operator. This, together with the fact that a suitable optical system 
permits increasing the height of the operator with respect to the level 
of the plates, makes it possible for the operator himself to have the 
plates under examination right under his eyes and reasonably close. 
The three coordinatoaeters groups are identical among themselves 
and are radially oriented towards the center of the aforementioned 
quadrilateral. 
The construction of each of these coordinatometers is also more or 
less identical to each one of the coordinatometers of the Analytical 
Plotters whose characteristics have previously been described and, 
therefore, we shall limit ourselves to summarizing them briefly. 
The two movements of the direction of the coordinates X and Y are 
independent from one another: one of these is placed on the carriage 
which also carries the opaque mark, the other carriage transports the 
photograph. The mark is 25 microns referring to the photograph scale 
and can easily be removed and substituted by others of different sizes 
as required. Since the stereocomparators require a higher level of 
precision, we have applied on the measuring screws a screw corrector 
cam equal to the one which gave such excellent results on the TA3 
stereocomparator, the previous model. 
This screw corrector is utilized not so much to correct local 
screw errors -which in most recent materializations are most negligible - 
but because of the difficulty in obtaining sets of six screws which are 
absolutely identical in length. 
Prom this point of view the corrector simplifies the problem by 
eliminating the necessity of constructing an enormous number of screws 
from which to choose, and in addition to this it is useful for correcting 
those small deviations from the correct measurement which do occasionally 
occur. 
The placing arrangement of the coordinate axes with relation to 
the observer is completely arbitrary and depends exclusively on the 
electrical connections between the drive mechanism and transmission 
systew in the lead screws. The three plate holders can, for example, be 
placed so that the coordinate X will be, in all three photographs, 
parallel to the observation base. In such a case, the coordinate X is 
entrusted to the plate carriage in photographs 1 and 3, and is entrusted 
to the observation system in carriage 2. Or one can arrange for 
coordinate Y always to be entrusted to the plate carriage (radial 
disposition) and in this case the coordinate X is oriented parallel to 
the base only for the coordinatograph No. 2. 
I would like to point out that the latter placement becomes 
necessary when we intend to use the stereocomparator for observations