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emitted by incandescent solids, such as those contained in natural and artificial illuminants, although in the majority of cases Τ is not the actual temperature of the material, but is a temperature determined by the distribution itself and known as the color temperature, this being the actual temperature of a theoretical black body which would yield that same relative distribution in the visible spectrum. Table 7 and Fig 5 give the relative intensities for various representative wave-lengths for Planckian distributions at a considerable variety of temperatures.

Fig. 5

A. Average Noon Sunlight.—The most important standard of energy distribution, from the point of view of colorimetrics, is that which characterizes “daylight,” since it is with respect to deviations from this distribution that the chromatic processes of vision have been adjusted by nature. Unfortunately, however, the form of this distribution is highly variable. There is, in the first place, the radical difference between sky-light and direct sunlight, the former exhibiting that marked deficiency in long-