Footnote 15
Part I, §13.  See fig. 3, Plate III.  Each plate turns on a hinge, just like the `sights' of a gun.  One is drawn flat down, the other partly elevated.  Each plate (tabella vel pinnula) has two holes, the smaller one being the lower.  This Rewle is named in Arabic Alhidada or Al'idada; in Latin Verticulum, from its turning easily on the centre; in Greek Dioptra, as carrying the sights.  The straight edge, passing through the centre, is called the Linea Fiduciæ.  It is pierced by a hole in the centre, of the same size as that in the Mother.