BELL. — THE PHYSIOLOGICAL BASIS OF ILLUMINATION. 81 



intensity reached in the ordinary course of artificial lighting, though 

 acuity may be seriously interfered with by dazzling and consequent 

 rapid retinal exhaustion at intensities of a few hundred meter-candles, 

 and the same secondary cause also impairs shade-perception long before 

 its final decline. 



It must be clearly understood that in specifying 10 or 20 meter- 

 candles as the intensity physiologically necessary to bring the eye into 

 its normal working condition, these intensities are those which become 

 visible to the eye, and not merely those that reach the objects under 

 observation. 



The light reflected from any object is Ih where / is the incident 

 illumination and k the coefficient of reflection. Then, if a is the 

 normal illumination just indicated, the required incident illumina- 

 tion is 



Taking, for example, a = 15 meter-candles, and assuming that one is 

 observing white or very light colored backgrounds for which k would 

 have a mean value in the vicinity of 0.6, the value of / should be about 

 25 meter-candles. If the background is dark fabric for which k would 

 not exceed 0.2, /would rise to 75 meter-candles, and for black fabrics 

 one could hardly get too much light. A typical application of the 

 principle may be taken in a draughting room where tracing has to be 

 done, and the drawing must be well seen through the tracing cloth, k 

 for tracing cloth is about .35, and the illumination which makes the 

 drawing visible is reflected from the drawing paper behind and passed 

 back through the tracing cloth. The drawing paper probably reflects, 

 if slightly off" white, as is common, about 60 per cent of the incident 

 light, and the final coefficient of the combination falls to about 0.25. 

 Taking the same value of a as before, /= 60 meter-candles. Ordinary 

 draughting rooms are found to be well lighted at this intensity. It 

 should be noted that draughtsmen generally use hard pencils, which 

 make marks contrasting rather weakly with the paper, so that strong 

 illumination is needed at all times. 



In illumination out of doors, as upon the street, where no weak con- 

 trasts or fine details need to be made out, a may be taken very much 

 lower, but k is also low, and the minimum of about .25 or .30 meter- 

 candle often allowed between lamps is, as the curves show, consider- 

 ably too small for good seeing. 



E^iect of Puinllary Aperture. The iris serves as an automatic stop 

 behind the cornea, adjusting itself so as to protect the retina from 

 too violent changes of brilliancy. It may vary in diameter of aperture 



VOL. XLIII. — 4 



