Artificial Lighting 183 



For special uses, as in printing establishments, for photography, 

 in accounting offices, where color discrimination is not required, 

 certain colors may be absent or one color predominate, as is the 

 case with the mercury vapor light, which has no red rays. Such 

 a light may have hygienic advantages. For exterior illumina- 

 tion as in a street, where quantity of light is of first importance, 

 a single color may strongly predominate, as in the yellow flam- 

 ing arc. This gives far more light than the red or the white 

 arc light. 



Seventh. These items indicate that a thoroughly safe, 

 useful, hygienic light must in its limited range approximate 

 the light of the sun in steadiness, diffusion, intensity or bright- 

 ness, and quality or whiteness. 



The standard of lighting. A room that is well lighted by 

 day may be taken as the standard by which the artificial illumi- 

 nation of the same room may be determined. Study the way 

 in which diffused sunlight is distributed in a room with ample 

 windows and preferably a northern exposure. Note that the 

 ceilings are only moderately lighted, but that the lower part of 

 the room is abundantly and brightly lighted. Then compare 

 with this the artificial lighting of the room, and observe es- 

 pecially the light distribution and concentration. 



SUMMARY 



Progress in artificial lighting is indicated by the use of such means 

 as the torch, fat, oil, gas, and electricity. 



Illuminating gas was first used in England about 1805. 



The Welsbach mantle increases the illuminating efficiency of gas. 



Illuminating gas is either water gas or coal gas. 



Water gas consists of hydrogen and carbon monoxid and is the 

 product of steam and hot carbon. 



Coal gas is a product of the destructive distillation of bituminous 

 coal. 



Coal-tar is the material from which are produced by synthetic 

 chemistry, dyes, oils, perfumes, drugs, and explosives. 



Two types of electric lamps are the incandescent and the arc lamp. 



The filament of the incandescent lamp is of carbon or metal. 



