444 FERREE AND RAND— LIGHTING 



1. A determination was made of the average illumination of the 

 test room under each of the installations of lighting used, and of the 

 distribution of light in the room. The room was laid out in 3 ft. 

 squares and measurements were made of the horizontal, vertical 

 and 45° components of illumination at 66 of the intersections of 

 the sides of these squares, and at the point of work. In all cases in 

 which the variation of intensity was not the special point of inves- 

 tigation, the illumination for each installation was made as nearly 

 equal as possible at the point of work. 



2. A determination was made in candlepower per square inch 

 of the brightness of prominent objects in the room, such as the test 

 surface and reading page ; the ceiling spots above the reflectors for 

 the indirect installations ; the reflectors and the ceiling spots above 

 the reflectors for the semi-indirect installations ; the reflectors, open- 

 ings of reflectors and the lamps in so far as they were visible for 

 the direct installations ; the specular reflections from surfaces, etc. ; 

 and the surfaces of lowest brightness to get the range. 



3. Since the angle of presentation is an important feature in the 

 effect on the eye, a determination was made also of the angle of ele- 

 vation of some of the more important surfaces such as the reflector, 

 opening of the reflector, etc., above the plane of the observer's eye 

 when held in the working position. 



4. Photographs were taken of the room from three positions 

 under each system of illumination. 



In the selection and use of observers for the work the following 

 are some of the precautions that were taken : Care was exercised in 

 the first place to choose only those who had shown already a satis- 

 factory degree of precision in other work in physiological optics and 

 whose clinic record showed no uncorrected defects of consequence. 

 All were under 30 years of age. Before being allowed to take part 

 in the actual work of testing each observer was trained to a satis- 

 factory degree of precision in the 3-minute record under a given light- 

 ing condition and in the 3-hour test under several of the conditions 

 to be tested. In the actual work of testing the results were com- 

 piled from several observations and the precision was checked up 

 by the size of the mean error. No results were accepted as signifi- 

 cant unless the variation produced by changing the conditions to be 



