. Physical researches on sensation [by] Frank Allen [and others]. •55 €0 Wave Lergln. Figure 5. Luminosity Curves. It, now, for aiiv one wave-length we take the reciprocalsof the corresponding values of the persistence ot visionfor each of the nine curves in Fig. 4. they will, when plotted,give a series of curves as shown in Fig. 6. 1 hese curvesconform to the equation I = klogl. Each of these curves consists of two straight lines ot differentslope, the change in direction occurring very abruptly. Theslope of the lower parts of the curves varies in an extraordin-ary manner, that tor the wave-l
. Physical researches on sensation [by] Frank Allen [and others]. •55 €0 Wave Lergln. Figure 5. Luminosity Curves. It, now, for aiiv one wave-length we take the reciprocalsof the corresponding values of the persistence ot visionfor each of the nine curves in Fig. 4. they will, when plotted,give a series of curves as shown in Fig. 6. 1 hese curvesconform to the equation I = klogl. Each of these curves consists of two straight lines ot differentslope, the change in direction occurring very abruptly. Theslope of the lower parts of the curves varies in an extraordin-ary manner, that tor the wave-length ^ being quitehorizontal. 13 AMERICAN JOURNAL OF PHYSIOLOGICAL OPTICS. Cos a. (^Lamincsity). Figure 6. The Relationship between the Reciprocal of the Persistence ofVision Values and the Luminosity. The explanation of this curious phenomenon is affordedby the duphcity theory of Von Kries, according to whichthe retinal cones are the parts that function in the percep-tion of very bright light and colors, while the rods performsimilar functions for dim light. The upper part of eachcurve therefore is for cone vision and the lower is for rodvision. The change in direction of the curve shows that thechange from one type of vision to the other takes place veryabruptly. From the fact that the. lower part of the curvefor the wave-length is horizontal it would appearthat below a certain intensity a change in luminosity doesnot affect the persistence of vision to any extent. 14 PRIMARY COLOR SENSATIONS ALLEN
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