With the sensitive organism the appearance and structure of the zone
edge was the same with all sizes of zone tested, except with certain very narrow
zones. Typically, three zones could be distinguished. The innermost zone, A
(Pl. 1, fig. I), was only visible with the hand-lens, and had a sharp and regular
boundary with the middle zone B, visible to the naked eye (one of the many
types of ‘ghost-zone’ which may be encountered in assay plates). The third
zone, C, had a clear and regular margin at its common boundary with zone B
but its outer limit was vague and ill-defined. The B/C boundary is of course that
usually measured in assay work.