VIH INTRODUCTION. 



one one-thousandth part of a millimetre (0.001 mm ), and is 

 designated by the Greek letter // ; it is equal to the 

 0.00039 ( 2 g* 40 ) of an inch. Ten micras are marked 10^, 

 four and a-half micras, 4.5^. 



According to the old system, 10/z would be called 

 "one one-hundredth of a millimetre," 4.5^ would be 

 forty-five ten-thousandths of a millimetre. It is, there- 

 fore, much simpler to apply the term micra in all micro- 

 scopical measurements. 



