English (4) A knob for ...

林頌賢：
I believe ' a knob for reticle illumination' simply means a turnable knob for
adjusting the reticle(= reticule=graticule), a network of illuminating fine lines,
wires, etc., placed in the focal plane of an optical instrument to assist
measurement of the size or position of objects under observation.

It means the round end of an aid of light.