CH. I] STEREOSCOPIC SCREEN IMAGES 37 



exceeded in every way by the moving picture that it is hardly worth 

 while to go to the trouble to get together the apparatus and slides 

 to show these small "effects" when such wonderful ones are shown 

 daily in every moving picture theater. 



The moving picture was originally invented to illustrate scientific 

 facts; and the indications now are that it is to become a great 

 factor in education by its striking portrayal of the processes of 

 nature. (See Ch. XI). 



STEREOSCOPIC SCREEN IMAGES 



50. For a stereoscopic screen image the same fundamental 

 law must be observed as for any other stereoscopic effect. That 

 is, there must be two slightly different images corresponding with 

 the image seen by the left eye and that seen by the right eye. 

 These images must be projected on the screen so that they nearly 

 coincide, then by some means the left eye sees its left-eye image, 

 but not the right-eye image; and the right eye sees the right-eye 

 image, but not the left-eye image. The two images are then 

 combined in the brain and the stereoscopic effect follows as with 

 ordinary naked eye binocular vision or when using a stereoscope. 



With the magic lantern this effect has been produced in three 

 principal ways: 



(1) By the aid of prism spectacles. Lantern slides of a stereo- 

 scopic pair are projected on the screen so that they nearly coincide 

 by the use of two lanterns. When this is done some people can 

 get the stereoscopic effect by looking at the pictures with the naked 

 eye, but for most people it is necessary to look through prism 

 spectacles so that the right eye shall see only one image and the 

 left eye only one. 



(2) By the aid of polarized light and Nicol-prism spectacles. 

 According to this method two lanterns are used and two lantern 

 slides, making a stereoscopic pair. For one lantern there is used a 

 Nicol-prism or a glass pile and the projection is made with the 

 ordinary polarized light. A similar prism or pile is used for the 

 other lantern, but the extraordinary polarized light is used for 

 projecting its image. These two images are projected so that they 



