| cathodal | Of, pertaining to, or emanating from a cathode. Synonym: cathodic. (05 Mar 2000) |
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| cathodal closure contraction | An obsolete term for the momentary contraction of a muscle under the influence of the negative pole when an electrical circuit is established. (05 Mar 2000) |
| cathodal closure tetanus | An obsolete term for a tetanic muscular contraction occurring during the time the circuit is closed, the current then running, while the negative pole is applied. (05 Mar 2000) |
| cathodal duration tetanus | An obsolete term for a tetanic contraction occurring on application of the cathode or negative pole, while the circuit is closed. (05 Mar 2000) |
| cathodal opening clonus | An obsolete term for a clonus produced near a cathode when the flow of current is stopped. (05 Mar 2000) |
| cathodal opening contraction | An obsolete term for the momentary contraction of a muscle under the influence of the negative pole when the circuit is broken. (05 Mar 2000) |
| cathodal opening tetanus | An obsolete term for a tonic contraction in a muscle, to which the cathode is applied; when the circuit is opened, the contraction is suddenly interrupted. (05 Mar 2000) |
| cardioid dark field condenser | <microscopy> A condenser designed with two reflecting surfaces, the first, a spherical surface which reflects the rays to a second, cardioid (heart-shaped) surface. The virtue in such an arrangement is that, if the cardioid surface is of true figure, the lens is both achromatic and aplanatic. It has a limiting numerical aperture of about 1.0. Thus objectives of a greater numerical aperture cannot be used successfully with it. A true cardioid figure is the trace of a point on the circumference of a circle rolling around an equal, fixed circle. (05 Aug 1998) |
| 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) |
| 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) |
| dark adaptation | The adjustment of the eye occurring under reduced illumination in which the sensitivity to light is greatly increased or the light threshold is greatly reduced. Dark adaptation is slower than light adaptation. During dark adaptation rhodopsin is built up in the retinal rods. (12 Dec 1998) |
| dark-adapted eye | An eye that has been in darkness or semidarkness and has undergone regeneration of rhodopsin (visual purple), which renders it more sensitive to reduced illumination. Synonym: scotopic eye. (05 Mar 2000) |
| dark cell | Cell's in eccrine sweat glands having many ribosomes and mucoid secretory granules. (05 Mar 2000) |
| dark current | <physiology> Current caused by constant influx of sodium ions into the rod outer segment of retinal photoreceptors and that is blocked by light (leading to hyperpolarization). The plasma membrane sodium channel is controlled through a cascade of amplification reactions initiated by photon capture by rhodopsin in the disc membrane. (18 Nov 1997) |
| 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) |