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From the time of Newton, attempts have been made to mix the spectral colors. About seventy years ago, Maxwell devised a method of separating them and plotted a curve around an equilateral triangle, at the apexes of which were the three primaries, red, green and blue. From this he obtained color equations and was able to express numerically the proportions of the primaries in all spectral colors.
Since his time many observers, beginning with Helmholtz and later Abney and Koenig, especially the latter, have obtained more accurate results with improved methods. All were handicapped by inadequate apparatus. Several colorimeters, made by Zeiss and others, are obtainable, but the author had one constructed which is far in advance of all others because of its perfect selectivity and its practically complete elimination of all stray light. Another marked improvement is the purity of his white light. This was obtained