- 3 - 
The micrometers, plate rot; tions and reseaux readings are all in 
dicated by circular mechanical cdgitisers or encoders, using a cyclic 
permuting code with conductive segments and sensed by brush contacts 
(Fig. b). 
By changing the decoding and transmitting system to transistor 
or solid state switching, a number of very great advantages have been 
obtained. The "read" time is extremely short, being only milliseconds, 
and a bistable store has been included so that the digitiser readings 
are transferred virtually instantaneously to the store, allowing the 
operator to continue to his next setting whilst the store is scanned 
at the appropriate rate of operation of the output device and suitably 
encoded for decimal card punching or tape code. Using transistors, 
it is possible to apply very low electrical currents to the digitiser 
sensing brushes, which avoids arcing and gives long life to brushes 
and coded discs. The buffer store holds the information in binary 
form, and must be encoded to suit the output device. This makes it 
possible to insert an encoder and drive unit to suit any form of card 
punch or teleprinter which an operator may specify. 
The standard internal program of the recording console is given 
in Table I, and typical punched card and teleprinter outputs in 
Tables II and III. ' 
Electrically-operated shutters are now made to occlude the views 
of left and right photographs at the appropriate times in the operating 
cycle when making monocular pointings to the reseaux. That is to say, 
the right photograph will be occluded when pointing to the reseau 
cross nearest to the point of detail in the left photograph, and then 
vice versa. The automatic shutters can be switched off if not wanted* 
In order to derive the photo - co-ordinates from the recorded 
readings a short computor program is required. A numerical example 
of the reduction is given in Table IV, which includes a plate holder 
rotation. In this case the axes of the system of reseaux crosses are 
no longer parallel to the measuring axes of the machine. Both micro 
meter and reseau. measurements must, therefore, be transformed into the 
directions of reseaux axes (Fig. 5). This must be done in two parts, 
fi: st for the reseau dimensions to obtain the correct centimetre 
reference numbers, and then for the X and Y dimensions, which are 
then added to obtain the full co-ordinates.