114 THE ART OF PROJECTING. 



ABSORPTION BANDS. 



If a piece of colored glass be held in the path of 

 the beam of white light before it enters the lens, Fig. 

 89, a part of the light will be absorbed and black 

 bands of greater or less breadth will appear upon the 

 screen. The glass may be held between the prism 

 and the screen with about the same result. Some 

 of the pieces of colored glass, which are quite com- 

 mon, will give very distinct absorption bands. It will 

 be well to try red, yellow, green, blue, and violet 

 glasses. If the color is very deep a greater width 

 must be given to the slit else the spectrum will be seen 

 with difficulty. 



The chemical tank (see page 34) may be used to 

 hold solutions of various kinds in this place. A wedge- 

 shaped tank is also very convenient, as it enables one 



fig. 90. 



to pass the light through any required thickness of a 

 solution, and thus to note the effects of thickness upon 

 absorptive action. This tank may be made five inches 

 long, four inches broad, and an inch thick at its broad 

 end. A piece of thick rubber cut as in the figure will 

 answer for bottom and edges of this tank. 



