The co-ordinate measurement is based on a reseau of crosses at 
centimetre intervals, and the displacements in X and Y of a point of 
detail from a nearby reseau cross are measured by short lead screws 
directly to microns. In the case of photography taken by a "reseau" 
camera, the reseaux is already there on the diapositives. In the case 
of non-reseaux photography a register plate carrying reseaux markings 
is used for each diapositive. 
The production model is illustrated in Figs. 1 and 2. The co 
ordinate measuring unit is mounted on the top of a pedestal unit with 
X, Y lead screw driving handwheels placed conveniently on either side. 
The viewing angle through the stereoscopic eyepieces is slightly down 
wards, 6°, and the operator’s position is, in fact, very similar to 
that of many stereoplotters. All electronics are housed in the con 
sole standing to the right of the machine, excepting a digitiser 
selector unit and circuitry associated with the operation of the 
machine itself, such as lamps, buzzers and zero return motors, which 
are housed in the base of the pedestal. 
The mechanical design of the instrument permits free scanning in 
X and Y by grasping and pressing release knobs on the X and Y carriages, 
the X motion being on the plate carriage and the Y motion on the tele- 
microscope carriage. Relative motion in X Is provided for each photo 
carriage, and In Y for each sighting microscope. Rack and pinions, 
instead of friction drives, have now been fitted to these relative 
motions, and the driving drums can, therefore, be graduated for the pur 
pose of taking repetitive readings. A diagrammatic arrangement of the 
X and Y movements is shown in Fig. 3 (the numbers indicate ball bearings). 
On the prototype, provision was made for observing and recording 
base inwards and base outwards, but without plate rotation. Now that 
this latter has been provided it is only possible to observe base in 
wards if more than a few degrees of rotation are applied, for to avoid 
this restriction it would have been necessary to Increase the separ 
ation of the viewing microscopes, requiring a major re-design. The 
diapositive holders are located geometrically to be interchangeable 
on the carriages. The plate rotation Is effected by a lead screw, the 
rotation of which is proportional to the sine of the angle through 
which the plate is turned. A digital encoder indicates sin 9 x 10^ + 
5000. The optical system, which had an elbow Joint containing rotating 
optical components to provide the Y scanning motion, has been changed 
so that the light is collimated between the fixed and moving parts of 
the optical system: the variable magnification system has been simpli 
fied but reduced in range to 1:1*5 differential: the nominal magnifi 
cation can be x 8 or x 12 by changing eyepieces. 
Each optical system now includes an image rotating prism, of the 
Pechan type, between the floating mark and the eyepiece. The prisms 
are mounted on bearings so that they can be rotated about their respec 
tive optical axes, and linked together by driving gears. The control 
knob is on the top of the optical casting to the rear of the eyepieces, 
and can be turned into positions which are spring-located corresponding 
to 90° rotations of the fields of view, thus allowing stereoscopic or 
pseudoscopic observation with the conversion of Y to X parallax for each. 
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