at an efficiency of approximately 8 lumens per watt. This corresponds fairly closely with the color of an ordinary mazda B tungsten light burning at 1.25 watts per mean horizontal candle power. Other selectively absorbing glasses designed to make possible the production of artificial daylight include the Ives-Brady glass, a very satisfactory but rare product, the Luckiesh “Tru-tint” glass, and the Corning “Daylite”’ glass.
C. Normal Gray Light.—The conception of “white light” is one which is of fundamental importance to many of the purposes of colorimetrics, for example in colorimetry by the “monochromatic” method, in defining complementaries, etc. There are a vast number of characteristic intensity distributions of radiant energy which can be used with practical success to meet this need. Although the one most frequently employed is that of average noon sunlight, to be of the greatest theoretical as well as practical significance, the definition of “white light” should evidently determine a spectral distribution which will generate a pure gray by its action on the normal human visual apparatus in a normal condition. Since there are an infinity of conceivable distributions which would satisfy this requirement, it seems advisable to limit the general form of the distribution to a species of which only one member can excite a gray. Distributions of the Planckian type meet this requirement (32, 198-202), and are further to be recommended because of the approximate conformity of all natural and artificial radiant sources to the Planckian law, and the comparative ease with which distributions of this sort can be reproduced in the laboratory. Preliminary determinations by Priest, (82), using a system of nicol prisms and quartz plates as a filter to yield Planckian distributions representative of temperatures lying between 4200 and 6200°K, indicate 5200° as the value for the gray stimulus. This figure is regarded at present as highly tentative, on account of the small number of subjects tested and doubt as to the normality of certain of them. Criteria for the selection of the pure gray, other than that of the simple introspective judgments used by Priest, may also be advisable.
