the unit, can easily be corrected, inasmuch as the three casings superimposed 
to the bridges can be directionally adjusted. | 
Finally, we have the true observation unit composed of a single binocular 
whose lenses can be substituted by means of a revolving device to change the ma- 
gnification. 
Also this last unit is completely independent from the others and can be com- 
pared to a normal variable magnification binocular. 
As can easily be noted, the advantages obtained from this new structure are 
numerous and important: 
  
  
  
  
  
  
  
Fig. 5 - Optical diagram of the TA3/A « Nistri » Stereocomparator 
1) First of all, whenever a two-stage instrument instead of a three-stage instru- 
ment is required, it is sufficient to completely remove the central coordinatometer 
with relative bridge and floating mark box, remove the corresponding Amici 
prism, substitute the optical commutator and eliminate the controlling sinchro step 
motors. 
[n such conditions, it is, therefore, possible to transform the TA3/A into a 
TA2/A and save, without having to use new parts. 
2) The individual optical casings (the three floating mark casings, prisms casing 
and the observation binocular) being completely independent among themselves 
inasmuch as they are conjugated in a parallel ray regime, can be calibrated sepa- 
rately and thereafter assembled on the instrument. 
After this there remain simple and quick tuning up operations to complete 
the calibration of the instrument, operations relative to the direction of said bundle 
of parallel rays exclusivelv and not to the calibration of the lenses. 
3) It was, therefore, possible to eliminate the complex system of air conditioning 
inside the instrument base, a system that existed in the first models, so as to reduce 
to a minimum the movements of the coordinatometers in respect to the observa- 
tion system. If the base itself becomes warm and moves, this fact has no impor- 
tance inasmuch as even if the coordinatometers displace themselves in respect to 
sv