STEREOSCOPIC PHOTOGRAPHY 



573 



3. Accommodation. — In viewing space the accommodation or focusing of the eyes 

 varies with the distance to the object and consequently gives some indication of depth. 

 Change in accommodation involves a change of shape of the eye lenses, whereas change 

 in convergence involves a change in direction of the eye axes. 



All the factors, except that of motion, entering into depth judgment in monocular 

 vision are equally operative in stereoscopic vision. In addition the following factors 

 are present: 



1 . The presence of two views is the same as in binocular vision. 



2. The theorjr of convergence is the same as in binocular vision. However, the 

 two rays of light to the two eyes do not originate from a point in space but from the 

 two views of the point on the stereograph. Thus the actual convergence may differ 

 from that of the original space scene by varying separation between the two views or 

 by the use of various stereoscopes. 



3. Accommodation in stereoscopic vision is fixed either at the distance from the 

 plate to the eyes or at infinity by the proper use of lenses. This is an important 

 fundamental difference between binocular and stereoscopic vision. 



Leff (Bl 

 image\-'\ 



Right 

 Image 



'Right lens \<- 



-^-H 

 -->i 



Fig. 1.- — Diagram of three-dimensional object for which conditions of stereoscopic vision 



are derived. 



4. The presence of a fixed plane as represented by the location of the print or 

 transparency. 



5. The presence of a definite visible limit to the edges of the two views acting as an 

 enclosing frame which restricts the sense of size and depth. 



Apparent Location and Size of Stereoscopic Images. — Geometrically, convergence 

 gives an accurate method of determining the distance of any point from the two eyes. 

 Consequently it should give the brain a perfect depth-sensing device. 



The following formulas are derived from the geometry of convergence. It should 

 be clearly imderstood that they are based upon the fundamental premise that the 

 apparent location of the stereographic image of a point can be obtained by projected 

 rays from the eyes through the respective stereographic views of the point, the image 

 being located at the intersection of the rays. In other words they are based on the 

 assumption that the convergence of the lines of sight is an absolute determinant of the 

 location of the image. 



It can be stated definitely, however, that the absolute convergence of a single point 

 never determines its apparent depth in a stereoscopic image. Its relative convergence 

 with other points is used by the brain to locate it with regard to those points after the 

 general position of the image is fixed. 



Thus the formulas do not accurately indicate the absolute apparent location or size 

 of a stereoscopic image in space as seen through a stereoscope. They do determine the 

 mechanical conditions under which stereoscopic viewing should take place and the 



