JOURNAL 



OF THE 



ROYAL MICROSCOPICAL SOCIETY. 



OCTOBER, 1910. 



TRANSACTIONS OF THE SOCIETY. 



XIII. — Comparative Micrometric Measurements. 

 By Dr. Marshall I). Ewell, Chicago, Illinois. 



(Read May 25, 1910.) 



The measurements recorded in this paper were made at the sug- 

 gestion of the writer by Mr. Frank J. Keeley, of Philadelphia, 

 who is widely known as an accomplished microscopist, and by the 

 writer, with the view of ascertaining how nearly independent 

 measurements of the same spaces under different conditions would 

 agree, and the probable accuracy to be attained in such measure- 

 ments with medium and high powers. 



The results are recorded to the second decimal place, not 

 because confidence is felt in so high a degree of accuracy, but in 

 order better to compare results. Upon this subject Mr. Keeley, in 

 a letter to the writer, stated in substance that if we accept • 1 fx 

 as the limit of accuracy in micrometry, it would seem useless to 

 record decimals beyond the nearest • 1 fi ; and while this is pro- 

 bably good practice in giving measurements of various objects, with 

 the above understanding it does no harm in the comparison of 

 micrometers to use a larger number. Four or five decimals may 

 well be used in stating the value of one division of the filar micro- 

 meter, when such value has been determined by taking the mean 

 of many readings. 



In 1889 the writer touched upon this subject in a paper pub- 

 lished in the Proceedings of the American Society of Microscopists 

 for that year, on pages 64 to 66. The measurements there re- 

 corded were made in 1885 and 1889, with powers varying from a 

 £-in. dry objective, in winch one division of the filar micrometer 

 equalled 0'126/a, to a 2 I 5 -in. homogeneous immersion objective, 

 Oct. 19th, 1910 2 o 



