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ROYAL SOCIETY OF CANADA 



Two sets of observations were made at two regions on the temporal 

 side of the retina, about ten and twenty degrees from the centre. 

 Measurements were attempted at other regions farthci- out, but the re- 

 sults 'were too unceiiiain to be of any use. 



The measurements are shown in table 3, and are plotted in figure 

 4, in comparison with a curve for the centre of the eye. 



The curves show that the duration of all colour impressions dimin- 

 iisbes 3& the distance from fthe centre of the retina increases; or, iji other 

 words, the peripheral portion of the 'retina is more sensitive to fluctua- 

 tions in the intensity of the ligjht than the centre. 



The ten and twenty degree curves also show a remarkable eleva- 

 tion in the part oorrespondiiig to the yellow and greenish yellow. In 

 direct vision the light rays fall upon thelmaculia lutea or yellow spot 

 where the greatest distinctness of vision occurs. This spot, however, 

 is small and would not be siiniulatod by the light falling on the outer 

 regions. As the lowest point of the persistency curve corresponds to the 

 brightest part of the spectrum, the curves show that for the outer regions 

 of the retina under consideration the most luminous point of tho spec- 

 trum is nearer the green than it is for the macula lutea. 



An elevation in a persistency curve Iras alwaysibeen found a.ssociatod 

 with some induced or natural ahnormality of colour vision. ITere, how- 

 ever, it is found in the normal persistency curves for the outer parts of 

 the retina, and indicates some slight modification in the perception of 

 yellow. The mjacula lutea contains a yellow pigment 'and no visual 

 purple, while the retina in other parts has only the visual purple. This 

 latter substance .absorbs light of all parts of the spectrum, but in amounts 

 varying with the wave length, the absorption being very slight in the red 



