14 THE ART OF PROJECTING. 



as will be seen. It has not been found possible yet to 

 make an electric light with less intensity than several 

 hundred candle-power, while with the new gramme 

 machine before mentioned, it is expected that not less 

 than twenty thousand candle-power will be obtained. 



Sun light has about four times the intensity of the 

 most powerful electric light that has yet been meas- 

 ured. When it is reflected from a good mirror, it loses 

 its brilliancy somewhat, but is second to nothing but 

 the direct sun light. Hence, the desirability of em- 

 ploying the latter whenever it is possible, for both effi- 

 ciency and cheapness. 



LANTERNS. 



When sun light is used, the room must be darkened, 

 and the light only admitted through the opening of the 

 porte lumiere ; but when artificial lights are used, it be- 

 comes necessary, not only to have the room dark, but 

 to inclose the light, so that what is not used shall not 

 interfere with what is used. The refractive power of 

 lenses is made available for the purpose of securing a 

 larger amount of light than is possible without. This 

 will be understood by the diagram. Let a be a point 



of light, and 

 b c an object 

 three inches 

 long and dis- 

 tant six or 

 eight inches 

 from the light. 



Fig. 4. 



