| positive afterimage | An afterimage in which the lightness relationship is the same as the original one; if chromatic, it appears in the same colour. (05 Mar 2000) |
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| positive afterpotential | A spontaneous or inducible increase in transmembrane potential of a cardiac or nerve cell following the completion of repolarization. In the heart, this usually corresponds to the electrocardiographic U wave. (05 Mar 2000) |
| positive anergy | A reduction of the normal or usual immunologic response resulting from a reaction to a specific allergen. Synonym: specific anergy. (05 Mar 2000) |
| positive control | Mechanism for gene regulation that requires that a regulatory protein must interact with some region of the gene before transcription can be activated. (18 Nov 1997) |
| positive convergence | Inward deviation of the visual axes even when convergence is at rest, as in cases of convergent squint. (05 Mar 2000) |
| positive cooperativity | Cooperativity in which successive ligand molecules appear to bind with increasing affinity. (05 Mar 2000) |
| positive electrode | <microscopy> The electrode to which a major flow of electrons takes place internally (as in a cathode-ray tube) or to which an external positive voltage supply is connected. (05 Aug 1998) |
| positive electron | A subatomic particle of mass and charge equal to the electron but of opposite (i.e., positive) charge. Synonym: positive electron. (05 Mar 2000) |
| positive electrotaxis | Electrotaxis by which an organism is attracted toward a cathode or repelled from an anode. (05 Mar 2000) |
| positive, false | A result that is erroneously positive when a situation is normal. An example of a false positive: a particular test designed to detect cancer of the toenail is positive but the person does not have toenail cancer. (12 Dec 1998) |
| positive feedback | <physiology> The return of some of the output of a system as input so as to exert some control in the process. (18 Nov 1997) |
| positive focal length | <microscopy> Any lens which converges parallel rays to a focus at the back of the lens is a positive lens and has a positive focal length. The focal length is measured from the second principal point of the lens to the point on the lens axis where the rays from an infinitely distant point are brought to focus. See: negative focal length. (05 Aug 1998) |
| positive G | Gravity or acceleration in the usual head-to-foot direction in flying or in standing upright; the reverse of negative G. (05 Mar 2000) |
| positive image | <microscopy> A developed image in which dark areas correspond to dark areas of the original subject. (05 Aug 1998) |
| positive lens | <physics> Any lens with a positive focal length. Such lenses are thicker in the centre than around the circumference. There are three types of positive lenses, double convex or biconvex, planoconvex, converging concavoconvex or converging meniscus. (05 Aug 1998) |