Also spider’s line. [SPIDER sb.] One of the threads or filaments of spiderweb used to form the reticle of various optical instruments, esp. of micrometers, and serving to obtain minute measurements; also loosely, any slender thread or wire used for this purpose.
1829. W. Pearson, Pract. Astron., II. 323. The spider’s lines, or wires, are usually laid parallel to one another on a circular plate of brass.
1866. Parkinson, Optics (ed. 2), 209. Such a set of threads are commonly called cross-wires or spider lines.
1888. Rutley, Rock-Forming Min., 14. Within the focus of the eye-glass, two fine wires or spider-lines are inserted.
attrib. 1829. W. Pearson, Pract. Astron., II. 245. Binocular Spider’s-line and Glass-disc Micrometers.
1875. Knight, Dict. Mech., 1431/1. A substitute for the spider-line micrometer.
