Dioon, a genus of Cycads, natives of Mexico, where meal is prepared from the starchy seeds.
Diophan´tus of Alexandria, the first Greek writer on algebra, flourished, according to some authorities, about the middle of the fourth century after Christ. He is called the Father of Algebra, and left behind him thirteen books of Arithmetical Questions, of which only six are extant; and a work on Polygonal Numbers.
Diop´side, a calcium magnesium silicate, of the pyroxene series, occurring in igneous rocks and altered limestones, with a vitreous lustre, and of a pale-green, or a greenish- or yellowish-white colour.
Diop´sis, a genus of two-winged flies of which the species are native to India and Africa. Each side of the head is drawn out into a long lateral horn, which bears the eye and antenna at its extremity.
Di´optase, emerald copper ore, hydrated silicate of copper, a translucent mineral, occurring crystallized in six-sided prisms. It occurs in Siberia, Hungary, and Chile. It has been used as a gem-stone, notably in Persia and Russia.
Diopter, the unit in terms of which the power of a lens or curved mirror can be expressed. It is obtained by taking the reciprocal of the focal length of the lens or mirror in metres. Thus, a lens with focal length 1 metre has a power of 1 diopter. If the focal length is 2 metres, the power is ½ diopter. The power may also be obtained by dividing 39.37 by the focal length in inches. The unit is employed in classifying spectacle lenses.
Di´orite, a coarsely crystalline igneous rock, sometimes of a whitish colour speckled with black or greenish-black, sometimes very dark
in colour, consisting of hornblende and calcium sodium felspar. Dark mica sometimes takes the place of hornblende (mica-diorite). The 'greenstones' of older authors are mostly diorites.
Dioscoreaceæ, a nat. ord. of monocotyledons, with alternate reticulate-veined leaves, tuberous root-stocks, and twining stems. The flowers are small and unisexual. There are 6 genera, with about 100 species. The typical genus is Dioscorea, which includes the yam. Black bryony is the only British representative.
Dioscor´ides, Pedanius, a Greek physician, born in Cilicia in the first century of the Christian era. He was the author of a celebrated work on materia medica, in five books, particularly valuable in regard to botany.