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Photonics Dictionary

cross wire

Fine lines, wires or threads used in the focal plane of many optical instruments to point out and locate particular objects in the field of view. They were formerly made from a single strand of spider web; later, fine-drawn threads of glass, quartz, silk, or plastic were substituted. Today it is more common to employ etched reticle lines on a glass plate. These can be illuminated from the side and they act in every way as a true cross wire, with the advantage of greatly increased rigidity and permanence.
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