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— 12 — 
The reference points C' and D' were mechanically obtained by means of 
two small discs on a standard fixed at right angle to a crosspiece provided 
with legs, which stood at point almost corresponding to the support points 
A and B of the carriage on the rail (Fig. 8). 
Inverting this device on its legs and repeating the measurement of the 
d difference in the conjugate position, a value was obtained which, averaged 
with the former, enabled to completely eliminate the perpendicularity error 
between the AP supporting line and the C' D' reference line. 
The measurement of the length variation d was taken by a dial gauge 
reading to 0.001 mm connected to the Z carriage. 
The results of these checks are reported in the following tabel: 
Errors of perpend. between Y motion and X, rail = 14.1" + 05" 
) ) ) ) X; rail ^» » = 1207 4057 
» )) » » Y. )) )) Z. » = 13.0" i1 0.5” 
» » » » Y » » 7 » z 3.0 1 0.5% 
d 
The above mentioned errors affect the most accurate photogrammetric 
operations to an absolutely negligible extent. 
3) Eccentricities between the axes of the bridge base Cardan couplings, 
Before measuring eccentricities between the axes of the Cardan couplings 
and those of the rod, the bending and the rotation of the bridge base as and 
when the position on it of the two Cardan coupling units varies, were cheked. 
The elastic bending of the base crosspiece resulted immaterial. 
The maximum deflection of 5 u m takes place only during the passage 
from the inward to the outward base position. 
The crosspiece rotation due to asimmetrical movements of the Cardan 
coupling units was instead noticeable. 
The necessity arises therefrom to always simmetrically change the po- 
sition of the couplings during the setting of the base. 
As can be gathered from Fig. 9, the eccentricities in each coupling to be 
measured are three: one between the primary and the secondary axes of the 
Cardan coupling; another between the primary axis of the coupling and the 
rod axis; and a third one between the secondary axis of the coupling and the 
  
— —». X 
Fig, 9 - Geometric diagram of the link connecting the space rod to the base bridge 
and of its eccentricities. 
  
  
  
  
   
  
  
   
  
   
      
    
   
    
  
  
  
  
  
  
  
  
  
  
  
  
  
   
   
  
  
     
       
  
  
    
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