Jim, Rivett608, and Company --
Spider silk was the most commonly used fiber for reticles in traditional surveying and other scientific instruments for two reasons: 1. The typical fiber diameter is in the 0.00025 (read that as "a quarter of a thou") inch range, allowing high-magnification eyepieces without making the reticle look like a telegraph pole. 2. Spiders are reasonably common everywhere except in the arctic, allowing expedient replacement of broken reticles in the field.
(Even today, some survey equipment repair shops keep a few spiders in a terrarium, on call to make silk as needed. There will often be a "slingshot frame" made of bent wire or cut from wood nearby . . . the repairman catches a spider, sets it on one leg of the Y, and flicks or blows the spider off its perch. As the spider descends on his web, the repairman will wind the filament around the Y's until he has collected several feet of web.)
W & L E Gurley of Troy, NY was probably the first major instrument maker to generally abandon spider silk by making drawn platinum wire their standard filament in (IIRC) the closing years of the 1800's. Using platinum reticle wires overcame the hygroscopic length change of the silk, which could actually get long enough to sag or short enough to break if the change of humidity was severe enough.
One late-comer to the survey-equipment manufacturing business, Brunson Instrument, used glass fiber for many of their fiber reticles. I've heard from two different sources that Brunson drew their own glass fiber, using a single child's glass marble as their source for hundreds (or maybe thousands) of yards of fiber.
But glass's major use in reticle making has been as a see-through substrate for photo emulsion, vacuum-deposited, or etched-and-filled reticle lines. The patterns on the first-generation glass reticles simply emulated the taut-filament patterns of conventional reticles, but somebody figured out that "straightness" of the reticle lines no longer depended on tension in a fiber.
This realization soon led to stubby tic-mark stadia, and filar-bifilar, varying-width, wedge-on-one-side-only, and open-center reticle patterns for the instrument's principal line-of-sight.
I've only repaired reticles on "user" instruments, and favor glass fiber although I've also used a filament unravelled from a silk thread taken from an old necktie. Fingernail polish is the easiest to use "glue" for attaching the fibers to the reticle ring that I've found.
Incidentally, some old survey instruments featured "disappearing stadia hair" reticles that had the crossing filaments for the principle telescope line-of-sight on one side of the reticle ring, the parallel filaments for the stadia reticle on the opposite side of the ring.
The limited depth of field of the eyepiece (which is, in reality, a microscope) made it impossible to see both the main and stadia reticles simultaneously . . . which totally prevented the classic "wrong hair" pointing / reading blunder.
Incidentally Mk II, at least one traditional survey textbook identifies the "brown [recluse??] spider" as the preferred source of silk for reticles while almost dismissing black-widow silk as second-rate but easy to find.
John