| dark-field condenser | An apparatus for throwing reflected light through the microscope field, so that only the object to be examined is illuminated, the field itself being dark. (05 Mar 2000) |
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| dark field illumination | <microscopy> Any method of illumination which illuminates the specimen but does not admit light directly to the objective. It may be by substage (dark field) condensers, by stagespot lighting, by special condensers fitted around special objectives for reflected illumination or by the slit ultramicroscope. (05 Aug 1998) |
| dark field imaging | <microscopy> Using a single diffracted beam to form the image in a transmission electron microscope. This causes all regions of the specimen not of the same crystal structure and orientation as the region which produced the diffracted beam to be represented as very dark in the final image, allowing phase differentiation visually in the transmission electron microscope. (05 Aug 1998) |
| dark-field microscope | <instrument> A microscope that has a special condenser and objective with a diaphragm or stop that scatters light from the object observed, with the result that the object appears bright on a dark background. (05 Mar 2000) |
| dark field microscopy | <procedure> A system of microscopy in which particles are illuminated at a very low angle from the side so that the background appears dark and the objects are seen by diffracted and reflected patches of light against a dark background. (18 Nov 1997) |
| dark field objective | <microscopy> Certain objectives for high-power, dark fieldwork equipped with iris diaphragms or funnel stops so that their apertures may be reduced to correspond to the dark field con-denser with which they are used. (05 Aug 1998) |
| dark field slides | <microscopy> Owing to the exacting demands of dark field illumination, not only must the microscope slide be especially clean, but also the glass of which the slide is composed must be optically clear under dark field conditions. The glass should not fluoresce. (05 Aug 1998) |
| dark field stop | <microscopy> A central stop for obtaining a dark field effect for low-power objectives. It is customarily used with a high numerical aperture, bright field condenser. (05 Aug 1998) |
| depth of field | <microscopy> The depth or thickness of the object space that is simultaneously in acceptable focus. The distance between the closest and farthest objects in focus within a scene as viewed by a lens at a particular focus and with given settings. The depth of field varies with the focal length of the lens and its f-stop setting or numerical aperture, and the wavelength of light. Depth of fields only a small fraction of a micrometre can be achieved at 546 nm with microscope lenses of N.A. Greater than 0.9. (05 Aug 1998) |
| image field | <microscopy> Any field showing a focused image. There are a number of such fields in the complete microscopical system. The term may also denote the field of view, or the image field at the focal plane of the camera, generally the field where the final image is formed. (05 Aug 1998) |
| individuation field | The field within which an organiser can bring about the rearrangement of primordial tissues in such a manner that a complete embryo is formed. (05 Mar 2000) |
| object field | <microscopy> A position lying in the front focal plane of the objective. (05 Aug 1998) |
| electric field | <radiobiology> A property of a patch of space which causes the acceleration of electric charges located at that patch of space. The acceleration is given by a = qE/m, where q is the charge, E the electric field vector, and m the mass of the particle. Electric fields are generated by the presence of charges and/or the time variation of magnetic fields (09 Oct 1997) |
| electrophoresis, gel, pulsed-field | Electrophoresis in which the direction of the electric field is changed periodically. This technique is similar to other electrophoretic methods normally used to separate double-stranded DNA molecules ranging in size up to tens of thousands of base-pairs. However, by alternating the electric field direction one is able to separate DNA molecules up to several million base-pairs in length. (12 Dec 1998) |
| electrostatic field | <radiobiology> The region surrounding an electric charge in which another charge experiences a force. (16 Dec 1997) |