HISTORICAL EXHIBITION 
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in x and y may be made to an accuracy of approximately 0.01 mm. 
(Lent by the R. E. Equipment Branch, War Office, England). 
Exhibit 60* Panel illustrating the survey of Niagara Falls, 1927. 
(Lent by the Surveys and Mapping Branch, Department of Mines and 
Technical Surveys, Canada). 
Exhibit 61 Topographical stereoscope (precision model), type ZD 15, c 1929. 
A precision grid stereoscope for 9 in X 9 in photographs developed by 
the War Office in conjunction with Barr and Stroud Limited, Glasgow, 
for interpolation of contours between a control of spot heights and for 
parallax-difference measurements in x and y to an accuracy of approxi 
mately 0.01 mm. 
(Lent by the R. E. Equipment Branch, War Office, England). 
Exhibit 62 Barr and Stroud tilt-finder, c 1930. 
For use in conjunction with Barr & Stroud’s stereoscopic air-plotter, ‘Big 
Bertha’. 
It reproduces the geometrical conditions under which the air photograph 
was exposed, in the following way: 
a. The map screen and its adjustable eyepieces represent the ground with 
four control points at known positions and heights, and identifiable on 
the photograph. 
b. The pin-hole projector and air photographs represent the camera and 
plate, with optical path reversed, an image of the photograph being pro 
jected on to the area of the map screen. 
The projector and map screen are adjusted until the photographs of the 
control prints, as projected on the map screen, are centered in their 
corresponding eyepieces. 
(Lent by the Science Museum London). 
Exhibit 63 Boer Declivometer, 1928. 
Intended for determining the position of the nadir point of an aerial photo 
graph taken with a special camera. The instrument itself was never taken 
into use. 
The camera, which is not exhibited, has in front of its objective lens a 
system of mirrors, identical with that of the declivometer. This system 
of mirrors produces on the photograph oblique images of the outside part 
of the area to be surveyed. In these parts a number of heliotropes or 
searchlights are placed on the ground and pointed continuously at the 
aeroplane, their elevations being recorded at certain equal intervals of 
time. 
The photograph is put in the frame of the declivometer above the system 
of mirrors which project an image on to a set of ground-glass screens. 
Each screen is engraved with a system of curves of equal heliotrope- 
elevation. The picture is adjusted until the image of the various heliotropes 
fall on to the appropriate elevation curves. In this position of the photo 
graph the central point of the frame of the declivometer indicates the 
nadirpoint. 
(Lent by the Geodetic Institute, Delft Technical University, Delft). 
Exhibit 64* Panel illustrating Melvill Jones process for controlled mosaics, 1928. 
A feature of the process was the use of elastic bands for the distribution 
of residual errors. 
(Lent by Fairey Air Surveys, Ltd.) 
Exhibit 65 Nistri Photomultiplex, 1930. 
4 Archives I