NATURAL AND ARTIFICIAL LIGHTING 65 



lights, have a strong tendency to cause the pupils of the eye 

 to close, thus, in a measure, shutting out the light and pro- 

 ducing the same effect as that due to poor illumination. 

 With indirect lighting all bright spots and glaring effects are 

 avoided and shadows are few; the diffused light coming from 

 all parts of the ceiling makes this an ideal system of artificial 

 lighting. 



75. Amount of Light Which Should be Provided. It is 

 impossible to give any but very general rules governing the 

 amount of light which should be provided fora room, and there- 

 fore the number and size of the lamps which should be installed. 

 The color and the nature of the wall coverings and the char- 

 acter of the furniture and the decorations, as well as the use 

 to which the room is to be put, all have important bearings 

 upon the lighting. A white wall paper or a "white finish" 

 wall will reflect 80 per cent, of the light; a red, dark brown, 

 or dark green wall will reflect only about 15 per cent. A light 

 buff or yellow wall will reflect 45 per cent, of the light; a light 

 apple green will reflect about 40 per cent. The decorations 

 of a room determine largely the illumination of the room with 

 a given amount of lighting. 



Intensity of illumination is measured in FOOT-CANDLES. 

 A FOOT-CANDLE is the amount of light, or illumination, received 

 by a plane surface 1 ft. from a l-candlepower light. The 

 illumination varies inversely with the square of the distance 

 from source of the light to the surface illuminated, i.e., if the 

 distance from the lamp be doubled, the illumination will be 

 but one-fourth as great ; if the distance be made three times as 

 great, the illumination will be but one-ninth as great, etc. A 

 16-candlepower lamp illuminates a surface 1 ft. distant with 

 an intensity of 16 foot-candles, but it illuminates a surface 4 ft. 

 distant with only 1 foot-candle intensity; a 32-candlepower 

 lamp illuminates a surface 1 ft. distant with an intensity of 

 32 foot-candles, but it illuminates a surface 4 ft. distant with 

 an intensity of only 2 foot-candles. 



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