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Since a three dimensional study is essential for most purposes, the balance 
will obviously be greatly improved by a stereoscope which meets all or many of 
the interpreter's requirements. 
It is my privilege to present to you a new interpretation stereoscope, the 
“O.D.S.” designed and manufactured by the Optical Industry, "De Oude 
Delft" in Delft, Holland. 
Its unconventional construction (Fig. 1) reflects several new features which 
are important in photo-interpretation. 
The principal features are the following: 
It is possible to choose one of two magnifications by simply pulling a small 
lever. These magnifications are in the proportion of 1 : 3, namely 1'/» and 4!/ 
  
Fig.2. Two O.D.S. stereoscopes in use for simultaneous 
viewing by two interpreters. 
or, using a different set of eyepieces, 2 and 6 thus covering a wide range of 
power. When the magnification is changed the center of the field of view does 
not move with respect to the image, so stereoscopic view is mainta‘ned auto- 
matically. 
The field of view, the size of which naturally depends on the magnification 
applied, can be moved in x- and y-direction to scan the entire overlap of 9 X 9 
inch pictures without shifting either the stereoscope or the pictures. This is 
made possible by an ingenious system of mirrors and prisms, invented and pa- 
tented by the Optical Industry’s Director, Prof. Bouwers, and actuated by two 
knobs, one for each hand. Moving the field of view over the spatial model gives 
the interpreter a curious impression of flying by helicopter and, when stopping 
the motion, of hovering over the landscape. When he wishes to have a closer 
view for pinpoint reconnaissance of a certain area, he goes down to one third 
of his initial flying height by just switching over to a higher magnification with- 
  
  
  
  
  
  
  
  
  
  
   
  
    
  
  
  
  
  
  
  
  
  
  
  
  
    
  
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