are two refracting telescopes mounted side-by-side and aligned to point in the same direction, allowing the viewer to use both eyes (binocular vision) when viewing distant objects.
Crown glasses such as BK7 have a relatively small dispersion (and can be used roughly between 330 and 2500 nm), while flint glasses have a much stronger dispersion for visible light and hence are more suitable for use as dispersive prisms, but their absorption sets on already around 390 nm.
In some lenses, the curved surfaces are replaced with flat surfaces, with a different angle in each section. Such a lens can be regarded as an array of prisms arranged in a circular fashion with steeper prisms on the edges and a flat or slightly convex center. In the first (and largest) Fresnel lenses, each section was actually a separate prism.