The analysis of an interferogram

It has been frequently suggested that it should be possible to analyse mathematically the interferometer pattern produced by a lens, in order to obtain a measure of the aberrations from the coefficients of the terms in the various orders of x and y . x and y are here the coordinates of a point on the interferogram, whose optical path difference relative to the central ray of the lens is known at once by counting the fringes. This method has now been tried several times, using the utmost experimental precautions to secure accuracy, but the figures do not agree very closely with those obtained for the same lens under identical conditions by the oblique Hartmann test. Possible reasons for this disagreement are suggested.