The figure describes the optical system of each bridge. At their exit we have 
two bundles of parallel rays superimposed and respectively forthcoming from the 
photographs and from the luminous spot representing the luminous mark. There- 
fore, according to the general principle already accepted for the OMI Photostereo- 
graph Beta 2, all that takes place after this phase does not impair the reading accu- 
racy. It must also be noted that should there be some slight movement of the 
coordinatometer in respect to the principal base, the measuring optical unit will 
exactly follow the same movement and, therefore, there will not be any loss of 
accuracy. 
  
Fig. 4 - Prisms box and viewing group 
As it can be seen in the figure, the three optical bridges are so placed that 
a single central casing 
g, containing the Dove prisms and the optical commutator, 
can receive the information from the three coordinatometers, and alternatively 
transmit it to the observation unit. 
The above mentioned central casing contains exclusively the reflecting system 
(prisms) and, therefore, the accuracy of its position in respect to the others is of 
no importance. The small angular differences which may occur in assembling