418 ALTERNATING CURRENTS 



areas under consideration are very close to the source. With 

 mercury tube lamps, Moore tubes, and lamps having certain 

 types of reflectors, the law of inverse squares must be applied 

 with great caution. 



Example. A drawing board directly under an incandescent lamp and 

 4 ft. distant has an average illumination of three foot-candles. What is 

 the illumination on the drawing board when the lamp is raised 2 ft.? 

 E 2 E z (4) 2 16 

 E l 3 (6) 2 36 

 48 

 36 



E 2 = = 1.333 foot-candles. Ans. 



184. Absorption; Brightness. When light falls on a surface 

 or object, a certain amount of the light is either absorbed or 

 transmitted and the rest is reflected. No substance reflects all 

 the light that it receives, although highly polished surfaces reflect 

 a very large proportion, as is shown in the table which follows. 

 A white surface reflects a high percentage of the light falling on 

 it and reflects all colors equally well. A pure black surface 

 should reflect no light at all, but practically all black surfaces 

 reflect a certain amount of light. When illuminated with white 

 light, the color of an object as seen by reflected light is deter- 

 mined by its ability to absorb, transmit and reflect the various 

 colors of the spectrum. For example, a green object as seen by 

 reflected light has the property of reflecting green and of ab- 

 sorbing or transmitting practically all other colors. Hence the 

 object appears green by reflected light. 



The color of an object as seen by transmitted light is frequently 

 different from its color as seen by reflected light. For example, 

 the color of thin gold leaf as seen by reflected light is yellow, 

 whereas its color as seen by transmitted light is greenish. 



The unit of brightness in the metric system is the lambert, 

 which is one lumen per square centimeter. Brightness may also 

 be measured in candles per square inch. These units are used 

 when the brightness of a luminous source, such as an incan- 

 descent filament, is under consideration. 



The brightness of a surface is the number of lumens per unit 

 area which the surface emits in the direction of the normal. 

 Let it be expressed by E'. E' is always less than the illumination 

 E, as the surface absorbs some light. The ratio E'/E is called 



