Comparative Micrometric Measurements. By M. D. Ewell. 541 

 Combining these results with those obtained by Mr. Keeley, 



These differences are also quite insensible. 



The first t ^q mm. spaces of the first T ^ mm. were next measured 

 by Mr. Keeley, using the same instruments, except that a -r^-in. 

 Wales water-immersion objective of 1 • 03 N.A. was used instead of 

 a lower power. With this combination, one division of the filar 

 micrometer equals • 03645 [m. Thirty readings were made on each 

 end of each space, with the following results : — 



The same spaces were measured by the writer in 1908, using 

 the Spencer stand, Zeiss filar, and a Spencer 4 mm. objective of 

 0*65 N.A. With this combination one division of the filar micro- 

 meter equals 0'0388 /x. Ten readings were made on each end of 

 each space, with the following results : — 



The same spaces were again measured in 1909 by the writer, 

 using a Spencer stand, the 4 mm. Spencer objective, and a Gaertner 



