224 The Microscope. 



is firmly held in its then position. The shaft, which has 

 meanwhile continued to revolve, by means of the eccentric 

 above described, now drops the diamond into contact with the 

 plate, and moves the diamond-holder, ruling a long line on the 

 plate, immediately at the close of which process the diamond is 

 automatically lifted a short distance above the plate and by the 

 continued motion of the eccentric carried back to its original 

 position ready for another stroke. The index meantime has re- 

 corded one line. At this point of time the second circuit is 

 broken, the first circuit is closed, the long arm of the lever, 

 which has now by 'means of the cam, regained its original po- 

 sition, is again moved between the stops, and the plate is again 

 advanced 1-100 mm., and another, and this time a short line is 

 ruled, and this process is repeated again and again as many 

 times as desired, every 5th and 10th line being made longer 

 than the others, as above described. 



So much for the merely mechanical part of the process. 



This, however, alone will not insure the making of a correct 

 micrometer. Before ruling a micrometer the errors of the pre- 

 cision screw must have been investigated and the necessary cor- 

 rections determined. These errors are of two sorts: periodic 

 errors of a single revolution, and lineal errors, each of which 

 must be investigated. Briefly, the periodic errors are investi- 

 gated by ruling lines every 5th or 10th division of the index- 

 wheel for one complete revolution, and comparing these spaces 

 with each other upon a separate comparator, or by using the 

 same part of the precision screw of the ruling machine as a com- 

 parator, in which latter case, the corrections can be determined 

 in terms of the divisions of the eye-piece micrometer of the 

 microscope on the index wheel. The lineal errors are of course 

 investigated by comparison of ruled spaces with a standard upon 

 a comparator in the usual way. 



Assuming these errors to have been investigated, their 

 corrections ascertained, and a table of corrections prepared, if it 

 is proposed to rule a standard micrometer, the first thing to be 

 done is to prepare a table of settings of the index so as to apply 

 these corrections. The next step is to find a ruling crystal upon 

 the diamond. This is a process requiring great practice and ex- 

 perience. The empirical method of cut and try, is the method 



