124 THE ART OF PROJECTING. 



The outlines of flowers, butterflies, letters, etc., are 

 drawn upon paper with a lead-pencil, and then painted 

 with substances that exhibit different colors by fluores- 

 cence. When these pictures are used they may be 

 pinned to the screen and the light allowed to fall upon 

 them as before. Examine the pictures or other things 

 by light transmitted through red, yellow, green, blue, 

 and violet glass. The kind of light that induces 

 fluorescent action will then be apparent. 



When fluorescent substances are to be examined in 

 the light of a solar spectrum they may be made to pass 

 through it from the red towards the violet, and con- 

 tinuing beyond the visible part, for the ultra-violet rays 

 are capable of powerfully exciting fluorescence in 

 some substances. Stokes found this invisible spec- 

 trum that was competent to induce such action to be 

 as much as three or four times the length of the visible 

 spectrum. 



The following substances manifest fluorescent ac- 



These substances are generally prepared in solutions 

 or decoctions for this purpose. 



Chlorophyl may be prepared by boiling tea-leaves 

 until water will remove nothing more, and then soak- 



