BELL. — THE PHYSIOLOGICAL BASIS OF ILLUMINATION. 



93 



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 



'A 



o 



o 

 o 



o 



05 



o 



00 



o 



CO 



o 



o 



8 a s 



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. 



