Bryn Mawr College monographs . X20 Gray No. 32, 2X20. 03 0 3 I3 I4 33 84 6 5 55 32 83-5 4-54-5 38 814 918 823 3-3 3-3 24 53 10 10 212 512 318 824 II 30 13 35 a. But if a Contrast Effect, Evidence that it May Take Placev^hen the Inducing Color is Unsensed As already stated, the strip frequently develops before thesquare; it invariably lasts longer than the square; returningseveral times after the square has finally disappeared; and influctuating its phases rarely coincide with those of the square,i. e.^ it is frequently visible w^hen the square is invisible, andconversely. Thus it is immate
Bryn Mawr College monographs . X20 Gray No. 32, 2X20. 03 0 3 I3 I4 33 84 6 5 55 32 83-5 4-54-5 38 814 918 823 3-3 3-3 24 53 10 10 212 512 318 824 II 30 13 35 a. But if a Contrast Effect, Evidence that it May Take Placev^hen the Inducing Color is Unsensed As already stated, the strip frequently develops before thesquare; it invariably lasts longer than the square; returningseveral times after the square has finally disappeared; and influctuating its phases rarely coincide with those of the square,i. e.^ it is frequently visible w^hen the square is invisible, andconversely. Thus it is immaterial for our thesis which interpretation begiven to the phenomenon. For {a) if the after-color in thestrip be a contrast sensation, our results show that it may beset up when the inducing excitation is not directly conditioningsensation; and {b) if it be an after-image sensation, they showthat it may be aroused by a previous excitation which did notitself directly give rise to sensation. AFTER-IMAGE AND CONTRAST SENSATIONS 235. 236 C. E. FERREE AND GERTRUDE RAND III. ExplanationA later paper will contain a history of the observations onthe effect of brightness changes, general and local, upon colorphenomena, beginning with Purkinje.^ Purkinjes observa-tions cover the following points, (i) The relative differencein the brightness values of the spectral colors at full and de-creased illumination. (2) The difference in the effect of changein brightness upon the saturation of colors. (3) The changesin color-tone produced by changes in brightness. After-imageThe explanation of the possibility of getting color as anafter-image from a stimulus in which no color can be sensedrests in general with the second point of Purkinjes observa-tions; namely, the difference in the effect of change in thebrightness of different colors upon their saturation. In everycase in which a colored after-image was obtained from a color-less stimulus, it was gotten at a degree of brightness whichworked again
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