128 HOW TO SEE WITH THE MICROSCOPE. 



no conveniences of your own, take this to the machin- 

 ist, or the watchmaker, either of whom will allow you 

 the use of a steel " straight edge," with which you will 

 be enabled to ascertain with tolerable exactness whether 

 one side of the micrometer is truly a plane. This done, 

 by the aid of a suitable "guage," you will also deter- 

 mine as to the other face. If, in a process of this kind 

 the micrometer betrays defects, it should be discarded 

 and another one chosen in its place. In our own prac- 

 tice we always use a Nobert test-plate, which has been 

 found to be very reliable. 



To avoid the third source of error, view the lines, 

 selecting as close a band of lines as possible, and using,, 

 as before named, nearly central light, and the two-inch 

 eye-piece, with the micrometer placed horizontally and 

 vertically on the stage, examining the bands from end 

 to end as they appear in the field. Kepeat the experi- 

 ment, but reversing the micrometer in each position end 

 for end. Any error due to the pose of the stage is thus 

 made manifest. 



Should you have reason to suspect trouble from the 

 eye-piece used, repeat the entire test on the stand of 

 some friend, always using the two-inch, or lowest eye- 

 piece. This ocular furnishing the more severe test. 



Finally, let it be known that all the eminent makers, 

 both American and English, furnish glasses that are 

 not to be rejected for non-flatness of field. We would 

 rather trust to the reputation of these gentlemen, than 

 to the test conducted by the novice. A little practice 

 on the other hand on the part of the latter will not be 

 a waste of time. 



