. Physical researches on sensation [by] Frank Allen [and others]. y designated the wave-length .4^0^ as thethird transition point. When the retina was fatigued with the violet of wave-length .400;i a persistencv curve was obtained with but oneelevation in the violet, the rest of the curve coinciding withthe normal. This is shown in Fig. 18. .Again, comparingthis single elevation curvx with Fig. i;, which is a curve ofthe double elevation type, it is clear that a fourth transitionpoint must lie between them. This was found to be at thewave-length .420^, which when used as the fatiguing colorgav
. Physical researches on sensation [by] Frank Allen [and others]. y designated the wave-length .4^0^ as thethird transition point. When the retina was fatigued with the violet of wave-length .400;i a persistencv curve was obtained with but oneelevation in the violet, the rest of the curve coinciding withthe normal. This is shown in Fig. 18. .Again, comparingthis single elevation curvx with Fig. i;, which is a curve ofthe double elevation type, it is clear that a fourth transitionpoint must lie between them. This was found to be at thewave-length .420^, which when used as the fatiguing colorgave the persistency curve shown in Fig. 19. This shows noelevations in any part, but coincides with the normalthroughout its length. We can represent these curves in a different way so as toshow them all in one figure. There is evidence to provethat the persistence of vision, or the duration of visual im-pressions on the retina, is dependent only on the luminosityor brightness of the color, except for very low intensities. AMERICAN JOURNAL OF PHYSIOLOGICAL OPTICS. ■4-Oju 50 -60 Wawe Length. Figure 17. Retina Fatigued with Wave-Length If, therefore, the brightness of all parts of the spectrum wereuniform, the persistency curve would be a straight hori-zontal line instead of the now familiar parabolic form. Re-ducing, then, the normal curve to such a horizontal straightline, and the fatigue curve in the same proportion, the rela-tion of the two curves will be shown perhaps more clearly,especially in regard to the characteristic elevations. This 28 JKIMARY COLOR SENSATIONS ALLEN has been done for the nineteen sets of curves which I havejust described and they are all shown in a single figure, viz:Fig. 20. The wa\c-lengths of the fatiguing colors are indi-cated by the ; on th^ right of the diagram and are
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