Radiopaque substances were injected retrobulbarly and applied to the apex of the cornea of animals. Roentgenograms were taken and the shadows of the radiopaque substances were readily perceptible and well defined; they delineated the posterior half of the eyeball and the apex of the cornea. The antero-posterior diameter of the eyeballs in the roentgenograms was measured with a vernier caliper. The eyeballs were enucleated and after a droplet of radiopaque substance had been applied to the apex of the cornea and to the sclera at the posterior pole, roentgen films were taken of the enucleated bulbs. The antero-posterior diameter of the roentgen shadows of the enucleated bulbs was measured with a vernier caliper. The enucleated bulbs were measured with a metric micrometer caliper. The roentgen measurements, corrected by triangulation, of fifty cats' eyes in situ and following enucleation are compared with the micrometer caliper measurements of the corresponding enucleated eyeballs. The instruments devised for this work are described and six tables summarize the results of the investigation. From the Divisions of Ophthalmology and Roentgenology, The University of Chicago. Read before the American Association for Research in Ophthalmology, at Atlantic City, June 11, 1935.