A hole to see through, esp. in a surveying or other instrument.

Optics. The space through which light passes in any optical instrument (though there is no material opening). Also attributive. aperture number, aper…

Mechanics. A thin lamina or plate serving as a partition, or for some specific purpose; sometimes… A thin plate or disk used as a partition…

= reticle, n. 2.

A grid or other pattern of fine threads, wires, lines, etc., in the focal plane or eyepiece of a telescope or other optical instrument in order to…

Either of the thin metal wires fixed horizontally and vertically at the focus of a telescope, to aid with the observation or tracking of an object…

In a filar micrometer: (originally) any of several lengths of spider silk used to measure distances or mark the position of an object being…

A flat vertical panel of fabric, paper, wood, etc., typically used in scientific experiments to intercept light, heat, or other radiation; a…

Any of various types of part attached to the end of the eyepiece of an optical instrument; spec. (a) a tubular attachment which alters the image that…

One of the threads or filaments of spider-web used to form the reticle of various optical instruments, esp. of micrometers, and serving to obtain…

A single thread spun by a spider. (Used in optical instruments.)

A straight and narrow cut or incision; an aperture resembling a cut of this description. A narrow, usually straight aperture in an optical…