A method is proposed for the magnification calibration of the electron microscope to compensate for the calibration errors caused by mechanical and electrical variations of the instrument. This method uses microscopic glass spheres of predetermined size, mounted directly on the specimen supporting film. A single calibrating sphere is then exposed on the same photographic plate with the sample. Any change, therefore, in the magnification of the specimen also causes a corresponding change in the magnification of the sphere, thus eliminating errors in calculating magnification arising from instrumental variations. The spheres are graded to a given size by controlled levigation and calibrated internally on a grating replica at a low magnification with a very low electron‐beam intensity. By the use of low intensity of the beam, deformation of the replica of the grating is avoided. Also at the lower magnification many more lines of the grating replica may be included in the field for greater accuracy in determining the calibrating magnification. The spheres are placed on the specimen supporting film by evaporation from a water suspension. It is estimated that an accuracy of about ±3.0 percent is attained by the measurement of the image of the calibrated sphere on the same plate as the sample.