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. (SeeCh. 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