Summary: I. It is shown that freshly-prepared rhombic crystals of the mixed Seignette salts exhibit abnormal interference-figures. The type of dispersion observed in freshly-prepared crystals is that belonging properly to the monoclinic system, viz. crossed dispersion, of the ‘borax’ type. The extent of crossed dispersion varies with the composition of the mixed crystals and rises to a maximum of 75° for the 45 % sodium-potassium tartrate mixture.
II. The effect observed in freshly-prepared crystals does not persist, but gradually disappears, the process occupying from one week to three or four months.
III. Increase of temperature hastens the process of change. The most pronounced cases of crossed dispersion disappeared on uniform heating to 30° or 40° C. in less than a week.
IV. The effect is shown to be independent of the concentration around the crystal of either ammonia or water-vapour.
V. An interesting feature of the change to be observed on leaving these crystals to stand is that there is a change in optic axial angle for all colours in addition to a closing in to coincidence of optic axial planes. The maximum change of optic axial angle for any section observed was that of 40° for yellow-green light with the 60 % sodium-potassium tartrate mixture. The observations indicate that there is no uniaxiality for any colour so long as crossed dispersion persists.