STEREOSCOPIC PHOTOGRAPHY 589 



Projection Systems. Fundamental Problems. — The projection of stereographs 

 involves certain problems not present in individual viewing. These are: 



1. The projected stereograph must be viewable from a number of different posi- 

 tions. This presents two major problems: that of keeping the stereographs at equal 

 distances from the eyes and that of distortion due to oblique viewing. 



If the projected stereographs are large and side by side they will be at different 

 distances from the eyes of all observers, except those near the normal to the center 

 of the screen. This difference in size increases as the observer moves farther to the 

 side or nearer to the screen. If fusion can still be maintained, the question of eyestrain 

 arises. 



This difficulty cannot be cured by devices involving mirrors, prisms, or lenses at the 

 eye unless each observer has a device suited to his particular position. It is com- 

 pletely cured by systems which permit the superposition of the two views. Anaglyphs 

 and the polaroid system are of this type. 



The problem of distortion is much the same as that in a motion-picture theater 

 complicated and emphasized by the presence of depth. Theoretically there is but one 

 correct viewing point. From this point there is an increase of depth as the observer 

 moves back and a decrease as he moves forward. In addition, as he moves to the 

 side, the scene appears narrower and twisted, objects on the near side of the screen 

 moving toward the observer with respect to objects on the far side. 



Such distortions are, in general scenes, not too annoying. In the event of three- 

 dimensional movies on a large scale, theaters may be made somewhat narrower than 

 is now the custom. 



2. The stereographs must be made mutually exclusive to the eyes of a number of 

 individuals. This necessitates some form of stereoscope or spectacles at the eye of 

 every observer. This introduces the problem of expense, and, as a commercial proposi- 

 tion, the overcoming of the slight annoyance entailed. 



The only possibility of separating the stereograms for every observer at the screen 

 rather than at the eyes seems to lie in the Ives' system (see page 590) in which other 

 difficulties are almost insuperable. 



3. The increased difficulty of getting the proper separation between views. This 

 problem does not arise if the system permits superposition on the screen. If views are 

 placed side by side, one above the other, or in any other position, mirrors or prisms 

 must be used at the eyes to effect fusion. 



4. If motion pictures are contemplated, the additional problems of synchronization 

 of shutters and films presents itself together with the doubling of a number of expenses. 



No detailed discussion of the above problems or of the following systems is given, as 

 the field is still in the laboratory stage. 



The anaglyph system is the same as that discussed on page 583. Its advantages 

 and disadvantages are the same as set forth there. Its chief advantages for projection 

 work lie in the ability to superimpose the views and in its cheapness. Its chief dis- 

 advantages lie in the loss of light due to the colors employed and its inadaptability to 

 color work. 



In anaglyphic movies the edges of color around moving objects against a light field 

 causes an annoying flashing of color. However, anaglyphic movies have been success- 

 fully shown as a novelty in American and Continental theaters. 



The Polaroid system is the same as that discussed on page 584. The two views are 

 projected through Polaroid and superimposed on the screen. The advantages of the 

 system lie in the possibility of superposition and of using color. On the other hand 

 powerful light sources are needed owing to the cutting out of a large portion of the 

 light in polarizing. This demands that an adequate system of cooling the bulbs be 

 devised. The Polaroid Corp. is rapidly conquering the difficulties in this system. 



