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INCE the beginning of 1953 experiments at the Laboratories of the Hydro- Electric Power Commission of Ontario with the method of van Heel (see references) for precision alignment have shown it to be very suitable for measuring small deflections, such as movements of rocks and glaciers. The basic idea is that three marks are placed on a line with, say, 400 feet between the outer two. They are: a single slit, about 1/32 inch wide, illu- minated by a 100-watt projection lamp with condenser lens (Fig. 1); a double slit (Fig. 2), and a 3x eyepiece with a scale (Fig. 3). The light, originating at the single slit, undergoes diffraction at the double slit, and results in an interference pattern consisting of coloured bands which can be observed with the eyepiece. The set-up resembles the well-known "High School" Fresnel experiments for demonstrating that light is a wave phenomenon. The applications are based on the fact that the coloured lines are very sharply bounded and that the pattern is symmetrical about its centre. The centres of the first slit, the second slit, and the pattern can always be lined up with great accuracy. Once adjusted the marks can be left in the field for days or months. Any sideways deflection (p) of the double slit (Dl) (Figs. 4 and S) results in a corresponding sideways deflection (q) in the pattern at the eyepiece scale (P) where c1 + cz q= c1 q = 2P

A Method for Measuring the Movement of Rocks and Glaciers with Simple Equipment