Comparative Micrometric Measurements. By M. D. Ewell. 539 



micrometer (using one fixed wire and one parallel movable wire, 

 and pitch of screw fa in.), a Tolles /^-in. objective of 0-42 N.A., 

 and a Bicknell achromatic condenser diaphragmed to about one- 

 half the aperture of the objective. With this combination one 

 division of the filar micrometer equals "26289 fx. 



Forty readings were made at each end of each space, and the 

 mean only is recorded. 



The instruments used by the writer on the ■ 1 mm. spaces were 

 a Spencer stand (of excellent construction, but of moderate weight), 

 a Zeiss filar micrometer (pitch of the screw, \ mm.), and a Leitz 

 No. 3 objective, of 18 mm. equivalent focus and 0-28 N.A. One 

 division of the filar micrometer with this combination equals 

 0*169 ja. Ten readings were made at each end of each space 

 measured. The results of the measurements by the writer were as 

 follows : — 



The results of Mr. Keeley's measurements were as follows :- 



Combining the above results, giving them the weights 1 and 4 

 respectively, that being the ratio of the square roots of the number 

 of observations respectively, we have the following as the relative 

 corrections : — 



2 o 2 



