2) the optical observating system, the task of which is only that 
of supplying the operator with the binocular vision, for the spacial colli- 
mation of the mark over the points of the model. 
From fig. 5 representing the complete diagram of the optical tram 
of the instrument, one can easily see the whole operation of the two tele- 
gonioscopic units (right and left), symmetrically mounted with respect 
to the middle longitudinal plan of the instrument. 
Considering the left system, one can remark that the collimating axis 
is represented by the directrix F S; and it can be assimilated to the two 
pieces PN, and N,m (see fig. 5). The first, inside the camera, represents 
the direction of the point image which is considered; the second, when 
the mark is in correpondence with the considered point image, represents 
the direction of the rays emerging from the mark itself. These two di- 
rections melt in the « Nistri» roof prism (which in fig. 5 is represented 
by 1) giving place to a new direction, along which comes to coincide 
the pencil of rays emerging from the face of the prism, and in which are 
already melted in one image only the mark and the image of the consi- 
dered point issuing from the photogram. This pencil is taken to the ocular 
observation through the optical train shown always in fig. 5. 
One has to notice that the possibility of permutation of the images 
observation, for carrying out the aerial triangulation, has been accoun- 
ted for. 
In fig. 6 is reproduced the scheme of « Nistri’s» prism, with the 
diagrammatic indication of the fusion of said pencils and their correspond- 
ing emergence. 
Obviously the direction of the pencil coming from the mark indicates 
the point to be collimated, in virtue of the property of the prism, and of 
the possibility of changing at will the direction of the telescopic mark by 
moving in the space the collimating rods. One has to notice here that 
« Nistri » roof prism is kept in an steady position respect to the direction 
of the rays emerging from the collimating rod. 
At this point the observation optics, having but the task of transpor- 
tation, start playing. 
In the fig. 7 is schematically represented the telegonioscopic device. 
Here one sees in detail how the ray emerging from the prism 1” is made 
to converge by the lens 4, and then is averted laterally by the losange 
2’ along the secondary axis bb of the gimbal. The rays emerging from the 
prism 2’ are collected inside the prism 3, of trapeze section, and then 
convoyed on to the focal plane f, determining thus a first formation of 
the image of the considered point and of the mark. 
On this focal plane the collecting lens 5 makes the beam of extra- 
axial pencils narrower, and the isosceles triangular prism 4’ deviates the 
beam itself along the axis of the bridge. 
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