gray: micrometer microscopes 45 



dary from Cape Muzon to the Arctic Ocean, a distance of about 

 1600 miles, nieasur-ed along the frontier is in progress. 



All honor to the distinguished statesmen of both countries who 

 successfully strove to find a peaceful settlement of the many con- 

 troversies which have been barely touched upon by me. Their 

 justification lies in the homely adage, that, "Good fences make 

 good neighbors." 



PHYSICS. — Micrometer microscopes. Arthur W. Gray. Bureau 

 of Standards. To appear in the Bulletin of the Bureau of 

 Standards. 



The purpose of this communication is to outline a simple and 

 rapid method of applying the proper corrections to the readings 

 of micrometer microscopes when making length measurements 

 of precision, and also an accurate graphical procedure for comput- 



ing convenient correction tables. The numerous sources of error 

 that need attention will not be discussed in this brief paper. 



In order to emphasize the advisability of examining a microscope 

 intended for accurate measurements Table 1 is introduced, which 

 shows the magnitudes of the periodic errors found in some micro- 

 scopes. On the assumption that the micrometers read correctly 

 at the beginning and at the end of a turn, the corrections in ten- 

 thousandths of a turn that must be added to the reading at the 

 end of each fifth turn indicated in the first column are recorded in 

 the other columns. Under A are the corrections for one turn of a 

 microscope that had been repaired as well as possible after its 

 micrometer thrust bearings had been discovered scored by a grain 

 of dirt. Under B are those found for a turn of a microscope used 

 in making important length measurements. The instrument had 



