The physical theory, of testing optics by the Ronchi sine grating interferometer is developed. In the experiment a sine grating splits the third-order wavefront of the zone plate under test into three displaced wavefronts, which then interfere with one another to form interference fringes. The intensity distribution of the interference pattern can be recorded for various positions of the sine grating. By subtracting the contribution of the inherent diffractive aberration of the sine grating, the value of the spherical aberration associated with the third-order wavefront of the zone plate under test can be determined. Theory and experiment are compared.