The later addition of a second sighting vane of another length, with a second scale calculated for it, provided a means to determine if the stave was in fact correctly placed relative to the eye by setting both vanes to the same angle marks on their respective scales and comparing them. But this could not actually be done during a sighting. A second problem was that the scale was not linear; the divisions are wide apart at low angles when the vane is well forward on the staff, but compacted when the vane is closer to the eye when high angles are being measured. These defects were inherent in the design of the instrument, and no later attempts at improvement could completely address them.