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 spectral distribution. Other methods accomplish this result in terms of a complex stimulus, having general characteristics varying with the method, but seldom spectrally identical with the stimulus to be measured, which nevertheless is found empirically

to evoke the same color as the latter. The most important of these methods which depend upon simple color matching are as follows.

B. .—In this method the variable stimulus is composed of heterogeneous radiation, which by itself evokes white or gray, combined with homogeneous radiation of variable wave-length. The total intensity, ratio of homogeneous