4 
the images through a 90° angle so as to transform vertical parallax 
into a horizontal parallax. The three above mentioned prisms are 
independently controlled manually by readily accessible control knobs. 
The optical axes of the three groups just described are located in 
the same plane and are placed in such a way that they may be selected 
two by two by the appropriate prisms which in turn are placed by means 
of a rotating support which can assume three different positions. The 
figure shows the three possible positions: 1-2, 2-3, 3-1. To these 
positions correspond respectively to the models visible to the observer 
and are made up of these pairs: 1-2, 2-3, 3-1. This placing is analogous 
to that of the TA3/A and TA3/D and permits, as it is well known, the 
successive substitution of plates in the triangulation process and the 
possibility of examining two models at the same time and thus of 
checking the points under examination as they appear in each model as 
well as the accuracy of the reading. 
The optical beams picked up by the prisms of the rotating group 
are reflected in a vertical direction by means of appropriate prisms 
and then, by means of another pair of prisms, bent towards the 
observation group which is tilted 30° from the horizontal plane. 
The enlargement due to the first lens is 0.9 while the enlargement 
due to the lens of the observation group is 1.45, so that the telescopic 
group enlarges 1.3 times. The comparator is equipped with two pairs of 
eyepieces which enlarge respectively, 10 and 7 times, so that the final 
available magnifications are 9 and 13 times. 
In case less magnifications are preferred, this is made possible 
by substituting the mark carrying cones with others that enlarge to 0.7 
instead of 1. Thus the magnifications possible through the use of 
substitute eyepieces are 6.5 and 9. Of course different enlargements 
from this series are also possible by using cones and eyepieces 
calculated to special order. 
The observable field naturally depends on the magnification. It 
varies between 28.5 mm in diameter for a magnification of 6.5 to 14.7 
mm in diameter for magnification of 13. 
The resolving power obtained from these optics is considerable; we 
reach on an average, with a high contrast target, a resolving power of 
about 125-135 lines mm. 
The illuminating lamps of the photographs are easily replaced and 
their intensity is controlled by appropriate rheostats within the 
operator's reach.