6 THE ART OF PROJECTING. 



the room : for instance, the projection of the Fraunho- 

 fer lines in the spectrum of the sun, and the phenomena 

 of diffraction. For these, and the like, the darker the 

 room the better. 



The curtain in the window that holds the porte lumi- 

 ere will need to have a hole cut in it large enough to 

 allow the beam of light to come through, and to permit 

 the hand to give proper motions to the mirror. A flap 

 should hang over this when sunlight is not wanted, and 

 the electric light or the lime-light is used instead. 



THE SCREEN. 



The white surface that receives the projected picture 

 is called the screen, and it may be a white finished 

 wall, or white cloth properly mounted. The back of a 

 large wall-map makes a good screen if the light is used 

 in front of it, and only a small disk of light is needed, 

 but the backs of such maps are apt to get discolored, 

 and to become so dark as to be useless. They maj> 

 then be made white by painting them with whiting, 

 mixed in a thin solution of glue. 



For a parlor exhibition, a common sheet may be 

 hung against the wall, or between the folding doors, 

 and the lantern used on either side. If the lantern is 

 placed back of the screen, the latter should be kept 

 wet, as it is made more translucent, and the pictures 

 will appear brighter. 



When the porte lumiere, the electric light, or the oxy- 

 hydrogen lantern is used, a much larger screen will be 

 necessary. They are sometimes made twenty-five feet 

 square or more, but for most purposes a screen fifteen 

 feet square will be large enough. Common bleached 

 sheeting, ten quarters wide, can be bought in most 

 towns. A strip of this, ten yards long, cut into two 



