The determination of the co-ordinates of the steel balls was done with the 
aid of a stereo-microscope. This microscope had two measuring marks, one in 
each ocular holder. Fig. 18. These marks were adjusted to give one visual floa 
ting mark on the object to be measured. This device was a setting microscope 
with the floating mark defined in all three dimensions. The object to be measu 
red was placed on a holder on the base carriage of the stereo-autograph Wild 
A7, while the stereo-microscope with the floating mark was fixed to the frame- 
Fig. 18. 
Optical parts of the stereomicrosco 
pe with a floating mark. A: Oculars. 
B: Image planes with two index 
marks on glass plates. C: Roof angle 
inverting prisms. D: Lenses in the 
optical tubes. E: Objective. F: 
Floating mark formed by projection 
of the index marks in B, through the 
optical parts C, D and E. 
work of the autograph. By moving the base carriage the co-ordinates of diffe 
rent points on the object could be measured directly by the model co-ordinate 
counters in the autograph. See also Torlegard [31]. 
As the steel balls were located on both sides of the plexiglass plate only 9 
points could be determined in one set-up. The object was then turned over and 
the other 9 points were measured. To relate the two sides to each other three 
needles were fixed on the edges of the plexiglass so that they defined points that 
could be measured in both positions. These three points were then used for a