Alidade for the plane table. Alidade-Length 20.25" (51.4cm). Brass, graduated on upper surface 30-90, 25-90, 20-14, 16-11, 12-8. Also a diagonal scale. Sight vanes-Two sight vanes, attached to alidade by brass screws, 4.8" (12.2cm) long. One vane has a vertical slit with rectangular aperture above (complete with thread), the other vane has a rectangular aperture (complete with thread) with a vertical slit above.