The joint influence of partially coherent illumination and secondary spherical aberration on microscopic resolution has been theoretically studied. The aberration function used is that given by the Zemike—Nijboer expansion. The results are presented in terms of the contrast in the images of two point- objects and the resolution according to the Rayleigh criterion. It is seen that for certain states of illumination, the presence of spherical aberration leads to a slight improvement in resolution, although the image contrast is reduced. Analytical expressions for the point-image intensity distribution and Strehl’s ratio in the presence of secondary spherical aberration are also given. © 1971 Taylor & Francis Group, LLC.