CH. XII] TRANSLUCENT SCREENS 461 



On the curved surface of the diagram are given the degrees of inclination of 

 the light. On the diameter, and on the radius at right angles to the diameter 

 are given the percentage of apparent brightness. Magnesium oxide is taken 

 as the standard and called 100%. 



The data shown on the diagram are given in figures in the table, 632. 



Curve i. Screen coated with magnesium oxide. It is to be noted that, it is 

 only in the central region that the full 100% of reflection occurs. 



Points 2222 Plaster of Paris screen. 



Curve 3 Cardboard screen. 



Points 4 4 Screen painted with white lead. 



Points 5 5 Screen painted with Artists' Scenic White. 



Points 6 6 Screen painted with zinc white. 



Curve 7 Cardboard screen painted with aluminum. 



Curve 8 Zeiss metallic screen. 



For 9 see the table, 632. 



For the Mirror Screen, see the table, 632. 



Points 10 10 10 Reflection and transmission of a white muslin screen. 

 Note its uniformity ( 632). 



Points ii ii ii Reflection and transmission of white gauze (Griswoldville 

 gauze, No. 10). With this screen more light is transmitted than reflected. 



Point 12 Transmission of ground-glass. 



Point 13 Reflection of bristolboard. 



conditions. At an angle of 30 degrees and upward the metallic 

 screens appeared almost black, and the white screens pure white. 



631. Translucent screens. For the old phantasmagoria and 

 for many appearances given by shadow pictures it is necessary to 

 have a translucent screen like ground-glass or translucent cloth 

 or paper. The paper or cloth is rendered as translucent as desired 

 by the use of water, water and glycerine, or oil. Tracing cloth 

 makes good translucent screens of moderate size. 



With a translucent screen the apparatus is entirely out of sight 

 behind the screen and only the picture shining through the screen 

 is seen by the audience. This is not so good and effective a method 

 of showing projection images as the opaque white screen or the 

 metallic screen, for much more light is lost (fig. 248). It is still 

 used in some institutions, as it entirely eliminates the projection 

 apparatus and the operator from the auditorium ( 631 a). 



The ground-glass screen is excellent, but this, like a metallic 

 screen restricts the brilliant image to a rather narrow angle (see 

 630, 632 and fig. 250). The ground surface should be fine or there 

 is given the appearance of looking toward a bright light in a snow 

 storm, this is especially marked if one is near the ground-glass and 

 looking nearly along the axis. 



