Comparative Micrometric Measurements. By M. D. Evjell. 545 



Using the formula; — 



r =0*6745 



and 



in which 

 we have 



r =0-6745 sj IM.^ 



1 [y 2 ] = 1.82 



r = + 0*3 div. = +0-06/* 

 r = ± • 1 div. = ± ' 02 /i 



Using the Xo. 5 Leitz objective (with which one division equals 

 0*067 fi), on the first jfa mm. of the first j 1 ^ mm., we have the 



following :• 



r = 



r n = 



0= ZL 



±0-7 div. 

 + 0-16 div. 



+ 0-05 fi 

 ± 0-01 fi 



