3) The mechanical complication of the magnification system changing with conti- 
nuity has suggested the advisability to suppress the system and to substitute it 
with a certain number of discrete magnifications; 
The new system consists in having placed, over each coordinatometer and 
rigidly bound to it, a bridge containing the observation lens as well as the whole 
unit of the luminous floating mark and relative semitranslucid mirror. 
  
  
  
  
Fig. 1 - Left-hand mark box 
  
  
  
  
  
  
  
  
Fig. 2 - Central mark box 
  
  
  
Fig. 3 - Right-hand mark box