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The question of optical efficiency is discussed in relation to various forms of scanning. The "split-focus" principle and the smaller size of scanner thereby made possible are described with the aid of perspective drawings. Figures relating to the design of bar and slit sizes for the supersonic cell are given and the method of calculating the light flux on the screen is described. Tolerances on the accuracy of the scanners are discussed and the adjustment for the effect of temperature on the acoustic speed of the liquid is mentioned.