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4» 
There is in fact the possibility of checking and eventually 
correcting all the rotation components with a single equipment. An 
illuminated sight and a 10" level perpendicular to its axis are mounted 
on the carriage, and finally a theodolite is placed in order to be able 
to collimate the grid of the sight itself. By sliding the carriage 
forward and backwards the movements of the image of the grid and of the 
level are observed. Movements of the vertical line of the grid indicate 
rotation of the carriage due to backlash or eccentricity of the bearings 
or due to guides which are not rectilinear. Up and down movements of 
the horizontal line indicate similar faults either in the bearings or 
in the guides. Movements in the level will show rotation around the X 
axis due to an error in parallelism between the support tubes and the 
guide tubes of the same co-ordinate. 
The distance between the axes of these tubes is 232 millimeters 
depending on the sensitivity of the level used, the residual errors of 
non parallelism can be eliminated with the precision of 0.01 millimeters. 
In order not to lose parallelism between the guide tubes and the 
measuring screws, previously obtained, the position of the support tube 
with respect to the above mentioned tube must be rigorously rectified 
by regulating its pair of vertical bearings. 
As to the eventual errors shown up by the collimator grid, the 
guide tube must be rotated around its own axis until the best position 
is found. 
It is possible to notice displacements of the grid line, of 
quantities approximately as big as one half of its width. 
These displacements correspond to rotations of the mobile optics 
carriage of about 10" to 12". 
The tolerance for the rectil inearity of the tubes, which are 
chromium plated and then ground, is 8 microns along the whole length. 
This tolerance corresponds approximately to the sensitivity limit of 
the measuring system described above. 
It is interesting to note that among all the different instruments 
built (Analytical Plotter of military and civilian types) it has never 
been necessary to substitute any tube and that the adjusting operations 
of the X movements do not lead to a loss of time of more than 30'. 
The four couples of bearings rolling on the guide tube must have 
an eccentricity tolerance of less than three microns. 
It should be also noted that, from experience gained on the 
prototype, it has been felt useful to increase up to 181 mm, the 
distance between the two groups of bearings responsible for the correct 
sliding of the mobile optical unit.