7 
The two faces x, and x, of the dihedral intersect said bundle of nuclear planes according 
to two bundles of nuclear beams having the centers in the respective nuclear points N, and 
N,. Between the said bundles there necessarily exists a projectivity of which the edge of 
the dihedral is the axis. It is easy to image that the homography, in which the bundles of 
   
/ 
—«6— 
/ 
    
\ \ 
\ Cod A 
UN a me WC me Cen 
\ 
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Fig.4. Nuclear beams projectivity and correspondent zones on the two take frames. 
nuclear beams correspond, is identically preserved, when the take dihedral is developed 
on a plane, which can take place either with faces adjacent (dihedral open) or with the 
faces overlapping (dihedral closed as one can see in Fig. 9—6). 
Let us now consider the take dihedral developed open-faced as in fig. 5, there will 
immediately be seen that each image point supplied by one of the frames must necessarily 
have its homolog on the other frame upon the nuclear beam corresponding to that which 
passes through the said image point. (Projective principle of Hauk). 
Obviously this only necessary condition it not sufficient to identify the pairs of 
homologs. But if the point of which the homolog is searched for belongs to an image line 
to which corresponds its conjugated image line upon the other frame, each one of the points 
of a pair is always the intersection of the line to which it belongs with its own nuclear 
beam, so that if a point is set upon one of the two lines, its homolog results determined by 
the intersection of the nuclear beams corresponding to its own, with the other line. The 
condition also becomes sufficient as in the case of the stereogram of a spatial line, for 
example, the trajectory of a moving point in threedimensional space. 
d) Difficulty of the analogical translation. 
Once a point is considered on one of the two images of the linearform object, its homolog 
is determined projectively only if there is established the practical procedure with which