These parts bear the ball-joints providing connexion with the end 
section of the optical collimation system; therefore the latter system 
results to be movable with respect to the photograph that, instead, 
remains unmoved during plotting. 
So Santoni has reverted to this arrangement after experience of 
long standing fully taught him its advantages and its defects. 
As far as the advantages go, we might mention not only better 
balancing of the movable parts but also another fact: the solution 
adopted here is particularly well suited, as will be shown later, for 
the introduction of such auxiliary devices as hinted to in our fore 
word. Moreover, for the purpose of removing defects, the movable 
section of the optical system was conceived as follows: 
there is an arm (27 fig. 2 and 3a) susceptible of rotation around an 
axis (m) parallel to the axis of the photograph (axis of perspective); 
the latter part bears, at its free end, another arm (28) that may 
rotate around an axis (n) parallel to the first axis (m). The free 
end of the second arm bears a right prism (29) with one side face 
turned towards the photograph ph n . 
From the side opposite to this face, the arm (28) is connected, as 
said before, to the freely extensible section (bi) of the guide-rod (b) 
by means of a ball joint (30). 
Both arms are hollow and they house those parts of the optical 
system which convey the partial image of the photograph (cor 
responding to the right prism) to the level of the collimation mark 
(34) first and towards the eyepiece (0 3 ). As shown in the drawings, 
the mark is located in the second movable arm (the one closer to 
the photograph) so that collimation is safe from errors that are cau 
sed by reciprocal rotation of the optical parts borne by the two arms. 
It is also to be noted that the flatness of the surface described by the 
center of the ball joint (30) depends entirely on parallelism between 
the rotation axes (m-n) of the two arms. It is easy even for the user 
to check on parallelism (or in case, to make the necessary adjust 
ments) with the aid of appropriate levels. 
Another cause of error due to a movable optical system is that 
there is no coincidence between the ball joint center (30) and the 
perpendicular from the point (p) collimated in the photogram onto 
the plane determined by the joint itself.