The Colour of Magic

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It is stereotypical that in fantasy fiction (e.g. Jack Vance’s Dying Earth stories) and role-playing games (e.g. Advanced Dungeons & Dragons) spells are often named after their ‘creator’, e.g. ‘Bigby’s Crushing Hand’. And indeed, in our universe Augustin Fresnel was the 19th century inventor of the Fresnel lens, often used in lighthouses to concentrate the light beam. A Fresnel lens consists of concentric ring segments; its main advantage is that it is not as thick as a (large) normal lens would be. The disc Rincewind is referring to is a transparent lens twenty feet across.