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mobile optical unit. All these operations and the following 
connections between the two groups with the relative bringing 
together of the axis bearings of the guide shaft are 
particularly delicate. If these operations are carried out very 
carefully it will be easy both to keep the image of the mark 
still during X motion and to arrive quickly at the centering 
of the pupils. 
(3) Having mounted the mark carrying carriage we fulfill the 
condition that the rays in the telescopic interval are strictly 
parallel. In order to carry out this operation one places the 
theodolite in front of the switching prism; the instrument will 
have been previously set to infinity v.ery carefully. We must 
be sure that the mark projected by the second lens of the 
telescopic system is perfectly focused without the slightest 
parallax. If this condition is not noted, we must readjust the 
position of the same lens by means of the counter rings of the 
moun ting. 
(4) The next rectification consists of making the mark immobile 
and carrying out the centering of the pupils. The conditions 
to be satisfied in order to achieve perfect immobility of the 
mark are the following: 
(a) the axes of rotation of the mobile optical unit and of the 
rotating prism support must be parallel and in particular, 
vertical, 
(b) the reflection surfaces of the two rotating prisms must 
also be parallel. 
Having placed the first reflecting prism, the one most 
responsible, for deforming the pupil, with its main surface 
horizontal, we pass on to rectify the upper prism of the 
mounting. By moving the optical group in the X direction we 
shall note the movement of the mark in any direction. By 
rotating the prism mentioned above, by means of its rectifying 
screws, in relation to the horizontal axis, we can correct the 
horizontal error component, by making small rotations around 
the vertical axis, using the appropriate rectifying screws 
again, we can correct the component for vertical errors. 
The immobility of the mark is achieved, operating in the way 
described above, with successive approximations, we must keep 
in mind that every adjustment to rectify and make immobile the 
mark must be followed up by a recheck of the pupil.