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 trans- 

 mits 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 some- 

 where 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 any- 

 thing 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 



