BELL. — THE PHYSIOLOGICAL BASIS OF ILLUMINATION. 



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main nocturnal reliance. Cone vision begins to come perceptibly into 

 play at a few thousandths of a meter-candle, and at a few tenths is 



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pretty well established, but does not become normal over the visual area 

 below five or ten meter-candles, and gains materially even beyond that, 

 especially in acuity, which is weak at the lower intensities. 



