Comparative Micrometric Measurements. By M. I). Ewell. 547 



The following are ten complete measurements by Mr. Keeley 

 on the first t ^q mm., using the I Tolles objective, with which one 

 division equals 0-17 /x : — 



r = ± 0"13 div. = ± 0-02/* 

 /•„= ± 0-04 div. = + 0-007 ft 



The following are a series of ten complete measurements on the 

 first j^q mm., using the -j^ Wales objective, with which one 

 division equals 0*03645 fi : — 



r = ± 0-3 div. = ±0-01/1 

 r = ± u-09 div. = ± 0-003 p 



The probable errors both of a single observation and of the 

 arithmetical mean furnish no indication whatever of the existence 

 or non-existence of constant errors involved in the measurements, 

 and have been here computed simply because that is the customary 

 practice in measurements of precision, and as an easy means of 



