SYNOPSISTwo methods of testing microscope object‐glasses are described and classified as “Static” and “Kinematic.” The effects of residual aberrations are demonstrated and then a quantitative relation between the factors at the disposal of the microscopist and the residual aberration is given. A definition of “Sensitivity” follows with numerical values for certain lenses. The effects of variations of cover‐glass thickness and refractive index and variation of immersion‐oil refractive index are discussed with the aid of the conce