82 THE ART OF PROJECTING. 



the portelumiere^ while converging B and diverging c rays 

 will be obtained by interposing a convex lens of any 

 size in the path of the parallel rays. 



Fig. 61. 



Transparent substances, like glass, some crystals, 

 gases, and water permit the rays a to go through them 

 and appear upon the screen. Translucent substances, 

 like paper, ground glass, milk, allow but a few scattered 

 rays to go through them, and a diffused light appears 

 on the screen. Opaque substances, such as metals, 

 thick pieces of wood, stones, etc., stop all the light, 

 reflecting some and absorbing the rest. 



INTENSITY OF ILLUMINATION. 



When the lens is interposed in the path of the beam 

 the light appears as a circular disk upon the screen, 

 and as the rays cross each other at the focus f y that 

 point may be considered as the source of light. Cut a 

 sheet of paper or a board s, one foot square,, and hold 

 it any distance from the focus, say two feet. Its shadow 

 upon the screen will be bounded by a, e, which may 

 be measured in square feet. Now move the paper 

 to /, twice as far from the focus, and again measure 

 the shadow l>, d, it will be but one -fourth the size 

 of the other, proving that at s the paper received 



