The reading and recording of the coordinates is limited to the micron even 
values. Also the accuracy of all the reading field and for each coordinate is in 
the order of circa 2-micron mean square error. 
The extended use of several instruments distributed in different countries, has 
supplied data used to improve and. perfect these instruments; in fact, while it was 
noted that the accuracy of the instrument performance has proven to be notably 
constant, in the minor details some drawbacks have been noted, such as the one 
pertaining to the recording system. 
The possibility of an imperfect positioning at the time of recording caused, 
even though rarely, the materialization of double contacts and, therefore, of erro- 
neous recording, inasmuch as this causes the recording of the code resulting from 
the addition of the codes of the two figures corresponding to said contacts. 
This drawback has been completely eliminated by not only improving the 
mechanic unit entrusted with the positioning, but also by materializing an automatic 
locking system of the recording each time that a double contact occurs; therefore, 
even if the positioner fails in its task, it is impossible to experience an erroneous 
recording. 
Special care has also been given to the fast speed system and, in the last 
instruments, was introduced a completely automatic and reliable unit which has 
proven to be completely satisfactory. 
[ shall not go into details pertaining to other minor improvements introduced 
in the instruments manufactured lately, and I shall now pass to the description 
of the new model of the Stereocomparator, being shown at the current exhibition 
of the Congress, which, especially in its optical part, embodies a most important 
innovation, responsible for great advantages. 
Inasmuch as the accuracy of the instrument has supplied a satisfactory per- 
formance in all cases when the instrument was properly used, the structure of 
the coordinatometers and their static and kinematic characteristics were left un- 
changed. Particular care was devoted to the observation optical system, which 
following careful studies and repeated tests was completely revolutionized. The 
transformation of the optical system was suggested by numerous considerations, 
of which the principal are the following: 
1) The grouping of the observation optics in one single mechanical unit, upheld 
by the main base, was responsible for the fact that it was not easy to reduce the 
instrument to only two stages as sometimes it would have been desirable for eco 
nomic reasons: 
2) The observation unit having to be upheld independently from the coordinatome 
meters (and that is to say by the main base) was apt to feel the temperature 
changes inside the instrument caused by the illumination lamps, motors and other 
electric accessories, which imposed the necessity to use the instrument only after 
having stabilized the temperature of the instrument, and that is to say feeding 
current to the instrument itself some hours before starting to measure;