264 



As the measurements of many microscopical objects (especially 

 the rotatoria) are given in fractions of an inch in English 

 literature, and in metrical measure in foreign works, the above 

 table has been drawn up to facilitate comparison. Its use is 

 obvious. Examples : -f inch = 3-63 mm., -^ inch = 438 micra, 

 or '4SS mm. For fractions smaller than -^Iro inch that 

 portion of the table between the figures 26 and 99 

 may be used, by cutting off the last figure for hundredths, and 

 the two last figures for thousandths. Examples : ^To " lcn = 

 94-1 micra, or "0941 mm. ; y^Vo inch = 3;21 micra, or -00321 

 mm. When that portion of the table between the figures 100 

 and 250 is used it is only necessary to cut off the 

 last figure for thousandths and the two last figures for 

 ten thousandths. Examples: y^Vo i ncn = 18*8 micra, or 

 •0188 mm., ye^j-jj inch = T54 micra, or '00155 mm. The 

 conversion of mm. into fractions of an inch is performed in the 

 same manner ; thus, 529 micra or "529 mm. = -^ inch ; 397 

 micra or -0397 mm. = ^to incn 5 2 "62 micra or "00262 mm. 

 = TtVo inch; T04 micra or "00104 mm. = -g- 4 5 00 inch; 977 

 micron or '000977 mm. = 26 j -g- inch, and so on. 



One millimetre contains 1000 micra, and one inch 25399*772 

 micra. 



When a slide rule is employed put 63 on the Slide (B line) 

 in a line with 1600 on the Rule (A line) ; then everything on the 

 Rule (A line) is inches, and everything on the Slide (B line) 

 is metrical. 



A micron is very nearly equivalent to 4 trans, striae of an 

 average Arnphipleura pellucida, and to 2| trans, striae of the 

 large Navicula rhomboides in Cherry field and Sozodont gather- 

 ings. Micra are usually denoted by the symbol yu. — [Ed.] 



