An arrangement using a radial shearing interferometer is described with which quantitative measurement of the aberration coefficients of microscope objectives can be conveniently made. Measurements can be made at an angle to the axis as well as on the axis, and for various wavelengths. The apparatus is relatively simple, stable and easy to handle. In addition, since tests are carried out under conditions approximating to those under which the microscope is actually used, the effect of changes in various parameters can be studied quite readily. Some results obtained with typical microscope objectives are presented.