cells" in addition to the normal cones and rods possessed by humans. (See tetrachromacy for the real-world equivalent.) Substances The Disc contains magical substances. One such is octiron, a dense black metal that is a large part of the Discworld's crust. Its melting point is above the range of metal forges. The gates of Unseen University are made out of it, as is Old Tom, the university's bell. It is used to make magic needles and bells. It releases magical radiation, but if it becomes negatively polarised, it can be used to absorb such radiation. It generates significant amounts of heat under pressure, accounting for most of the volcanic geological processes on Discworld. When struck (such as with Old Tom), instead of producing a sound it briefly silences anything around it. Another is fingles, insinuated in Eric to be an important part of human psychology. Their absence, according to the Creator, can cause psychological problems. "On the surface they were all right, but deep down they knew something was missing", as he put it, referring to the inhabitants of a world where he forgot to include any. Since fingles do not exist on Earth, it is implied that Earth is the planet the Creator is referring to, and humanity is the species that is fundamentally incomplete because of their absence. Another is slood. First mentioned in The Last Continent, slood is a natural substance that could be discovered by intelligent beings, but that humans on Earth have been too unintelligent to find; it is said to be much easier to discover than fire, and only slightly harder to discover than water. One of Rincewind's many accumulated positions is Reader in Slood Dynamics. The General Theory of Slood was discovered by Archchancellor Sloman, and a stained glass window representing this event is in the meeting room of the Unseen University college council. The University's plumbing system contains pipes for maintaining slood differential. The Disc The Disc itself is described as roughly wide, giving it a surface area two-fifths that of the Earth, which would make it roughly the size of the Pacific Ocean. In addition to its flatness, Pratchett gives it another principal geographic feature; Cori Celesti, a mountain that lies at its exact centre and is the point of origin for its standing magical field. Cori Celesti is also the location of Dunmanifestin, the home of many of the Disc's