| achromatic | <microscopy> Literally, colour-free. A lens or prism is said to be achromatic when corrected for two colours. The remaining colour seen in an image formed by such a lens is said to be secondary chromatic aberration. A truly achromatic lens would transmit light without decomposing it spectrally and there would be no colour fringing in the image. A doublet, composed of a positive and negative lens element, can be made achromatic for two colours which much improves the performance for most of the others. (05 Aug 1998) |
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| achromatic aplanatic condenser | <microscopy> A well-corrected microscope condenser lens, corrected for chromatic and spherical aberrations and satisfying the sine condition. (05 Aug 1998) |
| achromatic apparatus | The nonstaining asters and spindle fibres in a dividing cell. (05 Mar 2000) |
| achromatic lens | A compound lens made of two or more lens's having different indices of refraction, so correlated as to minimise chromatic aberration. (05 Mar 2000) |
| achromatic objective | <microscopy> An objective that is corrected chromatic for two colours, and spherically for one, usually in the yellow-green part of the spectrum. (05 Aug 1998) |
| achromatic threshold | Threshold of visual sensation, the minimal light intensity evoking a visual sensation. Synonym: achromatic threshold, minimum light threshold. (05 Mar 2000) |
| achromatic vision | A severe congenital deficiency in colour perception, often associated with nystagmus and reduced visual acuity. Synonym: achromatic vision, monochromasia, monochromasy, monochromatism. Origin: G. A-priv. + chroma, colour, + opsis, vision (05 Mar 2000) |
| aortic spindle | A fusiform dilation of the aorta immediately beyond the isthmus. Synonym: His' spindle. (05 Mar 2000) |
| central spindle | A central group of microtubules (continuous fibres) that course uninterrupted, between the asters, in contrast to the microtubules attached to the individual chromosomes (s. Fibres). (05 Mar 2000) |
| meiotic spindle | The meiotic equivalent of the mitotic spindle. (18 Nov 1997) |
| cleavage spindle | The spindle formed during the cleavage of a zygote or its blastomeres. (05 Mar 2000) |
| mitotic spindle | See: spindle and mitosis. (18 Nov 1997) |
| mitotic spindle apparatus | An organelle consisting of three components: 1) the astral microtubules, which form around each centrosome and extend to the periphery; 2) the polar microtubules which extend from one spindle pole to the equator; and 3) the kinetochore microtubules, which connect the centromeres of the various chromosomes to either centrosome. (12 Dec 1998) |
| muscle spindle | A specialised muscle fibre found in tetrapod vertebrates. A bundle of muscle fibres is innervated by sensory neurons. Stretching the muscle causes the neurons to fire, the muscle spindle thus functions as a stretch receptor. (18 Nov 1997) |
| His' spindle | A fusiform dilation of the aorta immediately beyond the isthmus. Synonym: His' spindle. (05 Mar 2000) |