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A description is given of the design, construction, and operation of an improved internal standard flame photometer that burns city illuminating gas and requires no electronic equipment. Sodium and potassium analyses are performed with lithium as the internal standard, lithium analyses with potassium as the internal standard, and calcium analyses with sodium as the internal standard. Data on the interference of high concentrations of sodium on potassium analyses and vice versa are presented together with comparisons of these and other interference effects that occur in several different types of flame photometers. The internal standard technique appears to reduce or eliminate errors due to interference as well as those due to variations in operating conditions. Considerably greater precision is attained with the internal standard instrument. This technique provides a satisfactory method for calcium analyses of inorganic samples rich in sodium. Some of the difficulties that prevent accurate results for calcium in biological samples are presented, and criteria for evaluating flame photometers are suggested.