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1) Connection the plate-holder and the rod. — The connection between 
the upper part of the rod, called «short rod » and the end portion of the 
parallelogram linkage, which is in turn joined to the small bridge traverse, is 
obtained by means of a spherical coupling. 
The main property of this is, as is well known, that it enables to replace 
the terminal ball of the rod with a non-spherical cam affording the automatical 
correction, by mechanical means, of the lens distortion of the surveying camera. 
The freedom from back-lash of this connection was repeatedly tested al. 
ternatively stressing the rod and the plate-holder in opposite directions, and 
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Fig. 10 - The parallelogram linkage for plate control. 
checking the possible non-elastic shiftings due to back-lash. The checking, 
which was conducted by means of comparator dial gauges reading to 0.001 mm, 
did not reveal any appreciable errors. 
2) Surface described by the plate-holder. — The surface described by the 
centre of the coupling must be a plane when the mechanical distortion correct- 
ing system used is spherical. This condition can be prejudiced by three causes: 
a) imperfect parallelism between the rotational axes of the first and se- 
cond arm sections of the parallelogram linkage; 
b) flections in the mechanical system, which vary according with the 
geometric characteristics of the two mobile sections of the parallelogram 
linkage; 
c) reaction to the component, normal to the plate plane, of the force 
needed to move the plate. 
In the direct observation of this surface no attempt was made to deter- 
mine its relative position since possible inclinations of the surface can be easily 
remedied by means of the orientation movements of the camera; the exami 
  
  
    
    
  
    
   
   
   
  
   
  
  
   
  
  
  
  
  
  
  
  
  
  
     
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