are placed sidervays in respect of the flux of the illumination in 
order not to affect the formation of the image of the ground. Fig. 1 
and 2 show two variants in straps disposition, fig. 3 shows a way 
for fixing the lenses to the straps. 
The experience in the use of reticolated slides has demonstrated 
that, when it is desired, as often happens, that there be an increase 
in the precision of the control beyond the limits of the precision 
with which the marks on the glass slide were engraved, it is there 
fore necessary with appropriate instruments of measurement to 
establish in respect to theoretical positions the positional errors 
(for example in x and y) of aech cut mark, so long as it is remem 
bered at the moment the photographs are utilized. With this premise, 
it is obvious that with the Santoni device more than give positions 
of the reperes, determined with approximage precision, it is neces 
sary to measure and know their position whatever it is even though 
these measurements are effected with adequate precision. To this end, 
using the Santoni device there can be made a photograph of a white 
face, conveniently illuminated, situated in front of the objective 
of the camera, having placed a photographic slide, of suitable thi 
ckness and flatness, in the place of the film. This slide can serve 
as a test to determine with adequate means of measurement (for 
example to a comparator) the position assumed by the single reperes 
with regard to the normal marginal reperes which have been assu 
med as a point of origin. Analogous measurements made on each 
photograph to be utilized for the plotting and comparing such of 
measurements with those resulting from the « test», (upon exami 
nation) there can be discovered the deformations formed on the 
photograph and relative corrections can be effected. This process 
presupposes a plotting of numerical character, which accordingly 
becomes realized in the use of Stereocomparator in the form of the 
analytic aerotriangulation. In this case an opportune scheduling 
for the electronic calculator is necessary. 
But even in the case of plotting with analogical instruments, it is 
possible to materially effect corrections applying on each of the 
two projection cameras of the instrument a slide « test » obtained 
as mentioned above, against which is laid the photogram to be 
plotted, generally a glass slide. For obvious reasons it is better to