GEOMETRICAL MEASUREMENTS 



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ceeding 1 mm would be permissible. However, it is best not to exceed 

 a 05 mm diameter exit pupil, corresponding to a one dioptric admissible 

 eye-accommodation variation, assuming the eye to be a perfect 

 instrument. 



3. MEASURING A LENGTH IN A FOCUSING PLANE BY MEANS 

 OF TWO TRANSVERSE SETTINGS 



The length to be measured in the focusing plane is shown (Fig. 5.3) 

 by the A^Ao interspace. A^ and A2 are imaged at A[ and A', in the 

 plane observed through the micrometer eyepiece. The reticle is aligned 



'^1 



Fig. 5.3. Measuring a length by means of two lateral settings. 



with A[ and A', by means of the graduated drum T that drives the 

 spider-line transversely. Provided the graduation of T is calibrated, 

 the actual length A^A^ within the object is readily ascertained by means 

 of the expression: 



a[a:. 



A^^^ 



g 



(5.7) 



where g is the magnification of the objective Oi. Such calibration is 

 achieved by means of an objective micrometer and allows the lengths 

 AiAo to be read off the drum T: the specimen is substituted for the 

 objective micrometer comprising n divisions per millimeter and the 



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