DEVICE FOR RECORDING THE NADIR 
POSITION DIRECTLY ON THE PHOTOGRAPH 
AT THE EXPOSURE 
The problem of recording directly on the photograph taken from the 
airplane all those elements which are needed in order to determine the 
attitude of the camera at the exposure represented from the very be- 
ginning of aerophotogrammetry the subject of deep and continuos study; 
and it could not be otherwise, when one considers the advantages that 
the knowledge of the outer orientation contributes to the solution of the 
photogrammetric problem. 
For sake of brevity, let us restrain from considering the many more 
or less satisfactory solutions, which have been devised or realized and 
which after all are well known, and let's just limit to notice that, being 
obviously preferable those solutions which allow to get the outer orien- 
tation from the smallest number of calculations or subsidiary operations, 
al! researches aim to this purpose. It was therefore unavoidable that one 
should preferably resort to the gyroscopes having three freedom degrees. 
It is known that several are the devices originating from this instru- 
ment. Among these the major part aims to produce directly the « rectifi- 
cation » of the photograph at the exposure, (or control the camera through 
servomotors) or correcting by means of mirrors the image beam entering 
the lens. 
The first system lacks of accuracy because of the many intermediate 
mechanical parts, and of the mass to be set in motion; the second, although 
less inaccurate, cannot be used for wide-angle exposures. 
The task which the Ottico Meccanica Italiana — Rome — put to 
itself was to solve the problem by aiming above all to obtain as much 
directly as possible on the photogram those elements sufficient to spot 
the « plumb-point » (nadir), i.e. that point where the vertical line passing 
through the second nodal point of the lens meets the sensitized surface. 
A first solution, consisting in projecting on the focal plane a reference 
by means of a small mirror connected with the gyroscope gave encour- 
aging results, owing to the remarkable accuracy which was achieved al- 
ready in the experimental stage of the model. It showed all the same 
some trouble dependant from the fact that, whilst the oscillation of 
the primary axis of the gyro was transferred directly to the mirror, that