458 WHITE IMAGE SCREENS [CH. XII 



cloth. Behind the screen the seams show as black or dark streaks, 

 as less light traverses the screen along the seam ( 627 a). 



628 Paper screens. The suitability of white paper screens 

 has been recognized for a long time. One of the best possible 

 screens is a large sheet of white cardboard. As shown by photo- 

 metric measurements, the reflections from a white cardboard are 

 almost as great as from the standard surface of oxide of mag- 

 nesium ( 632). The white cardboard is especially suitable for 

 the images of the high power projection microscope, and if it could 

 be had in sufficiently large sheets it would make an almost perfect 

 screen for large rooms. (In large paper stores one can get sheets 

 71x112 cm. (28x44 in.). The paper used for drawings by 

 architects and engineers and 69 x 102 cm. (27 x 40 in.) in size is 

 also excellent for screen purposes. It is not so easy to get a smooth 

 surface as with the cardboard). 



Finally, cloth is sometimes faced with paper to give a more 

 opaque and perfect screen. 



SCREENS WITH METALLIC SURFACES 



629 Dull white surfaces reflect almost equally throughout 

 the whole hemisphere (fig. 248) and therefore the image appears 

 almost equally brilliant in any position. Those near the axis of 

 the projection apparatus in the middle of the room do not see the 



627a. Screens for traveling exhibitions. When exhibitions must be 

 given in school-houses and in halls where there is no lantern and no screen, the 

 exhibitor must supply both. In traveling it is inconvenient to carry a roller 

 screen, and usually the screen is folded so that it can be packed in a small 

 space. This, of course, makes creases in the screen, and besides there is noth- 

 ing to support it so that it will hang smooth and even. 



For a traveling screen a heavy, seamless bed sheet is excellent. Bed sheets 

 in one piece as large as needed are to be had. To hang these sheets there 

 should be a strong cord along the upper edge either in a hem or in curtain rings. 

 From the corners of the sheet should be strong cords by which the sheet can be 

 stretched out smooth and held in position by passing the cords through screw 

 eyes or attaching them to other fixed supports. 



It is well also to have rings along all the edges to attach strings to, to pull 

 the edges taut, and to support the curtain at the upper edge. 



For temporary use, a sheet may be stretched and held in position by tying 

 strings to the corners and by fastening the strings along the edges by safety 

 pins. 



