A simple interferometric method capable of displaying quantitatively the wave front aberration of any image-forming optical system is described. Its application for testing and aligning grazing incidence reflection optics at the same conjugates as those of short-wavelength use is demonstrated. The image-forming wave front from the system being tested is compared with a true spherical wave front generated within the interferometer from a point at the intended focus. The differences are displayed as a fringe pattern superimposed on an image of the exit pupil. Each fringe corresponds to one wavelength of separation between the actual image-forming wave front and the Gaussian reference sphere. The principle originates from a paper by W. P. Linnik published in Russian and German in 1933. A translation is included as an appendix. Four variations on Linnik's design are discussed, one of which avoids the use of transmitting optics and normal incidence reflections altogether and could therefore be used at ultraviolet or soft x-ray wavelengths.