The second is a simple office instrument for normal prints. Two 7-power magnifiers 
with transparent "trousers" are mounted on a piece of 1/4 in. clear plastic which is 
attached to a simple drafting machine. The prints are base-lined accurately and 
oriented on the board at the correct separation, being secured, preferably by magnets 
on their outer corners set over metal inserts in the board. The lens stereoscope can then 
be oriented with respect to the base-line and will subsequently retain correct orientation 
as the overlap is scanned. A desk lamp in front of the board will provide adequate 
illumination. All of the overlap can be scanned if one of the prints is partially cut 
through (on the photo side and along a straight line) and folded under. 
This is the one 
disadvantage of this instrument. (See Fig. 3.) 
  
  
Fig. 3 
Normal stereo prints and 7X lens stereoscope on draughting machine 
The third is more versatile (and more expensive, and less portable). It was 
exhibited at the Canadian Institute of Surveying meeting in Ottawa last January through 
the courtesy of Canadian Applied Research Limited. It comprises a Wild Mirror 
Stereoscope with 3 X and 8 X binoculars mounted on a bridge that spans the light table 
and carries the top lighting. The prints, diapositives, or negatives are centred on two 
rotatable clear plastic discs and can thus be readily base-lined. The discs are mounted 
on similar squares that can be relatively adjusted x-wise for correct separation, and 
then this assembly can be moved in x and y below the stationary stereoscope, and over 
a masked light table, to scan the overlap. Precise base-lining can be obtained and 
repeated, and air photo bases and y parallaxes can be measured with the aid of super- 
imposed transparent scales that are aligned through, or parallel to, the disc centres. 
11 
2b