The exit slit of a monochromator is focused upon a pinhole located in the first focal plane of an objective utilized for viewing the multiple-beam fringes formed by inter-reflections between two inclined surfaces, one of which is stepped. The step is focused into the plane of an eyepiece whose reticule contains a pair of parallel lines between which successive fringes are set and measured by rotation of the wavelength drum. The preferred, and possibly novel, method of interpreting the observations is discussed.