Spirograph Bruno Abakanowicz invented the Spirograph between 1881 and 1900. It was used for calculating an area delimited by curves. Drawing toys based on gears have been around since at least 1908, when "The Marvelous Wondergraph" was advertised in the Sears catalog. An article describing how to make a Wondergraph drawing machine appeared in the "Boys Mechanic" publication in 1913. The Spirograph itself was developed by the British engineer Denys Fisher, who exhibited at the 1965 Nuremberg International Toy Fair. It was subsequently produced by his company. US distribution rights were acquired by Kenner, Inc., which introduced it to the United
Cone of curves extremal rays of the cone cannot accumulate. If in addition the variety formula_1 is defined over a field of characteristic 0, we have the following assertion, sometimes referred to as the Contraction Theorem: 3. Let formula_48 be an extremal face of the cone of curves on which formula_33 is negative. Then there is a unique morphism formula_50 to a projective variety "Z", such that formula_51 and an irreducible curve formula_8 in formula_1 is mapped to a point by formula_54 if and only if formula_55. (See also: contraction morphism). Cone of curves In mathematics, the cone of curves (sometimes the Kleiman-Mori