STEREOSCOPIC PHOTOGRAPHY 575 



following discussion that mechanical conditions for orthostereoscopic view ing, such as 

 proper focal lengths and bases, have been achieved. 



First consider the effects of viewing stereographs without a stereoscope. It is well 

 to restate that in such viewing absolute convergence is not of importance. In viewing 

 the stereograph of Fig. 2, for instance, the resultant image would not seem to be of 

 exactly the indicated size and shape and would not be located at the indicated distance 

 d' from the eyes. 



A possible reason for this can be seen if it is remembered that in binocular vision 

 there is a fixed tie between accommodation and convergence. Thus in looking at a point 

 2 ft. away the eyes are focused for 2 ft. and the axes of the eyeballs intersect at 2 ft. 

 Regardless of the distance of the fixation point, nearlj'^ the same angle of convergence 

 always goes with any given focus, only varying slightly for objects not directly in front 

 of the observer. Both change together as objects of different depth are viewed. 



In stereoscopic vision this established tie is broken. The two views of the point 

 on the stereograph determine the convergence. This convergence changes with 

 changes of separation between different points on the two views. However, the 

 accommodation remains constant, i.e., the focus of the eyes is either fixed for the dis- 

 tance froin the eyes to the stereograph or bj^ a distance dictated by the lenses of the 

 stereoscope — usually infinity. This split of the accommodation-convergence habit is 

 the outstanding difference between binocular and stereoscopic vision. Since the eyes 

 are presented with a situation foreign to binocular vision, the psychological interpreta- 

 tion tends to differ from that of binocular vision. 



In very simple stereographs containing only separate points and lines ^ and viewed 

 without a stereoscope, the resultant interpretation tends to place one prominent point 

 or plane of the picture — called the "fixation" point or plane — in or near the plane of 

 the plate. Thus one point is approximately determined from the accommodation. 

 The depth of the rest of the points of the picture are interpreted from this point in 

 accordance with both the relative convergence with this point and the other depth 

 factors relating the points and objects to each other. 



This accounts for the fact that a change in the separation of the two views of the 

 stereograph causes no perceptible change in the position or size of the resultant image. 

 Regardless of what this separation may be, so long as fusion is maintained, the eyes 

 "draw the views together" until the fixation point appears in the proper plane. The 

 relative convergence of various points remains the same in any case. 



The location of the final image differs for different observers. The resultant effect, 

 however, is that the image is drawn forward from where it should be on convergence 

 principles and is smaller with a proportionally greater decrease in the depth than in the 

 other dimensions. 



As the views are made more complex bj'^ the use of planes which obscure each other, 

 perspective elements, and objects of known size, these added factors tend to dominate 

 the interpretation of the size and location of the image, while accommodation and 

 convergence become of increasingly less importance. The use of relative convergence 

 becomes so bound up with perspective elements that its effect cannot be separated 

 from them. 



Regardless of the degree to which other depth factors are present, the tendencj' 

 to fix one point in the plane of the plate never entirely disappears, with the result 

 that the stereographic image, without the use of a stereoscope, always appears smaller 

 and less deep than the natural object. 



When a stereoscope is used for viewing, a change in separation of the two views 

 still causes no change in the resultant image. The accommodation, if proper focal 



1 French, J. W., Trans. Optical Soc. (London), vol. 24. 

 Trump, R. ,I., Trans. Optical Soc. (London), vol. 25. 



