Production of Artificial Colour. Uindness by Moonlight. 219 



at a scarlet poppy in a cornfield causes a measurable degree of red- 

 blindness for the next two or three minutes. In applying the spectro- 

 scopic method of measuring the colour sensations described in my 

 previous paper"^ it is necessary to guard against this source of error. 

 Last summer I had a case of a man who seemed at first to be very 

 nearly green-blind. His green sensation did not reach more than half 

 way between h and F, even after fatiguing for thirty seconds with blue 

 light, and it was correspondingly shortened on the red side. After 

 conversing wdth him for some time in the subdued light of the labora- 

 tory I repeated the measurements, and found that his green sensation 

 then extended considerably beyond F. It appeared that he had been 

 strolling about the Parks on the grass in the bright sunshine. I have 

 myself frequently experienced a temporary green-blindness from a 

 similar cause. The effect seems to be intensified by looking at a white 

 surface, as, for instance, in reading a book while sitting on the grass. 

 After a time the green leaves seem to lose their colour and become 

 greyish. This effect may be often noticed during a long walk through 

 the fields. If during this condition the eyes are directed to a small red 

 spot on a black surface, as, for instance, a single geranium petal on the 

 black cover of a book, and the observer walks with it quickly into a 

 dark shed or barn, the colour of the geranium petal will seem to 

 change from red to orange and then to yellow, and finally almost 

 whitish, owing to the subjective admixture with the red of the green 

 dazzle-tint. On coming out into the light again the red colour will 

 reappear. These changes are similar to those observed in the red end 

 of the spectrum during green-blindness on opening and closing the 

 slit ;t and as the experiment requires no apparatus, I have recom- 

 mended it in my lectures for the last two years. 



The retinal fatigue induced by white light under various conditions 

 forms the subject of a recent paper by Beck.t 



* ' Phil. Trans.,' B, vol. 191 (1899), p. 19. 



t <Phil. Trans.,' B, vol. 191 (1899), p. 8, and Plate I, figs. 4 and 5. 

 X Archiv fiir die ges. Physiologie,' vol. 76, p. 634. 



VOL. LXVI. 



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