12 
During the calibration readings, the screws are working in the same 
conditions in which they will be used in the Instrument. 
Every screw is checked twice, at different times. The screw is 
discarded if accidental errors exceed three microns on the 26 cm of 
useful length. The coupling of the two screws of the same coordinate is 
made with an error in length not exceeding three microns. From 
experimental testings carried out on the prototype it has been found 
that, while for the Y coordinate the calibration of the measuring screws 
is maintained to within a few microns, by going over the readings for 
the length on the instrument at the time of testing, for the X coordinate 
we find a systematic shortening in length of about 10 microns. This is 
due to the fact that while the Y guide tube is supported by a central 
support which eliminates any bending the X guide tube is not equipped 
with one and this owing to the weight of the mobile optical carriage, 
is slightly bent. 
It has been noted that even small bendings of the guide tubes can 
considerably effect this phenomeno. In other words it is very easily 
possible, rotating the tubes on their own axes, to obtain a regular 
running and very good accuracies in the grid readings. However it is 
possible in this way, to lose the correspondence between the length of 
the lead screws as measured on the test bench and the one measured at 
the instrument test. 
The final test and check of the whole instrument is carried out 
with the same criteria used for the stereocomparators. In other words 
the readings are made for a co-ordinatometer at a time. Always the same 
grid plate is used and the 25 characteristic points are read twice, 
backwards and forwards. 
As I told before, the readings are corrected for k and screw length. 
In this regard, it is to be pointed that the latter correction does not 
allow only for the screw length of the co-ordinatometer under test, but 
also for the screw length of the other co-ordinatometer. 
In this way the correction for the screw-length is the mean value 
of the two screws, and it can be actually introduced by the computer. 
The Root Square Mean difference admitted is 3 microns, with a 
maximum deviation of 5 to 6 microns. The results of the tests of the 
instruments built up to this time, both for the civilian and the 
military type, are listed in the following table: