| magnetic field gradient | In magnetic resonance imaging, a magnetic field that varies with location, superimposed on the uniform field of the magnet, to alter the resonant frequency of nuclei and allow recovery of their spatial position. Synonym: field gradient. (05 Mar 2000) |
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| paraboloid dark field condenser | <microscopy> A lens of parabolic shape. The vertex end is ground back so that its focus can be brought into coincidence with the specimen on the slide. A central stop is provided to block the central rays. It is used chiefly for medium- power work. (05 Aug 1998) |
| receptive field | That part of the retina whose photoreceptors (rods and cones) pertain to a single optic nerve fibre. The response of a neuron to stimulation of its receptive field depends on the type of neuron and the part of the field that is illuminated; an "on-centre" neuron is stimulated by light falling at the centre of its receptive field and inhibited by light falling at the periphery; an "off-centre" neuron reacts in exactly the opposite fashion; that is, it is inhibited by light falling at the centre of its receptive field. In either case, the net response depends on a complex switching action in the retina. When an entire receptive field is equally illuminated, the response of receptors at the centre of the field predominates. (05 Mar 2000) |
| visual field | The area simultaneously visible to one eye without movement; often measured by means of a bowl perimeter located 330 mm from the eye. (05 Mar 2000) |
| visual field test | <ophthalmology> A test which measures the extent of visual field loss. This test may be performed by a number of methods including confrontation, tangent screen exam and automated perimetry. Diseases that affect visual field include stroke, diabetes, hypertension, multiple sclerosis, glaucoma, hyperthyroidism, Jacob-Creutzfeldt disease and optic glioma. (27 Sep 1997) |
| reversed-field pinch | <radiobiology> A toroidal magnetic confinement scheme which could constitute an alternative to the Tokamak for building a fusion reactor. It is characterised by a magnetic field mostly generated by the plasma itself, with toroidal and poloidal components of comparable intensities, in contrast with the Tokamak where most of the field is toroidal and externally applied. The name of the configuration is given by the fact that the toroidal component of the magnetic field changes sign in the outer region of the plasma. The main attractivness of the Reversed Field Pinch is that, according to presently established scalings, it could reach ignition without the need of auxiliary heating. (09 Oct 1997) |
| rice-field fever | A febrile illness affecting workers in rice fields, reported in Po valley in Italy and in Sumatra, caused by infection with a species of Leptospira. (05 Mar 2000) |
| microscope, field emission | <microscopy> An image-forming device in which a strong electrostatic field causes cold emission of electrons from a sharply rounded point or from a specimen that has been placed on that point. The electrons are accelerated to a phosphorescent screen, or photographic film, giving a visible picture of the variation of emission over the specimen surface. (05 Aug 1998) |
| microscopic field | The area within which objects are visible with microscope oculars and objectives of various magnifying powers. (05 Mar 2000) |
| washed field technique | The cutting of cavity preparations in teeth utilizing a constant irrigant which is immediately removed from the mouth by means of a vacuum device. (05 Mar 2000) |
| Cohnheim's field | A polygonal mosaic-like figure formed by a group of myofibrils, as seen in the cross-section of a skeletal muscle fibre examined under the microscope; a shrinkage artifact of fixation. Synonym: Cohnheim's field. (05 Mar 2000) |
| Wernicke's field | The region of the cerebral cortex thought to be essential for understanding and formulating coherent, propositional speech; it encompasses a large region of the parietal and temporal lobes near the lateral sulcus of the left cerebral hemisphere; corresponding approximately to Brodmann's areas 40, 39, and 22. Synonym: sensory speech centre, Wernicke's area, Wernicke's field, Wernicke's region, Wernicke's zone. (05 Mar 2000) |
| wide field ocular | An ocular that gives a larger than usual field of view and a high eyepoint. (05 Mar 2000) |
| condenser, dark field | <microscopy> A condenser forming a hollow cone of light with its apex (or focal point) in the plane of the specimen. When used with an objective having a numerical aperture lower than the minimum numerical aperture of the hollow cone, only light deviated by the specimen enters the objective. Objects are seen as bright images against a dark background. The ordinary bright field condenser of low power, used with a central stop, makes a good dark field condenser. They all form a dark field while illuminating the specimen with a hollow cone of light. The lower limiting aperture of the condenser must be greater than the numerical aperture of the objective with which it is to be used. Thus, no direct light enters the objective, the specimen is seen by reflected or scattered light on a dark background. See: condensers See: special dark field condensers: paraboloid, cardioid and Cassegrainian. (05 Aug 1998) |
| constant field equation | An equation derived to predict membrane potentials in terms of the membrane's permeability to ions and their concentrations on either side. Synonym: constant field equation, Goldman-Hodgkin-Katz equation, GHK equation. (05 Mar 2000) |