A knowledge of the relative size of the red blood cells may be of definite value in differentiating between the types of anemia. This information may be obtained by a careful study of the red blood cells in a stained smear made from the patient's blood. Determinations of the mean diameter of the red blood cells by actual measurement of a number of cells, as suggested by Price-Jones,1 or by a modification of this method, have been made use of frequently; but this method is rather too time-consuming to be used extensively. Pijper2 and others have suggested a diffraction method for determining the size of the red blood cells. Haden3 suggested the determination of the volume of the average red blood cell by means of a simple calculation based on a knowledge of the red blood cell count and the total percentage volume of cells as determined by means of