IN ITS RELATION TO THE EYE. 



453 



concaved. When placed above the lamps employed, they served 

 merely to direct the light to the working plane. No protection from 

 the brilliancy of the light source was afforded to the eye. For the 

 seini-indirect system inverted alba reflectors ii inches in diameter 



POSITIOD A 



POSITIOH B 



Indirect 



6 lighting units :n field o:' view 



4 lightiig Tinits i.i. field of view 



>lreot 



POSITION C 



POSITIDN D 



2 lighting irnits 



Ln field 



Bo light 



Lng ^mita Ln field 



Indirect 



Direct 



-' S S 0' i 2! S 



Chart I. (Direct, Semi-indirect and Indirect Systems of Lightmg.) 

 Showing the power of the eye to sustain clear seeing under direct, semi- 

 indirect and indirect systems of lighting, and the efifect of varying the ob- 

 server's position in the room or the number of bright sources, primary and 

 secondary, in the field of vision. Power to sustain clear seeing before and 

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



PROC. AMER, PHIL. SOC, VOL. LVII, EE, SEPT. I4, I918. 



