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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.