Pi g. 
Generally the guide tube should not be readjusted so as not to lose 
the calibration already carried out in the first stage at the workshop. 
Therefore the level is placed in the Y. direction and the sliding is 
repeated. Because of the bending of the Y guide tube (this tube is 
715 mm long, while the X tube is 409 mm long) a gradual movement of the 
bubble in the level will be noted. This is adjusted therefore by applying 
tension on the central adjustable support of the tube itself. By sliding 
the carriage we can check to see whether the level bubble stops. At this 
point, having once more placed the comparator on the illumination arm, 
we complete the levelling of the glass of the plate-holder by making it 
slide in the X direction (moving the illuminator) and in the Y. direction 
(moving the plate-holder carriage) and working on the support screws of 
the glass. Finally with the collimator placed in the Y direction, we 
can check as we did before for the X axis, the sliding movement for Y. 
Any movement of the vertical line or the horizontal line of the grid of 
the collimator will show defects similar to those described above for 
the mobile optical unit and attributable to either the guides or the 
bearings. Even in such a case, we can correct it by rotating the guide 
tube on its axis or replacing any defective bearings. As far as the 
bearings of this group are concerned, it should be noted that only 
horizontal couples need to have a maximum eccentricity tolerance of 
three microns. In fact the tests carried out have confirmed that, with 
bearings sorted in such a way, the maximum errors produced by the only 
eccentricity are mainly important for the X direction, and their 
magnitude is about 2 to 3 microns. 
The vertical couples of bearings are necessarily less critical as 
far as the intrinsic precision of the instrument is concerned and in 
this case, bearings with an eccentricity tolerance of 5 to 6 microns 
are used. 
In fact the same tests have proved that both for the couples of 
bearings rolling on the guide tube and for the couple rolling on the 
support tube, bearings with an eccentricity, up to 15 microns can be 
used, without remarkably affecting the instrument accuracy. 
When the eccentricity error is within the above mentioned range 
the tendency to defocus the image which is produced by the eccentricity 
itself is actually negligible. 
On the other hand, the specifications for the couples of bearings 
of the X mobile optical group are more restrictive. All the couples of 
bearings both horizontal and vertical rolling on the guide tube must 
have a maximum eccentricity tolerance of 3 microns. The horizontal 
couple located near the support of the observation lens is particularly 
critical. 
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