The well-known diffraction figure of three equidistant slits may be considered as formed by the fringes of equal intensity caused by the outer slits, together with the coherent background from the middle slit, which alternately reinforces and suppresses these fringes. This effect disappears when the focus of the observing telescope is changed so as to cause a geometric path difference of a quarter wave between the middle and outer beams, resulting in two accurate settings of the drawtube inside and outside focus for which the fringes are all of equal brightness. A phase strip in front of the middle slit gives an outward or inward shift of these settings, which measures its path difference with an accuracy of a few thousandths of a wave-length. To compensate for the absorption of the strip the middle slit is made much wider than the outer slits. The effect is reversed if two phase strips are placed in front of the outer slits. By an arrangement with five slits, both possibilities may be alternately switched in, thus doubling the precision. The method is especially valuable for testing larger samples of the material prepared for cutting out the small phase rings to be inserted in microscope objectives. Applications of the same photometric setting to other interference measurements are indicated.
F. ZERNIKE, "A Precision Method for Measuring Small Phase Differences," J. Opt. Soc. Am. 40, 326-326 (1950)