•V 
10 
This operation is really rather delicate; normally the mark is 
held still for the complete run of the optical unit, along the 
-V axis with an approximation of one or two marks. We must 
remember however two very important facts; these movements come 
above all in the transport optical unit, that is after the 
fusion of the photogram image with the mark and therefore 
without jeopardising the precision of the instrument; secondly 
a residue of movements of about two marks will not be noticed 
by the operator mainly because the vertical component which 
mainly disturbs the vision is accurately eliminated. 
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(5) Having mounted the front casting and the eyepieces group we 
pass next to the operations involved in centering the mark with 
regard to the axes of the eyepieces themselves. For this 
operation, having removed the theodolite which we had previously 
worked with, we mount an auxiliary mark in the focal plane of 
the eyepiece (where the real image of the entire optical system 
is formed). Working on the adjusting screws of the lower 
rotating prism, we can exactly center the floating mark on the 
auxiliary mark and can check that when one changes the switching 
prisms from the "base in" to the "base out" the mark itself 
remains fixed in the center of the field. 
(6) We now go therefore to the mounting and rectifying of the Amici- 
Dove prisms. To carry out the rectification of the Amici-Dove 
prism it is necessary to fulfill two conditions; that is to say 
the mechanical axis of rotation of the mounting of the prism 
must be parallel to the two reflecting surfaces of the prism, 
in addition this mechanical axis must be coincident with the 
optical axis of the system. With the rectifying screws of the 
mounting it is possible in successive stages to fulfill the 
required conditions. By rotating the Amici-Dove prism 180°, 
the rotation of the mark on itself must not exceed its own 
dimension. 
(7) Having achieved all the conditions described above, the 
photogram image coincident with the plane containing the mark, 
remains to be made. To do this, the auxiliary mark is removed 
and instead a small three magnification telescope is mounted. 
These three magnifications allow us to achieve an even greater 
sensitivity. Sample grids are placed on the film plate carriers 
and the first lens is corrected until the required coincidence 
is obtained. 
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