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Historical and Theoretical Data 25
by the comparison of lineatures with homogeneous grey mixtures on the color top. For this purpose, a scale of five different luminosities should he prepared by means of black lines on white paper, the lines varying in breadth from 0.05 to 0.40 mm., and by measurement of the breadth of the lines and interspaces the luminosity of the tones can be obtained. Then by means of black and white circles of paper on the color top, the luminosity of the same can be determined, the angles of the white sectors measured and thence the brightness of the homogeneous greys calculated. The lined sheets should be observed at such distance that the lines are still distinctly visible; the surface does not appear homogeneous, but with a little experience and repeated experiment the comparison can be made without difficulty. The objective luminosity of the grey, formed on the color top, is known and the subjective sensation is obtainable from the equal brightness
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Fig. 2.
of the lined surfaces. The result should be a straight line at an angle of 45 degrees, but it takes the form of a logarithmic curve, so that the subjective luminosity sensation increases in logarithmic progression to the subjective luminosity. The result is shown in Fig. 2, and it will be seen that a grey consisting, for instance, of 3.5 parts of white and 6.5 parts of black, appears about twice as bright as it ought to, and a mixture of equal parts of black and white appears to have only one-fourth black.
The same phenomenon should appear with colors when mixed with black, but the brightness of the colors alters the law. Yet this is of considerable importance in color mixtures, because only a part of their black