56 THE PRODUCTION AND USE OF LIGHT 



Diffused light is usually obtained by one or the other of the 

 following methods: 



First, by causing direct light to be reflected from some 

 uneven or unpolished surface. Examples: (1) Light reflected 

 from a common plastered, or a " white finish" wall; (2) light 

 reflected from a light-colored papered or painted wall (Fig. 

 40); (3) light reflected from the sky, especially from the 

 portion .of the sky opposite the sun, in general, from the 

 northern sky. 



Second, by causing the direct light to pass through some 

 semi-transparent substance or ribbed or fluted glass. Ex- 

 amples: (1) Light which passes through the common white or 

 opalescent globes such as we used on gas mantle burners; 

 (2) light which passes through the frosted tips, or " frosted 

 bowl," of the common tungsten lamps; (3) light which 

 passes through thin cloth such as is commonly used for curtains 

 or light window shades; (4) light which passes through ribbed 

 or frosted glass commonly used in skylights. 



72. Obtaining Diffused Natural Light. Photographers have 

 long recognized the value of diffused light in their work. 

 Most photograph galleries are lighted by means of windows 

 placed in a slanting position and facing the northern sky. 

 Only diffused light from the northern sky can enter these 

 windows. Further to diffuse the light, the windows are often 

 fitted with ribbed glass or glass coated with a thin coat of 

 white paint. Still further to control the light, muslin curtains 

 or light, semi-transparent shades are hung before these 

 windows. 



It is becoming common practice for factories to be con- 

 structed with what are known as SAW-TOOTH ROOFS (Figs. 41 

 and 42). The windows are placed in a slanting position on the 

 north slope of the " saw-tooth" roof, thus admitting only dif- 

 fused light from the northern sky. By this method of lighting, 

 even large rooms may be evenly and effectively lighted with a 

 soft, mellow light. It is found in factories thus lighted, not 

 only that the employees can do more and better work on ac- 



