21 
b). Projection system. 
The projection system consists of mechanical rectifiers and the projection computer. 
The two units are interconnected by means of electrical synchro-transmissions. 
Mechanical rectifiers. 
The mechanical projectors are fixed and located behind each other in vertical positions. 
The photo-carriers can be removed for the centration and rotated about the principal 
points for orientation (x). 
The tracking devices (for photographs) are orthogonal cross-slides carrying the optical 
units. 
The principal distance can be set between 149 and 155 mm. Under each projector there 
is at a constant distance (100 mm) another cross-slide system, representing the rec 
tified (exact horizontal) photo-plane (x,y). This cross-slide is rotatable about the 
perspective centre for oo and <p (fig. 14). The upper and the lower cross-slides are 
connected via a space rod, rotatable about the perspective centre (central cardanic 
joint). 
The location of the cardanic joint (x,y) is controlled by the respective output of the 
projection (model) computer. 
Projection computer. 
The computer is based on projections of conjugate rays on the XZ- and YZ-coordinate 
planes. The arrangement, representing the projection on the XZ-plane, is shown in