SynopsisThe study of positive phase contrast commenced in the first paper of this series is continued, with special reference to the optimum characteristics of the phase plate. It is shown that, although it is always possible in theory to make a given object detail appear perfectly black, the construction of special phase plates for this purpose is difficult and not necessarily desirable. The behaviour of a number of phase plates designed to fulfil special conditions is compared with that of standard 90° phase plates and it is shown that the differences are slight, especially when the absorption of the phase plate is moderate or high. The effect of absorption in various phase plates is investigated and it is shown that for plates which introduce a phase change of about 75° the maximum contrast is essentially independent of absorption. This suggests a possible method of measuring phase change in the objec