of the secondary axis had necessarily to be transferred through a system 
of levers which — accurate as it was studied and realized — introduced 
a cause to error, on account of its unavoidable frictions. 
The difficulty was brilliantly overcome through a solution due to Nistri. 
Instead of one, two gyros are installed, with their primary axis at 
right angles to each other; only the oscillation of the primary axis, cf 
either of them is used, the second remaining completely free. On the 
  
sensitized surface are thus obtained the two components of the plumb point 
distance, from which one can immediately obtain the plumb point by 
measuring its coordinates or even, in the multiplex or similar instruments, 
by measuring it graphically. 
This disposition, which for simplicity is limited to one group only, is 
schematically represented in diagr. A. 
From the collimator-projector (1) emerges a pencil of parallel rays 
originating from the luminous cross-wire (2) placed on the focal plane 
4