mr 
4 
the horizontal pair of bearings in Question is adjusted. 
In order to regulate the measuring screws, using the test rod, the 
comparator is once more placed on the outer side of the plate-holder 
and the same sliding operation is carried out. At this point the X 
guide and measuring screws are parallel in the space to each other and 
perpendicular to the Y guides and Y tubes. 
All the operations of general mechanical calibration described 
above, are carried out with a tolerance of 2 hundredths of a millimeter. 
The next group of operations for general mechanical rectifying 
have necessarily the aim of making the movements of the optical groups 
and the plate-holders as exact and regular as possible. 
This is achieved providing that the X and the Y motions are 
obtained without any rotation component, and in particular those which 
might influence the accuracy of the readings due to the considerable 
distance between the rotation center and the collimation axis are 
eliminated. 
Thus for example the plate-holder carriage rotation around the Y 
guide axis would have negligible influence on the accuracy granted that 
the only effect would be a displacement in the focusing of the image. 
The precision would be more affected by the rotation of the optical 
group around the axis of the X guide tube; this because of the distance 
of 112 mm between the mark and the plate-holder plane. 
These factors, along with others that we shall examine later on, 
lead us to the different tolerances established for the support bearings 
of the X guide with respect to the Y. guide. 
The next group of general levelling and rectifying operations 
which I shall describe are carried out by putting the main frame of the 
co-ordinatometers on the basement (fig.4). The first operation consists 
in putting horizontal the plane containing the axes of the two guide 
tubes for the Y. co-ordinate. The verification rule is placed with a 4 H 
level, on these tubes and working on the four screws set' into the 
support feet of the instrument the two above mentioned guide tubes are 
made horizontal. 
The accuracy obtainable is in the field of 0.02 mm, in fact the 
maximum distance between the supporting points of the rule placed 
diagonally with respect to the two Y guide tubes, is approximately l 
meter. 
In virtue of the general mechanical calibration, also the tubes 
of the X movements have to be horizontal. Now, after having mounted the 
carriage of the mobile optics, holding also the illuminators, a final 
adjustment of the previous calibration is carried out.