30 
Fig. 22. Point marking instrument Kern PMG 1. 
Stereoscopic observation of overlaps between the 1st and 2nd, 2nd and 3rd and the 1st and 
3rd photographs is possible by corresponding switching of optical trains. Each photo-stage 
is provided with orthogonal cross-slides for setting the measuring marks on identical points. 
The three measuring marks, together with the corresponding saphire needles for pricking, 
are located on a yoke above the photo stages. For pricking, the yoke is moved down on the 
photo-planes by means of an electrical drive. After a visual verification of the correct pricking 
the yoke moves down for a second time to print rings around the previously pricked points. 
Fig. 23. Measuring mark of the PMG 1. 
The measuring mark is a luminous dot, matched with the diameter of the prick, and a 
luminous ring concentric to it (fig. 23). The dot or the ring can be switched off. For the point 
transfer, the dot is switched off and the ring is accurately set on the pre-pricked point. Then 
the ring is switched off and the dot is illuminated such that the prick is no more visible. Hence 
the prick does not disturb the stereoscopic fusion. 
The maximum optical magnification is x50.