STEREOSCOPIC PHOTOGRAPHY 585 



half. Thus an observer at A receives the reflected one-half of the horizontally polar- 

 ized light and the refracted one-half of the vertically polarized light. 



If the observer wears a pair of Polaroid spectacles in which the right glass is polar- 

 ized vertically and the left glass horizontally, the views are rendered mutually exclu- 

 sive and a stereoscopic fusion results. 



Theoretically there is only one possible position in which the observer may place 

 his eyes and see the views orthostereoscopically. This can be approximately found by 

 moving the head back and forth until the view appears most natural. However, where 

 the accurate judgment of depth is not of great importance there is a considerable area 

 within which a normal scenic view of objects or persons appears quite natural. This 

 area is large enough for six or more persons to view the scene at the same time. The 

 size of the area depends upon the size and the correct viewing distance of the plates. 

 The larger these are, the larger the area and the more persons who can view 

 the stereograph. 



As a device for showing stereographs to small groups of people, this system is 

 unequaled by any other unless projection is resorted to, as it enables more than one 

 person to view at the same time. 



A further advantage is that Polaroid offers the possibility of making stereographs 

 in color in which the illusion of reality is very startling. 



The only objection to the system is that the resultant view being enclosed in a 

 visible box tends to seem restricted in size to the size of the box. At the present time 

 it is fairly expensive. 



Processing, Mounting, and Viewing Stereographs. Developing and Printing. — 

 The photographic processes of developing and printing stereographs are exactly the 

 same as for ordinary photographic work. In developing negatives the photographer 

 should aim at sharp detail, low contrast, and lack of graininess. All these are par- 

 ticularly important in stereographs of small size where the magnification of the stereo- 

 scope is large. The elimination of graininess is particularly important in view of the 

 effect of the presence of the plane of the plate on the location of the image. 



For transparencies either glass plates or films may be used. The problem is exactly 

 the same as that of making lantern slides. 



Methods of obtaining the desired results with respect to detail, contrast, and 

 graininess are discussed elsewhere in the volume and need not be duplicated here. 



Reversal of Views. — In taking stereographs with a double-lens camera, the two 

 views must be transposed — interchanged, the right view being put on the left and vice 

 versa — before viewing. The reason for this can best be understood by remembering 

 that the negative in the camera has the image on it upside down and reversed from 

 right to left. The stereograph consequently has two upside-down views side by side. 

 If the plate is rotated through 180°, the two views come right side up and correct from 

 left to right, but the view taken by the right lens is now on the left and vice versa. 

 Consequently either the negative or the resultant positive must be cut apart and 

 remounted. 



If prints or negatives are separated and it is desired to determine which is for the 

 right eye and which for the left, this can almost always be done by inspection 

 as follows: 



Pick a sharp point or vertical line in the foreground which appears quite close to 

 some sharp feature of the background. The horizontal distance between these two 

 will differ in the two views, the difference being greater when the difference in depth is 

 greater. The view in which the foreground point is farthest to the right with respect to 

 the background is the left-eye view. 



Many of the methods for making stereographs are primarily for use with cameras 

 already on hand or readily available. Since the stereoscope should have the same focal 



