| least diffusion circle | In the configuration of rays emerging from a spherocylindrical lens system, the place where diverging rays of the lens first forming a line image are balanced by converging rays of the second lens. (05 Mar 2000) |
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| ambipolar diffusion | <radiobiology> Diffusion process in which buildup of spatial charge creates electric fields which cause electrons and ions to leave the plasma at the same rate. (Such electric fields are self-generated by the plasma and act to preserve charge neutrality.) (09 Oct 1997) |
| anomalous diffusion | <radiobiology> Diffusion in most plasma devices, particularly tokamaks, is higher than what one would predict from understood causes. The observed, typical diffusion is referred to as anomalous because it has not yet been explained. Anomalous diffusion includes all diffusion which is not due to collisions and geometric effects. While such effects were not understood when the term was coined, and most still are not, diffusion due to well-understood wave phenomena is still 'anomalous'. Classical diffusion and Neo-classical diffusion are the two well-understood diffusion theories, although neither is adequate to fully explain the observed anomalous diffusion. See: entries for classical diffusion and neoclassical diffusion. Anomalous resistivity (09 Oct 1997) |
| arterial circle of cerebrum | An anastomotic "circle" of arteries (roughly pentagonal in outline) at the base of the brain, formed, sequentially and in anterior to posterior direction, by the anterior communicating artery, the two anterior cerebral, the two internal carotid, the two posterior communicating, and the two posterior cerebral arteries. Synonym: circulus arteriosus cerebri, circle of Willis. (05 Mar 2000) |
| articular vascular circle | An anastomosis of vessels encircling a joint. See: articular vascular network. Synonym: circulus articularis vasculosus. (05 Mar 2000) |
| Baudelocque's uterine circle | A constriction located at the junction of the thinned lower uterine segment with the thick retracted upper uterine segment, resulting from obstructed labour; this is one of the classic signs of threatened rupture of the uterus. Synonym: Bandl's ring, Baudelocque's uterine circle, Scanzoni's second os. (05 Mar 2000) |
| bohm diffusion | <radiobiology> A rapid loss of plasma across magnetic field lines caused by microinstabilities. Theory formulated by the physicist David Bohm. Semiempirical formula for the diffusion coefficient given by Bohm in 1946 (noted by Bohm, Burhop, and Massey, who were developing a magnetic arc for use in uranium isotope separation). Bohm diffusion was proposed (not derived from first principles) to scale as 1/B rather than the 1/B^2 scaling predicted by classical diffusion. A 1/B scaling results from assuming that particles diffuse across field lines at an optimum rate (effective collision frequency=cyclotron frequency). The 1/B scaling is observed (approximately) in most reactors. See: diffusion, microinstabilities, field lines. (09 Oct 1997) |
| Pagenstecher's circle | In the case of a freely movable abdominal tumour, the mass is moved throughout its entire range, its position at intervals being marked on the abdominal wall; when these points are joined, a circle is formed, the centre of which marks the point of attachment of the tumour. (05 Mar 2000) |
| vascular circle | The circle around the mouth formed by the inferior and superior labial arteries. Synonym: areolar venous plexus. (05 Mar 2000) |
| vascular circle of optic nerve | <anatomy, nerve> A network of branches of the short ciliary arteries on the sclera around the point of entrance of the optic nerve. Synonym: circulus vasculosus nervi optici, circulus arteriosus halleri, circulus zinnii, Haller's circle, Zinn's corona, Zinn's vascular circle. (05 Mar 2000) |
| Ramsden circle | <microscopy> The circular spot of light formed at that distance above the eyepiece where the chief image forming rays cross, the back focal plane of the eyepiece. The objective back focal plane is in conjugate focus in this same plane. In visual microscopy, the point where the pupil of the eye is placed. (05 Aug 1998) |
| gel diffusion | Diffusion in a gel, as in the case of gel diffusion precipitin tests in which the immune reactants diffuse in agar. See: immunodiffusion. (05 Mar 2000) |
| gel diffusion precipitin tests | Precipitin test's in which the immune precipitate forms in a gel medium (usually agar) into which one or both reactants have diffused; generally classified in two types, in one dimension, and in two dimensions. Synonym: gel diffusion reactions. (05 Mar 2000) |
| gel diffusion precipitin tests in one dimension | Precipitin test's in which antigen solution and antibody incorporated in agar are layered in tubes, permitting effective diffusion in the vertical dimension; the antibody-containing agar may be overlaid directly with antigen solution (single (gel) diffusion in one dimension). (05 Mar 2000) |
| gel diffusion precipitin tests in two dimensions | Precipitin test's made in a layer of agar that permits radial diffusion, in both of the horizontal dimensions, of one or both reactants. Double (gel) diffusion in two dimensions (Ouchterlony test, technique, or method) incorporates antigen and antibody solutions placed in separate wells in a sheet of plain agar, permitting radial diffusion of both reactants; this method is widely used to determine antigenic relationships; the bands of precipitate that form where the reactants meet in optimal concentration are of three patterns, referred to as reaction of identity, reaction of partial identity (cross-reaction), and reaction of nonidentity. (05 Mar 2000) |