(230) 
so as to afford the preserving of the adjustment of the theodolite during its 
movements along the guides and the rotation of the beam. 
In any case the reading of the zenithal level, and the inspection through 
the small telescope (12) makes evident any change that may occur. 
The theodolite is a TC 1, manufactured by the O.G. at l cc accuracy; f = 
228 mm; its zenithal level is visible in the field of the eyepiece. Owing to the 
necessity of pointing vertically upwards, the normal eyepiece is substituted by 
a diagonal one. 
By removing the eyepiece and inserting a small lamp, with filter and con 
denser lens, to illuminate the cross-hair, then focusing the objective for infinity, 
the telescope of the theodolite is converted into a collimator. Since the emer 
gent rays are parallel, the established line of sight is in the same direction 
regardless of what portion of the objective is used, and the same line of sight 
will lay along the axis of the camera when the telescope is directed to the 
Zenith. It can be directed towards any other direction of the field according to 
the inclination to the telescope. To each of these directions corresponds an 
image on the camera’s focal plane. 
During the zenithal rotation of the telescope, the objective changes its 
position: it follows a translation of the pencil of light with respect to the 
entrance pupil of the camera lens. In order to recenter the line of sight, the 
theodolite slides along its horizontal guide and the guide itself may be brought, 
by rotation, along the selected direction of field. The small telescope (12), 
provided with a prism eyepiece, is fixed to an extremity of the beam: it serves 
to verify, by autocollimation, and with reference to a small glass (15) rigidly 
connected to the theodolite, if the latter, during its movements undergoes any 
azimuthal displacements. This control is necessary when the tangential distor 
tion is to be measured. 
The upper part of the battlement (2) supports, by means of three leveling 
screws (16) a strong frame, to which is applied an interchangeable plate (8) 
which plays as a camera holder, provided with a central hole, duly shaped. 
Each type of camera requests a special plate. 
Other measurements are taken for what is concerned with emulsion filter, 
apertures etc. in order to fulfil closely the same conditions which the camera 
will encounter in service. 
Testing proceeding. 
In manufacturing a new camera it is of greatest interest the establishment 
of the “Calibrated Principal Point” or Point of Symmetry about which all 
radial distortion are approximately symmetrical. To this determination is 
bound the consequent positioning of the fiducial marks. 
The first step of the procedure after the mounting of the objective is the 
research of the best focus setting, i.e. the focus setting which gives the best 
average resolution. Thereto follows the localization of the “Principal Point” 
according to its normal definition. About this point distortion curves are 
drawn and then the Point of Symmetry is computed. For those determinations 
the above described equipment is used. 
The camera is rigidly mounted on the plate holder, a thick optical plane 
parallel is placed on the camera focal plane and this latter is adjusted horizon-