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<p>The chief significance of this paper lies in the development and testing of an experimental method for determining an objective physical standard of 'white light.' The standard to be determined is the Planckian ('black-body') distribution of energy required to evoke the hueless sensation of brilliance, commonly called 'white' or 'grey,' under certain standard conditions stated in detail in the paper. Light from a lamp of known spectral distribution is modified by rotatory dispersion in a system of quartz plates and Nicol prisms in such a way that by rotating one of the nicols the light emerging from it can be made to assume the spectral distribution of a Planckian radiator at any desired temperature between 4000 and 7000°K. Such a system is,
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