SynopsisThe vector method developed in previous papers of this series has been applied to certain problems in negative phase contrast. Unlike positive phase contrast, increasing the absorption of the phase plate increases the range of phase change in the object over which contrast remains unreversed (i.e. bright). As absorption approaches 100 p.c., conditions change over smoothly to central dark‐ground illumination, in which all object details appear brighter than the background. In negative phase contrast it is preferable to have the retardation ψ produced by a phase plate greater than 90°, whereas in positive phase contrast ψ should be less than 90°. Maximum sensitivity occurs when ψ = 90°, in each case. The characteristic of the phase plate needed to give maximum contrast for a given phase detail are di