| negative feedback | This occurs where the products of a process can act at an earlier stage in the process to inhibit their own formation. The term was first used widely in conjunction with electrical amplifiers where negative feedback was applied to limit distortion of the signal by the amplification mechanism. Tends to stabilise the process. In contrast to positive feedback. (18 Nov 1997) |
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| negative focal length | <physics> The focal length of a negative lens. Parallel rays impinging on a negative lens can be traced to a virtual focus which exists on the same side of the lens as the impinging rays. The distance from the second principal point of the lens to this second focal point is measured on the same side of the lens as is the object. See: negative lens, focal length. (05 Aug 1998) |
| negative G | Gravity in a foot-to-head direction in flying, or in standing on one's head; opposite of positive G. (05 Mar 2000) |
| negative image | Continuation of visual impression after cessation of stimuli causing the original image. (12 Dec 1998) |
| negative lens | <physics> A lens with a negative focal length. The edge of a negative lens is thicker than the centre. The three negative lenses are, according to their figure: planoconcave, double concave or biconcave, and diverging concavoconvex or diverging meniscus. See: lens, simple. (05 Aug 1998) |
| negative meniscus | A convexoconcave lens in which the power of the concavity exceeds that of the convexity. Synonym: negative meniscus. (05 Mar 2000) |
| negative phase | The period during which the opsonic index is lowered following the injection of a vaccine. (05 Mar 2000) |
| negative pressure | Pressure less than that of the ambient atmosphere. (05 Mar 2000) |
| negative regulation | Negative feedback in biological systems mediated by allosteric regulatory enzymes. (18 Nov 1997) |
| negative S | Characteristic sedimentation behaviour of a lipoprotein fraction of plasma in a centrifugal field in a medium of appropriate density, achieved by adding a salt or D2O to the plasma. Synonym: negative S, Svedberg of flotation. (05 Mar 2000) |
| negative scotoma | A scotoma that is not ordinarily perceived, but is detected only on examination of the visual field. (05 Mar 2000) |
| negative stain | <technique> Stain forming an opaque or coloured background against which the object to be demonstrated appears as a translucent or colourless area; in electron microscopy, an electron opaque material, such as phosphotungstic acid or sodium phosphotungstate, is used to give detail as to surface structure. (05 Mar 2000) |
| negative staining | Microscopic technique in which the object stands out against a dark background of stain. For electron microscopy the sample is suspended in a solution of an electron dense stain such as sodium phosphotungstate and then sprayed onto a support grid. The stain dries as structureless solid and fills all crevices in the sample. When examined in the electron microscope the sample appears as a light object against a dark background. Quite fine structural detail can be observed using negative staining and it has been used extensively to study the structure of viruses and other particulate samples. (18 Nov 1997) |
| negative stranded RNA virus | <virology> Class V viruses that have an RNA genome that is complementary to the mRNA, the positive strand. They also carry the virus specific RNA polymerase necessary for the synthesis of the mRNA. Includes (Rhabdoviridae, Paramyxoviridae and Myoviridae (for example the T even phages). (18 Nov 1997) |
| negative strand virus | A virus the genome of which is a strand of RNA that is complementary to messenger RNA; negative strand virus's also carry RNA polymerases necessary for the synthesis of messenger RNA. (05 Mar 2000) |