On completion of the above work and before presentation of the material to the Society for applied Spectroscopy, the authors were kind enough to send us preprints for consideration. We offered some criticisms of the paper, one aspect being the constancy of the samples. There is no criterion of the transmittance constancy of the polystyrene film other than measurement by the instrument under test. The screen is a good test specimen, but its use so close to the slit image may be dangerous. The criticism showing the error from use of an 'unbalance' or 'drift' signal illustrated in Figure 13 above is valid if there is a drift in amplifier unbalance setting during the course of measurements. This error does not occur if the zero is properly reset after a <i>stable</i> unbalance condition is attained. The use of the unbalance condition is desirable since it gives the instrument 'sense' at the zero level and eliminates the difficulty of zero determination discussed by the authors for zero operation.
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