(1). Introduction: Description of fixed-focus and variable-focus electrostatic image tube; scope of paper. (2). Focusing properties: Measurement of the variation of image distances and magnification with object distance and ratio of applied voltages for both types of tubes; calculation of potential distributions and electron paths; comparison of experimental and theoretical results. (3) Aberrations: Classification of aberrations; measurement of tangential and sagittal image surfaces for fixed-focus tube; calculation of axial (chromatic and spherical) aberrations; calculation of field aberrations; comparison between measurements and calculation; reduction of field aberrations by curving the cathode.