The Present Stains of Micrometry. By M. D. Ewell. 691 



No. 21. " Centimeter A," the standard of the American Microscopical Society, 

 prepared in 1882 by the U.S. Bureau of Weights and Measures, and carefully 

 verified by Professor C. S. Pierce. Surface of platin-iridium ; centimetre 

 divided into millimetres, the first millimetre into tenths, and the first tenth 

 into hundredths. Three defining lines mark each sub-division, except the 

 hundredths of a millimetre ; the measurements of Professor Pierce were 

 made from the mean position of one triplet of lines to that of another, except 

 in the case of the hundredths of a millimetre, where the defining lines are 

 single. 



The following are the corrections of the separate hundredths of the first 

 one-tenth millimetre as determined by Professor Pierce in 1882, including 

 the hundredths mm., between the first and second and the eleventh and 

 twelfth defining lines of the first one-tenth mm. 



The corrections above recorded as made by the writer were determined from six series of 

 measurements, two of which were made in 1885, and the rest in 1889, with powers varying from 

 60 to 2250 diameters. 



