| ocular micrometer | A glass disk that fits in a microscope eyepiece and that has a ruled scale; when calibrated with a slide micrometer, direct measurements of a microscopic object can be made. (05 Mar 2000) |
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| caliper micrometer | A gauge with a calibrated micrometer screw for the measurement of thin objects such as microscope cover glasses and slides. (05 Mar 2000) |
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| micrometer | An instrument, used with a telescope or microscope, for measuring minute distances, or the apparent diameters of objects which subtend minute angles. The measurement given directly is that of the image of the object formed at the focus of the object glass. Circular, or Ring, micrometer, a metallic ring fixed in the focus of the object glass of a telescope, and used to determine differences of right ascension and declination between stars by observations of the times at which the stars cross the inner or outer periphery of the ring. Double image micrometer, a micrometer in which two images of an object are formed in the field, usually by the two halves of a bisected lens which are movable along their line of section by a screw, and distances are determined by the number of screw revolutions necessary to bring the points to be measured into optical coincidence. When the two images are formed by a bisected objects glass, it is called a divided-object-glass micrometer, and when the instrument is large and equatorially mounted, it is known as a heliometer. Double refraction micrometer, a species of double image micrometer, in which the two images are formed by the double refraction of rock crystal. Filar, or Bifilar, micrometer. See Bifilar. <mechanics> Micrometer caliper or gauge, a caliper or gauge with a micrometer screw, for measuring dimensions with great accuracy. Micrometer head, the head of a micrometer screw. Micrometer microscope, a compound microscope combined with a filar micrometer, used chiefly for reading and subdividing the divisions of large astronomical and geodetical instruments. Micrometer screw, a screw with a graduated head used in some forms of micrometers. Position micrometer. See Position. Scale, or Linear, micrometer, a minute and very delicately graduated scale of equal parts used in the field of a telescope or microscope, for measuring distances by direct comparison. Origin: Micro-: cf. F. Micrometre. Source: Websters Dictionary (01 Mar 1998) |
| slide micrometer | A scale made on a microscope slide with lines ruled in divisions, usually, of 0.01 mm; typically used to calibrate an ocular micrometer. (05 Mar 2000) |
| filar micrometer | An ocular micrometer with a line moved by a ruled drum such that a movement of the line of 5 um or less may be made in relation to fixed parallel lines. (05 Mar 2000) |
| accommodation, ocular | The dioptric adjustment of the eye (to attain maximal sharpness of retinal imagery for an object of regard) referring to the ability, to the mechanism, or to the process. It is the effecting of refractive changes by changes in the shape of the crystalline lens. Loosely, it refers to ocular adjustments for vision at various distances. (12 Dec 1998) |
| adaptation, ocular | The adjustment of the eye to variations in the intensity of light. Light adaptation is the adjustment of the eye when the light threshold is increased; dark adaptation when the light is greatly reduced. (12 Dec 1998) |
| albinism, ocular | Albinism affecting the eye in which pigment of the hair and skin is normal or only slightly diluted. The classic type is x-linked (nettleship-falls), but an autosomal recessive form also exists. Ocular abnormalities may include reduced pigmentation of the iris, nystagmus, photophobia, strabismus, and decreased visual acuity. (12 Dec 1998) |
| anterior ocular segment | That portion of the eye comprising the cornea, iris, lens, and their associated chambers and adnexa. (05 Mar 2000) |
| cardinal ocular movements | Eye rotations to the right and left, upward to the right and left, and downward to the right and left, to diagnose positions of gaze. (05 Mar 2000) |
| Ramsden's ocular | An eyepiece of a microscope, consisting of two planoconvex lenses with convexities turned to each other. (05 Mar 2000) |
| vestibular ocular reflex | <clinical sign> Reflex movement of the eyes in the opposite direction to that which the head is moved, e.g., the eyes being lowered as the head is raised, and the reverse (Cantelli's sign); an indication of functional integrity of the brainstem tegmental pathways and cranial nerves involved in eye movement. Synonym: vestibular ocular reflex. (05 Mar 2000) |
| reflex, vestibulo-ocular | A reflex wherein impulses are conveyed from the cupulas of the saemicircular canals and from the otolithic membrane of the saccule and utricle via the vestibular nuclei of the brainstem and the median longitudinal fasciculus to the oculomotor nerve nuclei. It functions to maintain a stable retinal image during head rotation by generating appropriate compensatory eye movements. (12 Dec 1998) |
| refraction, ocular | Refraction of light effected by the media of the eye. It also includes the determination of the refractive state and refractive correction. (12 Dec 1998) |
| goniometre ocular | <microscopy> A special type of ocular fitted with crosslines and a graduated head which rotates the crosslines to facilitate the measuring of angles, as in crystallography. (05 Aug 1998) |
| retro-ocular | <anatomy> Behind the pons. Origin: L. Bulbus = bulb (18 Nov 1997) |