A visual optical T-bench method for the measurement of optical path difference nA in units of wavelength is described. The method employs an optical T-bench equipped with nodal slide, a movable double-slit system, and an angle measuring telescope. Determinations of longitudinal spherical aberration in millimeters and optical path difference in units of wavelength are based upon measurements of angular deviations in small selected regions of the collimated beam emergent from the lens under test. The theory of the method is presented together with a description of the apparatus used and the technique of measurements. Results of measurement on a single lens are included.