Iris

(in electron and ion optics), an opening in a conducting plate; used to limit the cross section and vary the expansion (aperture) angle of a charged-particle beam. A round iris under a potential V and placed in an external electrical field is the simplest electrostatic lens. If E₁^ and E₂ are equal to the field intensity along different sides of the plate at a distance from the aperture, the focal length of such a lens is approximately f = 4ø/ (E₁ - E₂) where ø is the potential at the center of the iris. Depending on the sign of f, an iris may be a converging or diverging lens. Combinations of irises under different potentials are also electrostatic lenses.

REFERENCE

a genus of plants of the family Iridaceae. They are perennial rhizomatous herbs with sword-shaped or linear leaves. The flowers are large, with a brightly colored corolliform perianth; the ovary is three-celled and inferior; and the fruit is a trihedral, many-seeded pod.

There are about 200 species, distributed throughout the northern hemisphere; in the USSR there are about 60 species. The iris species that form tubers and bulbs are often subdivided into the genera Junona, Xiphium, Iridodictum, and Gynandriris. Irises are widely used for ornament, especially varieties of the species I. kaempferi, I. hybrida, I. spuria, and I. iberica. They grow best in sunny places with well-drained soil. The orrisroot is obtained from the rootstocks of some irises, and a coarse fiber for making brushes is produced from the leaves of the species I. songarica.

a part of the anterior portion of the eye in animals and man that is located between the vitreous and anterior chambers. The iris is a thin and movable diaphragm with a pupillary aperture in the center; light is regulated through the pupil and onto the retina by the contraction and dilation of the aperture.

The iris is composed of both retinal and uveal structures. The retinal, or posterior, surface consists of two pigmented epithelia. The posterior epithelium is covered by an internal restricting membrane and is a continuation of the retina and the ciliated epithelium. The anterior epithelium is a continuation of the pigmented epithelium of the retina and the ciliary body. The neuroepithelial muscles of the iris are the sphincter, which constricts the pupil, and the dilator, which widens the pupil. They are formed from the anterior epithelium. The sphincter is innervated by parasympathetic fibers of the oculomotor nerve and the dilator is innervated by sympathetic nerves.

The uveal, or mesodermal, surface of the iris is also called the anterior surface. It is a continuation of the vascular layer of the ciliary body and the vascular coat and is composed of exterior reticular and deep vascular layers. The uveal surface is covered with endothelium that extends from the cornea. At the level of the restricting membranes of the iris is the barrier that separates blood from the eye. The anterior surface of the iris is divided into a peripheral, or ciliary, zone, which contains both the reticular and vascular layers, and a pupillary zone, which is the lesser circle of the iris and contains the sphincter. The exterior layer of stroma atrophies in the pupillary zone.

The blood vessels of the iris originate in the vascular circulus major located along the margin of the peripheral zone and are positioned radially. In man, they anastamose into the arterial and venous arches of the vascular circulus minor, 1.5 mm from the pupil margin. No independent lymphatic system has been discovered in the iris. The stroma of the iris is composed of thin collagenic and elastic trabeculae. The predominate cells of the stroma are chromatophores, which determine eye color; man has only melanocytes, while birds, reptiles, and amphibians have iridophores and lipophores in addition to melanocytes. There are also fibroblasts and granular plasma cells in the stroma. The color and architectonic of the uveal part of the iris are determined by species and racial characters and change with age.

Inflammation of the iris (iritis) accompanies traumas and various infectious and metabolic diseases; as a rule, there is also inflammation of the ciliary body.