IN ITS RELATION TO THE EYE. 451 



countered in the lighting of rooms of low and medium height. The 

 difficulty decreases with increase of the height of the ceiling. In 

 rooms whose ceilings are very high in proportion to other dimen- 

 sions of the room, it seems safe to say that comparatively good 

 results could be gotten with almost any reflector of modern design ; 

 for it is much easier in such rooms to get the bright sources of 

 light, primary and secondary, out of the zone of most harmful in- 

 fluence on the eye. 



9. The loss of efficiency sustained by the eye in an unfavorable 

 lighting situation seems to be muscular, not retinal. The retina 

 has been found to lose little if any more in efficiency under one than 

 under another of the lighting systems employed. 



10. The observation of motion pictures for two or more hours 

 causes the eye to lose heavily in efficiency. The loss decreases 

 rather regularly with increase of distance from the projection 

 screen. It seems little if any greater, however, than the loss caused 

 by an equal period of steady reading under much of the artificial 

 lighting now in actual use. In making these tests care was taken 

 to choose a projection apparatus which gave a picture compara- 

 tively steady and free from flicker. 



11. In all the conditions tested a rather close correlation is 

 found to obtain between the tendency of a given lighting condition to 

 cause loss of visual efficiency and to produce ocular discomfort. 

 The tendency to produce ocular discomfort, as already stated, was 

 estimated by the time required for just noticeable discomfort to be 

 set up with the eye both working and at rest under the conditions to 

 be tested. The results of this work were also carefully checked 

 up by the determination of the mean error of the observation. 



II. Some of the Conditions Tested (Commercial Types of 



Lighting). 



The tests throughout the work were conducted in a room 30.5 

 ft. long, 22.2 ft. wide and 9.5 ft. high. In Fig. i this room is shown 

 drawn to scale : north, south, east and west elevations, and plan of 

 room. In the plan of room are shown by a cross and the appro- 

 priate numeral the 66 stations at which the illumination measure- 



