= 3 — 
and positioned under multiplex projectors. Pin holes were then 
punched in an opaque white card in several places and the card 
placed over this. At each pin hole on the surface of the light box 
a bright star appeared and these stars clearly defined the plane 
of intersection. A small portion of a map was successfully plot- 
ted with this device on the 29th of November, 1951. 
If this new form of light-dot grid could be enlarged to 
cover the full size of the multiplex model, it seemed to offer 
advantages over the conventional method of multiplex plotting. 
Such a piece of equipment was produced in 1952 and on this a 
1:50, 000 map of Belle Isle in the straits of Belle Isle off the north 
tip of Newfoundland was compiled. This instrument had a cast 
metal frame and three adjustable legs with micrometer scales 
reading to 0.001 inches (approximately .025 millimetres). The 
plate glass top measured 15 inches by 27 inches and six fluores- 
cent lights were used for illuminating the targets. For operation, 
this plotting unit was placed on top of the multiplex plotting table. 
In producing this larger model, some problems were en- 
countered in preparing the grid for the light dots. Although the 
illuminated targets do not have to be located with a high degree 
of precision, it is both simpler to prepare and easier to use if 
they are placed in a symmetrical pattern. 
Experiments indicated that a spacing of one centimetre 
would be satisfactory. When the symmetrical grid is placed 
parallel to the air base it is very easy to view different areas of 
the grid with each eye and thus appear to see a false plane dis- 
placed from the true plane of elevation. By sloping the grid at a 
ratio of one to four from the parallel, this tendency is eliminated, 
Since the spacing between similar parts of the grid is increased to a 
approximately four centimeters. 
The size of the pin holes is also quite critical and is largely 
dependent upon the strength of illumination: for very small tar- 
gets the eye tends to integrate brightness and area and apparent 
brightness becomes a function of the size of the illuminated area. 
The approach used, therefore, was to design the lighting 
first and then make the dots sufficiently large to form distinct 
targets. The size of pin hole used was approximately 0.15 milli- 
metres in diameter and about 1,500 were punched by hand in the 
  
  
  
  
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