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PROCEEDINGS OF THE AMERICAN ACADEMY. 



Konig and Brodhun* on acuity and Fechner's fraction respectively 

 give us safe ground on which to travel in these respects. 



In Figure 1 are shown the acuity curves and the shade-perception 

 curves of the normal eye for intensities up to 100 meter-candles. 

 Curves a and b give the values of Fechner's fraction for white light and 

 deep crimson light (X = 670 /^/a) respectively, while c and d give the acu- 

 ity curves for light orange (A=605 fx^) and yellowish green (A=575/x/>i) 



respectively. The ordinates in the first case are -y, and in the latter 



case are in arbitrary units. The most important feature of these curves 

 for the purpose in hand is that they are already becoming asymptotic 

 at low values of the illumination, and except for strong colors at about 



10 20 30 40 50 60 



Meter-candles 



Figure 1. 



70 



80 



90 



100 



the same point. At about 10 meter-candles they have turned well 

 toward the axis, and beyond 20 meter-candles the gain in shade-percep- 

 tion and acuity is very slow with further increase. Hence, when the 

 light reaching the eye has risen to 10 to 20 meter-candles, further in- 

 crease does very little in the way of assisting practical vision. 



Artificial illumination can be safely based on this amount as a work- 

 ing intensity. Visual acuity is the controlling factor in most indoor 

 lighting. It varies noticeably with color, but for practical reasons, 

 which will appear later, the actual visibility of colored objects depends 

 not on the differences here shown so much as upon their general light- 

 reflecting power, which for dark hues is always low. 



At great intensities both shade-perception and visual acuity consider- 

 ably decrease, the former at roughly 25,000 to 50,000 meter-candles, the 

 latter at much lower intensity. Neither function is likely to fail at any 



* Sitz. Akad., Berlin, 1888. 



I 



