HISTORICAL EXHIBITION 
43 
Exhibit 74 
Exhibit 75 
mate rectification for tilt and for changing the relative scale of photograph 
to map. 
(Lent by the R. E. Equipment Branch, War Office, England). 
Thompson pin-hole plotter, c 1944. 
An experimental model constructed to the designs of Professor E. H. 
Thompson, OBE, BA, incorporating the features of the Fourcade system 
of setting; all the setting motions are related to the air base. No lens 
system is used, the viewing pin-hole being located at the camera per 
spective centre. 
(Lent by the R. E. Equipment Branch, War Office, England). 
Precision grid stereoscope, c 1944. 
For contouring the terrain common to a pair of overlapping air photo 
graphs when the levels of a number of ground points are known. 
A glass plate covering each photograph carries a grid of fine lines en 
graved on its surface. When the two pictures are fused in the stereoscope 
the grid appears to float in space, rising or falling as the spacing of the two 
photographs is altered. 
(Lent by the R. E. Equipment Branch, War Office, England).