THE value of the glass electrode for determination of the hydrogen ion concentration in the vitreous1 led to its use for similar investigations on the aqueous humor. The studies were indicated in view of the variations in the physiologic values cited in the literature. The differences were often due to the unavoidable errors inherent in the applied methods. Early reports on the pH of the aqueous lacked reliability, mostly because of the escape of carbon dioxide during the procedure.2 After Goldschmidt3 and Baurmann4 drew attention to this error, it was usually avoided by methods employed in the examination of similar biologic fluids of moderate buffering capacity.5 In general the studies were conducted along four lines—by the colorimetric method with indicators ; by the calculation of the hydrogen ion concentration as a function of the carbon dioxide content ; by the calculation of the hydrogenion concentration on