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observation lens since the latter is independent of the thickness of 
the plate. 
(b) For the same reason it has become possible to illuminate the 
photographs from above and, therefore, eliminating the problem of 
overheating due to the light source. The illumination can be made much 
more powerful and certainly sufficient for whatever type of photograph 
is being used. 
(c) Since the accuracy of the reading may be considered to be only 
slightly greater than that obtained in the previous models, it will 
certainly have greater stability, that is, accurate calibration may be 
maintained much longer. This has been ascertained through previous 
experience and will involve a lesser amount of maintenance work. 
(d) The most convenient placing of the coord inatometers in a 
symmetrical arrangement has led to an optical system which is more 
efficient and of better quality.