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38 STEREOSCOPIC SCREEN IMAGES [Cn. I
nearly coincide upon the screen. The screen is covered with silver foil to prevent the depolarization of the reflected light. Now to look at the screen image and to make it possible for each eye to see only its own image, the observer must wear polarizing or analyzing spectacles with the prisms or piles corresponding, with the one supplying the light for its own image. For example, if the right eye image is made by extraordinary polarized light, then the right eye of the observer must have its prism spectacle so that it transmits the extraordinary polarized light, but extinguishes theordinary polarized light which produces the left eye image. And the left eye must have its prism so that it will receive the polarized light from its image, but extinguishes that from the right eye image. Each eye then sees its own image, but not the one for the other eye, and the conditions for stereoscopic vision are fulfilled.
(3) The two-color method. — For this method two complementary colors are selected — usually red and green.
(A) With two lanterns there are projected the two images of a stereoscopic pair so that they nearly coincide. There is put somewhere in the path of the beam of one lantern a plate of red glass and in that of the other lantern a plate of green glass. The observer must have spectacles or viewing glasses of corresponding colors. Then with one eye he sees the red image and with the other the green image. The combination of these colored images by the brain gives a stereoscopic image in black and white.
(B) With a single lantern the two-color stereoscopic effect can be produced as follows: The two pictures of a stereoscopic pair are printed by one of the color processes so that one is a red picture and one a green picture. These two are placed together so that they nearly coincide, then they are projected by one lantern. With the naked eye the pictures look like any two-color picture where the colors do not register, and such a screen picture is anything but satisfactory; but now if spectacles or viewing glasses of corresponding colors arc held before the eyes, one eye sees the green picture and one eye the red picture and the stereoscopic effect comes out very strikingly.
The simplest way to determine which color to put in front of the right and which in front of the left eye is to try first one color then