It should be observed that the micrometer drum readings 

 increase as the screw is turned backward, but in bringing the twin 

 wires to bisection on any division line the screw should always be 

 turned positi<vely so as to pull the diaphragm against the springs 

 which hold the micrometer screw in its bearings. 



All vernier contact edges whether on flat, beveled, or edge 

 graduated circles are exposed to more or less attrition through the 

 accumulation of dust and constant use. In Europe the vernier 

 scales are being replaced to some extent by a modified form of read- 

 ing microscope which has no micrometer drum, but a finely ruled 

 glass diaphragm in the common focal plane, by which the smallest 

 sub-division of the limb can be still further sub-divided. 



Thus in Fig. 86 the limb is divided into ^, which is not at 



all unusual for 5-in. instruments, and each degree line numbered. 



A space equal to 20', used at the diaphragm, is sub-divided 



into 10 equal microscopic spaces having a value of two minutes 



each. The long index line is the zero of the scale. If these index 



lines are not diametrically opposite in the microscopes, advantage 



of this fact may be taken by simply adding the readings in order to 



get the true result. Referring to the diagram let it be assumed that 



Micro I reads 69, 3.6 spaces, and that 



Micro II indicated 3.7 spaces, then 



Mean - 69, 7'. 3 = 



69 7' 18" 



If the fractional 

 values in both micro- 

 scopes agree, then we 

 should be required only 

 to multiply the micro- 

 spaces by two to reduce 

 to minutes and decimals 

 thereof. The method 

 is recommended for ac- 

 curacy that compares 

 favorably with vernier 

 scales and prolongs the 

 life of the graduated 

 circleindefinitely. Read 

 also Decimal Verniers, 

 p. 99 which dispenses 

 with calculations in- 

 volving minutes or 

 seconds. 



148 





