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Numerous unexplained departures from absorption theory in photoelectric spectrophotometry have been reported. The possible causes for such anomalies were studied. Physical reasons are proposed for results found in the literature, which may quantitatively account for the departures observed. Experiments that illustrate the precision attainable w-ith the Beckman quartz spectrophotometer, when the proper precautions are taken, are described. It is now possible to show what experimental conditions are necessary in order to limit the precision and accuracy obtainable by means of photoelectric spectrophotometry solely by the performance of the instrument, and under what conditions the instrunlent may give its best performance. I S C E Vandenbelt, Forsyth, and Garrett (30) called atten-

A convenient photometric linearity test that meets stringent criteria for accuracy and practicality is described. Experience with alternate, commonly advocated methods illustrates the need for an improved approach. The method described herein relies on direct application of physical principles. It combines use of Bouguer's law and superposition of optical fields. Each technique overcomes a deficiency in the other. Peak errors in absorbance were found to be 0.0011 or less in a commercial spectrophotometer equipped with a specially calibrated potentiometer and were primarily attributable to phototubes.

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