156 EXPERIMENTAL GENERAL SCIENCE 



133. Artificial Lighting. The light from artificial sources of 

 illumination is of varying intensity, which is usually expressed 

 in candlepower, the candlepower being defined as the amount 

 of light given by a sperm candle burning about 8 grams of wax 

 an hour. In lighting our dwellings and shops, the most satis- 

 factory light is that which comes from above in much the same 

 way that the light from the sun does. Diffused light is also 

 better than direct light. One should avoid too much light on 

 book or work and should never sit facing the light. In our 

 dwellings, lights are commonly surrounded by shades designed 

 to reflect the light in many directions and so diffuse it. An- 

 other method of securing diffused light is by the so-called 

 indirect lighting, in which the light is reflected from the ceilings 

 of the room. Since much of the light is absorbed by the 

 reflecting surfaces, this is an expensive though very desirable 

 method of lighting. 



Practical Exercises 



1. Why can we not see through a bent tube or around a corner? 



2. Can you think of a way of making objects around a corner visible? 



3. Why can a carpenter tell if a board is straight, by sighting along it? 



4. Trace the rays of light which enable you to see yourself in a 

 mirror. 



6. Why can we not see in absolute darkness? 



6. What makes a beam of light visible when it is thrown across a 

 dark room? 



7. Explain the appearance of "the sun drawing water." 



8. Why does it not become dark as soon as the sun sets? 



