Precise refractive index data on synthetic plagioclase feldspar glasses have been available for over forty years. However, no routine method based upon these data has come into general use for the determination of the natural feldspars. Such a method possesses certain advantages over the many other procedures presently followed. Its theoretical accuracy is about twice that obtainable with established refractive index methods. Index determinations, involving no particular refinements of technique, were made on the glasses of ten chemically analyzed natural plagioclase feldspars. The anorthite contents derived from these data agreed with those calculated from the chemical analyses within about two per cent. The method is simple and rapid, and calls for no costly equipment; some platinum foil for enclosing the feldspar sample, and a blowpipe capable of yielding a 1550° C. flame, are the only “extras” needed. It is therefore recommended for inclusion among the standard methods for the determination of the plagioclase feldspars.