The same figure pictures the cam-operating device. The movable 
section of the optical system is part of it, this being the main 
reason for which we chose this peculiar device. As a matter of 
fact (27) and (28) are respectively the first and the second rotating 
arm of the optical system; the free end of the second arm (28) 
being connected, as we said, with the guide rod. Levers (48) and 
(49) have been added to arms (27) and (28). Their free ends sup 
port cylinders (48') and (49') whose ends in turn are rounded off 
and rest on the cams (i. e. Ax, Ay, Sx, Sy). 
The points of contact between said cylinders and the cams, 
describe at 1 : 3 scale the same lines as the ones described by 
the end of arm (28) on planes parallel to the photograph. Of 
course, cams are driven so to perform the required shifts, such 
shifts being a function of the shape of the cams themselves. While, 
as has been shown, cams Ax and Ay cause the photograph to shift 
slightly on its own plane, cams Sx and Sy, by means of levers arranged 
in a manner similar to the parallelogram mentioned before, impart 
small rotations (around 2 orthogonal axes) to the optical plane (65) 
placed between the optical system and the photograph. Systematic 
projection errors due to the instrument are practically cancelled, 
for slight as they may be. Though the instrument in question is 
equipped with the well-known Santoni-type device for correcting 
lens radial distortion of the shooting camera, still the mentioned 
cams (Sx and Sy) could be made to perform jointly both the cor 
rection of distortion (in this case also tangential) and that of the 
remaining instrumental errors, if so desired. 
All devices described above might be excluded from the equip 
ment on supply, especially so with those applications where errors 
related to photographic images are acceptable. 
The user might request them at any later time and, if so, he will 
find it quite easy to fit them to the equipment himself. 
The optical plane belongs to the equipment anyway, so that it is 
possible at any time to measure vertical parallaxes to a very high 
degree of accuracy.