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tal by means of a level and with the aid of the levelling screws (16). 
After that, one passes to the arrangement of the theodolite on its support. 
The vertical axis (4) is lifted up until the telescope, directed to the Zenith, is 
very near to the camera lens. The theodolite must be set up with great accuracy 
and perfectly corrected. Besides the glass plate used for the levelling of the focal 
plane, it is useful to be provided with another glass plate carrying a cross wired 
reticule: this last plate will be used for the positioning of the fiducial marks 
and other controls. A test of the correct verticality of the axis of the theodolite 
can be obtained by placing the cross wired plate centered on the image pro 
vided by the lens when the telescope is towards the zenith, and by turning the 
theodolite around its axis. The centering of the plate must be kept at any 
position of the theodolite when observing with a microscope. In those condi 
tions the center of the target image of the collimator is, on the camera focal 
plane, the principal point. 
When this adjustment is accomplished, the parallel plane is replaced by a 
photographic plate and the operator takes a set of exposures, giving at each 
photograph a variation, generally by 5°, to the zenithal distance of the theo 
dolite, read on the vertical circle. Every time that the pencil of light results no 
longer centered with respect to the camera lens, the theodolite is accordingly 
slid along its guide. After the exposures are made along one diagonal, the 
support is rotated by 45° and new exposures are made, on the same plate, with 
the same proceedings. 
When the measuring of the tangential distortion is also carried out, it is 
indispensable, at any position of the theodolite, to correct its possible azimuthal 
displacement, which is revealed through the small telescope (12). Measurements 
are made on this negative in a comparator with 0,002 mm accuracy. Along one 
of the axis of the comparator is aligned a diagonal of the plate, the origin being 
the image of the principal point. Along the other axis can be read the tangen 
tial distortion. 
The distortion curves corresponding to the various semi-diagonals cannot 
be, in general, overlaid neither to one another nor with their middle curve 
because of the asymmetries of the lens. The computation of the Point of 
Symmetry is carried out according to the well known rules. In more complex 
cases the coordinates of the Point of Symmetry are so determined as to make 
the distortion symmetrical to a prescribed amount. The equipment allows the 
accomplishments of the various determinations and their reiterations without 
moving the camera. 
At last, always with the aid of the engraved glass plate, the adjustment of 
the fiducial marks is carried out. 
Advantages of the equipment. 
The above described equipment realises some favourable circumstances 
which can be summed up as follows: 
1) The camera, once situated on its support, does not require any further 
movement during the complete set of experiments. 
2) The operator can easily inspect the conservation of the system’s accu 
racy; and can impose the field angle with great accuracy. 
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