IN ITS RELATION TO THE EYE. 



457 



serted in the series to give gradations in density. These reflectors 

 were installed 30 inches from the ceiling in accord with the prin- 

 ciples of indirect lighting. Clear tungsten lamps were used as light 

 sources with each installation. These reflectors are numbered in 

 order of their density from greatest to least, that is, Reflector I. is 



Chart III. (Semi-indirect Reflectors Diflfering in Density.) Showing 

 the tendency of the six types of semi-indirect reflectors to cause loss of 

 power to sustain clear seeing. In A, power to sustain clear seeing before and 

 after work is represented on the ordinate and hours of work on the abscissa. 

 In B, percentage of drop in power to sustain clear seeing after work for the 

 different reflectors is plotted along the ordinate and brightness of reflector 

 in candlepower per square inch along the abscissa. 



5 By multiplying the above values by 486.8 they may be converted into 

 millilamberts, a term frequently used by engineers to specify small brightness 

 quantities. 



