. Physical researches on sensation [by] Frank Allen [and others]. [Vol. XXVIII. vation in the part corresponding to the yellow and greenish direct vision the light rays fall upon the macula lutea or yellowspot where the greatest distinctness of vision occurs. This spot is,however, small and would not be stimulated by the light falling onthe outer regions. As the lowest point of the persistency curvecorresponds to the brightest part of the spectrum, the curves showthat to the outer regions of the retina under consideration, the mostluminous point of the spectrum is nearer the green th
. Physical researches on sensation [by] Frank Allen [and others]. [Vol. XXVIII. vation in the part corresponding to the yellow and greenish direct vision the light rays fall upon the macula lutea or yellowspot where the greatest distinctness of vision occurs. This spot is,however, small and would not be stimulated by the light falling onthe outer regions. As the lowest point of the persistency curvecorresponds to the brightest part of the spectrum, the curves showthat to the outer regions of the retina under consideration, the mostluminous point of the spectrum is nearer the green than it is to themacula lutea. An elevation in a persistency curve has always been found asso-ciated with some induced or natural abnormality of color , however, it is found in the normal persistency curves for theouter portions of the retina, and indicates some slight modification Wh ITE CUR\ E! 1 1 \ 1 \ 1 c^ \ \\ \ \ M \ CO \ o \ / r \\ / r V V / // ^ > / / ^~- -^ E 1 w l-E IGUED WITH N 1 - .4 0/ .5 WAVE 1 LENGTHS. 3 .60 .7C 5 1 1. Fig. 5. Fig. 6. in the perception of yellow. The macula lutea contains a yellowpigment and no visual purple, while the retina in other parts hasonly the visual purple. This latter substance absorbs, and isbleached by, light of all parts of the spectrum but in amounts vary- No. I] rERSISTEXCE OF VIS/OX. 51 ing with the wave-length, the absorption being very shght in thered and greatest in the yellowish green, which has the most rapidbleaching effect. The region of the spectrum, therefore, which ismost absorbed by the visual purple coincides with the elevations ofthe persistency cur\es. I\. In the paper to which reference has been made an experi-ment was described in which a persistency curve was obtained whenthe eye was fatigued with white light from an electric arc. Thiscurve and its comparison normal curve are shown in Fig. 5 whichis reproduced from the paper cited. As white light, indistinguishable from ordinar) white,
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