In the account of an investigation of the optical properties of gypsum published by one of us (A. E. H. T.) in 1908 it was stated, as the result of direct measurement in sodium-light of the optic axial angle of a section-plate perpendicular to the first median line, which was heated in an air-bath, that the substance became uniaxial at 105.2° C. Three other section-plates examined subsequently gave somewhat higher values, the results ranging from 109.5° to 114.2° C. These figures were arrived at after a considerable correction, amounting to as much as 7°, had been applied for conduction along the crystal-holder, the temperatures actually observed being greater by this amount.